What changed?

--- 8.4
+++ 9.0
@@ -1,3 +0,0 @@
-
-
-When on, tables that are referenced by a query will be automatically added to the FROM clause if not already present. This behavior does not comply with the SQL standard and many people dislike it because it can mask mistakes (such as referencing a table where you should have referenced its alias). The default is off. This variable can be enabled for compatibility with releases of PostgreSQL prior to 8.1, where this behavior was allowed by default. Note that even when this variable is enabled, a warning message will be emitted for each implicit FROM entry referenced by a query. Users are encouraged to update their applications to not rely on this behavior, by adding all tables referenced by a query to the query's FROM clause (or its USING clause in the case of DELETE).

--- 8.4
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+vartype: string
+context: user
+boot_val: 
+unit: None
+enumvals: None
+
+The application_name can be any string of less than NAMEDATALEN characters (64 characters in a standard build). It is typically set by an application upon connection to the server. The name will be displayed in the pg_stat_activity view and included in CSV log entries. It can also be included in regular log entries via the log_line_prefix parameter. Only printable ASCII characters may be used in the application_name value. Other characters will be replaced with question marks (?).

--- 8.4
+++ 9.0
@@ -1 +1 @@
-The shell command to execute to archive a completed segment of the WAL file series. Any %p in the string is replaced by the path name of the file to archive, and any %f is replaced by the file name only. (The path name is relative to the working directory of the server, i.e., the cluster's data directory.) Use %% to embed an actual % character in the command. For more information see Section 24.3.1. This parameter can only be set in the postgresql.conf file or on the server command line. It is ignored unless archive_mode was enabled at server start. If archive_command is an empty string (the default) while archive_mode is enabled, then WAL archiving is temporarily disabled, but the server continues to accumulate WAL segment files in the expectation that a command will soon be provided. It is important for the command to return a zero exit status if and only if it succeeds. Examples: archive_command = 'cp "%p" /mnt/server/archivedir/"%f"' archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
+The shell command to execute to archive a completed WAL file segment. Any %p in the string is replaced by the path name of the file to archive, and any %f is replaced by only the file name. (The path name is relative to the working directory of the server, i.e., the cluster's data directory.) Use %% to embed an actual % character in the command. It is important for the command to return a zero exit status only if it succeeds. For more information see Section 24.3.1. This parameter can only be set in the postgresql.conf file or on the server command line. It is ignored unless archive_mode was enabled at server start. If archive_command is an empty string (the default) while archive_mode is enabled, WAL archiving is temporarily disabled, but the server continues to accumulate WAL segment files in the expectation that a command will soon be provided. Setting archive_command to a command that does nothing but return true, e.g. /bin/true (REM on Windows), effectively disables archiving, but also breaks the chain of WAL files needed for archive recovery, so it should only be used in unusual circumstances.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-When archive_mode is enabled, completed WAL segments can be sent to archive storage by setting archive_command. archive_mode and archive_command are separate variables so that archive_command can be changed without leaving archiving mode. This parameter can only be set at server start.
+When archive_mode is enabled, completed WAL segments are sent to archive storage by setting archive_command. archive_mode and archive_command are separate variables so that archive_command can be changed without leaving archiving mode. This parameter can only be set at server start. wal_level must be set to archive or hot_standby to enable archive_mode.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-The archive_command is only invoked on completed WAL segments. Hence, if your server generates little WAL traffic (or has slack periods where it does so), there could be a long delay between the completion of a transaction and its safe recording in archive storage. To put a limit on how old unarchived data can be, you can set archive_timeout to force the server to switch to a new WAL segment file periodically. When this parameter is greater than zero, the server will switch to a new segment file whenever this many seconds have elapsed since the last segment file switch. Note that archived files that are closed early due to a forced switch are still the same length as completely full files. Therefore, it is unwise to use a very short archive_timeout — it will bloat your archive storage. archive_timeout settings of a minute or so are usually reasonable. This parameter can only be set in the postgresql.conf file or on the server command line.
+The archive_command is only invoked for completed WAL segments. Hence, if your server generates little WAL traffic (or has slack periods where it does so), there could be a long delay between the completion of a transaction and its safe recording in archive storage. To limit how old unarchived data can be, you can set archive_timeout to force the server to switch to a new WAL segment file periodically. When this parameter is greater than zero, the server will switch to a new segment file whenever this many seconds have elapsed since the last segment file switch, and there has been any database activity, including a single checkpoint. (Increasing checkpoint_timeout will reduce unnecessary checkpoints on an idle system.) Note that archived files that are closed early due to a forced switch are still the same length as completely full files. Therefore, it is unwise to use a very short archive_timeout — it will bloat your archive storage. archive_timeout settings of a minute or so are usually reasonable. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Maximum time to complete client authentication, in seconds. If a would-be client has not completed the authentication protocol in this much time, the server breaks the connection. This prevents hung clients from occupying a connection indefinitely. The default is one minute (1m). This parameter can only be set in the postgresql.conf file or on the server command line.
+Maximum time to complete client authentication, in seconds. If a would-be client has not completed the authentication protocol in this much time, the server closes the connection. This prevents hung clients from occupying a connection indefinitely. The default is one minute (1m). This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Controls whether the server should run the autovacuum launcher daemon. This is on by default; however, track_counts must also be turned on for autovacuum to work. This parameter can only be set in the postgresql.conf file or on the server command line. Note that even when this parameter is disabled, the system will launch autovacuum processes if necessary to prevent transaction ID wraparound. See Section 23.1.4 for more information.
+Controls whether the server should run the autovacuum launcher daemon. This is on by default; however, track_counts must also be enabled for autovacuum to work. This parameter can only be set in the postgresql.conf file or on the server command line. Note that even when this parameter is disabled, the system will launch autovacuum processes if necessary to prevent transaction ID wraparound. See Section 23.1.4 for more information.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the maximum age (in transactions) that a table's pg_class.relfrozenxid field can attain before a VACUUM operation is forced to prevent transaction ID wraparound within the table. Note that the system will launch autovacuum processes to prevent wraparound even when autovacuum is otherwise disabled. The default is 200 million transactions. This parameter can only be set at server start, but the setting can be reduced for individual tables by changing storage parameters. For more information see Section 23.1.4.
+Specifies the maximum age (in transactions) that a table's pg_class.relfrozenxid field can attain before a VACUUM operation is forced to prevent transaction ID wraparound within the table. Note that the system will launch autovacuum processes to prevent wraparound even when autovacuum is otherwise disabled. Vacuum also allows removal of old files from the pg_clog subdirectory, which is why the default is a relatively low 200 million transactions. This parameter can only be set at server start, but the setting can be reduced for individual tables by changing storage parameters. For more information see Section 23.1.4.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the maximum number of autovacuum processes (other than the autovacuum launcher) which may be running at any one time. The default is three. This parameter can only be set in the postgresql.conf file or on the server command line.
+Specifies the maximum number of autovacuum processes (other than the autovacuum launcher) which may be running at any one time. The default is three. This parameter can only be set at server start.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the minimum delay between autovacuum runs on any given database. In each round the daemon examines the database and issues VACUUM and ANALYZE commands as needed for tables in that database. The delay is measured in seconds, and the default is one minute (1m). This parameter can only be set in the postgresql.conf file or on the server command line.
+Specifies the minimum delay between autovacuum runs on any given database. In each round the daemon examines the database and issues VACUUM and ANALYZE commands as needed for tables in that database. The delay is measured in seconds, and the default is one minute (1min). This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This controls whether a quote mark can be represented by \' in a string literal. The preferred, SQL-standard way to represent a quote mark is by doubling it ('') but PostgreSQL has historically also accepted \'. However, use of \' creates security risks because in some client character set encodings, there are multibyte characters in which the last byte is numerically equivalent to ASCII \. If client-side code does escaping incorrectly then a SQL-injection attack is possible. This risk can be prevented by making the server reject queries in which a quote mark appears to be escaped by a backslash. The allowed values of backslash_quote are on (allow \' always), off (reject always), and safe_encoding (allow only if client encoding does not allow ASCII \ within a multibyte character). safe_encoding is the default setting. Note that in a standard-conforming string literal, \ just means \ anyway. This parameter affects the handling of non-standard-conforming literals, including escape string syntax (E'...').
+This controls whether a quote mark can be represented by \' in a string literal. The preferred, SQL-standard way to represent a quote mark is by doubling it ('') but PostgreSQL has historically also accepted \'. However, use of \' creates security risks because in some client character set encodings, there are multibyte characters in which the last byte is numerically equivalent to ASCII \. If client-side code does escaping incorrectly then a SQL-injection attack is possible. This risk can be prevented by making the server reject queries in which a quote mark appears to be escaped by a backslash. The allowed values of backslash_quote are on (allow \' always), off (reject always), and safe_encoding (allow only if client encoding does not allow ASCII \ within a multibyte character). safe_encoding is the default setting. Note that in a standard-conforming string literal, \ just means \ anyway. This parameter only affects the handling of non-standard-conforming literals, including escape string syntax (E'...').

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: bool
+context: postmaster
+boot_val: off
+unit: None
+enumvals: None
+
+Enables advertising the server's existence via Bonjour. The default is off. This parameter can only be set at server start.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the Bonjour broadcast name. The computer name is used if this parameter is set to the empty string '' (which is the default). This parameter is ignored if the server was not compiled with Bonjour support. This parameter can only be set at server start.
+Specifies the Bonjour service name. The computer name is used if this parameter is set to the empty string '' (which is the default). This parameter is ignored if the server was not compiled with Bonjour support. This parameter can only be set at server start.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: enum
+context: user
+boot_val: hex
+unit: None
+enumvals: ['escape', 'hex']
+
+Sets the output format for values of type bytea. Valid values are hex (the default) and escape (the traditional PostgreSQL format). See Section 8.4 for more information. The bytea type always accepts both formats on input, regardless of this setting.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the target length of checkpoints, as a fraction of the checkpoint interval. The default is 0.5. This parameter can only be set in the postgresql.conf file or on the server command line.
+Specifies the target of checkpoint completion, as a fraction of total time between checkpoints. The default is 0.5. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Controls the query planner's use of table constraints to optimize queries. The allowed values of constraint_exclusion are on (examine constraints for all tables), off (never examine constraints), and partition (examine constraints only for inheritance child tables and UNION ALL subqueries). partition is the default setting. When this parameter allows it for a particular table, the planner compares query conditions with the table's CHECK constraints, and omits scanning tables for which the conditions contradict the constraints. For example: CREATE TABLE parent(key integer, ...); CREATE TABLE child1000(check (key between 1000 and 1999)) INHERITS(parent); CREATE TABLE child2000(check (key between 2000 and 2999)) INHERITS(parent); ... SELECT * FROM parent WHERE key = 2400; With constraint exclusion enabled, this SELECT will not scan child1000 at all. This can improve performance when inheritance is used to build partitioned tables. Currently, constraint exclusion is enabled by default only for cases that are often used to implement table partitioning. Turning it on for all tables imposes extra planning overhead that is quite noticeable on simple queries, and most often will yield no benefit for simple queries. If you have no partitioned tables you might prefer to turn it off entirely. Refer to Section 5.9.4 for more information on using constraint exclusion and partitioning.
+Controls the query planner's use of table constraints to optimize queries. The allowed values of constraint_exclusion are on (examine constraints for all tables), off (never examine constraints), and partition (examine constraints only for inheritance child tables and UNION ALL subqueries). partition is the default setting. It is often used with inheritance and partitioned tables to improve performance. When this parameter allows it for a particular table, the planner compares query conditions with the table's CHECK constraints, and omits scanning tables for which the conditions contradict the constraints. For example: CREATE TABLE parent(key integer, ...); CREATE TABLE child1000(check (key between 1000 and 1999)) INHERITS(parent); CREATE TABLE child2000(check (key between 2000 and 2999)) INHERITS(parent); ... SELECT * FROM parent WHERE key = 2400; With constraint exclusion enabled, this SELECT will not scan child1000 at all, improving performance. Currently, constraint exclusion is enabled by default only for cases that are often used to implement table partitioning. Turning it on for all tables imposes extra planning overhead that is quite noticeable on simple queries, and most often will yield no benefit for simple queries. If you have no partitioned tables you might prefer to turn it off entirely. Refer to Section 5.9.4 for more information on using constraint exclusion and partitioning.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This parameter enables per-database user names. It is off by default. This parameter can only be set in the postgresql.conf file or on the server command line. If this is on, you should create users as username@dbname. When username is passed by a connecting client, @ and the database name are appended to the user name and that database-specific user name is looked up by the server. Note that when you create users with names containing @ within the SQL environment, you will need to quote the user name. With this parameter enabled, you can still create ordinary global users. Simply append @ when specifying the user name in the client. The @ will be stripped off before the user name is looked up by the server. db_user_namespace causes the client's and server's user name representation to differ. Authentication checks are always done with the server's user name so authentication methods must be configured for the server's user name, not the client's. Because md5 uses the user name as salt on both the client and server, md5 cannot be used with db_user_namespace. Note: This feature is intended as a temporary measure until a complete solution is found. At that time, this option will be removed.
+This parameter enables per-database user names. It is off by default. This parameter can only be set in the postgresql.conf file or on the server command line. If this is on, you should create users as username@dbname. When username is passed by a connecting client, @ and the database name are appended to the user name and that database-specific user name is looked up by the server. Note that when you create users with names containing @ within the SQL environment, you will need to quote the user name. With this parameter enabled, you can still create ordinary global users. Simply append @ when specifying the user name in the client, e.g. joe@. The @ will be stripped off before the user name is looked up by the server. db_user_namespace causes the client's and server's user name representation to differ. Authentication checks are always done with the server's user name so authentication methods must be configured for the server's user name, not the client's. Because md5 uses the user name as salt on both the client and server, md5 cannot be used with db_user_namespace. Note: This feature is intended as a temporary measure until a complete solution is found. At that time, this option will be removed.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This is the amount of time, in milliseconds, to wait on a lock before checking to see if there is a deadlock condition. The check for deadlock is relatively slow, so the server doesn't run it every time it waits for a lock. We optimistically assume that deadlocks are not common in production applications and just wait on the lock for a while before starting the check for a deadlock. Increasing this value reduces the amount of time wasted in needless deadlock checks, but slows down reporting of real deadlock errors. The default is one second (1s), which is probably about the smallest value you would want in practice. On a heavily loaded server you might want to raise it. Ideally the setting should exceed your typical transaction time, so as to improve the odds that a lock will be released before the waiter decides to check for deadlock. When log_lock_waits is set, this parameter also determines the length of time to wait before a log message is issued about the lock wait. If you are trying to investigate locking delays you might want to set a shorter than normal deadlock_timeout.
+This is the amount of time, in milliseconds, to wait on a lock before checking to see if there is a deadlock condition. The check for deadlock is relatively expensive, so the server doesn't run it every time it waits for a lock. We optimistically assume that deadlocks are not common in production applications and just wait on the lock for a while before checking for a deadlock. Increasing this value reduces the amount of time wasted in needless deadlock checks, but slows down reporting of real deadlock errors. The default is one second (1s), which is probably about the smallest value you would want in practice. On a heavily loaded server you might want to raise it. Ideally the setting should exceed your typical transaction time, so as to improve the odds that a lock will be released before the waiter decides to check for deadlock. When log_lock_waits is set, this parameter also determines the length of time to wait before a log message is issued about the lock wait. If you are trying to investigate locking delays you might want to set a shorter than normal deadlock_timeout.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-If set, dumps information about all current locks when a DeadLockTimeout occurs. This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled.
+If set, dumps information about all current locks when a deadlock timeout occurs. This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the default statistics target for table columns that have not had a column-specific target set via ALTER TABLE SET STATISTICS. Larger values increase the time needed to do ANALYZE, but might improve the quality of the planner's estimates. The default is 100. For more information on the use of statistics by the PostgreSQL query planner, refer to Section 14.2.
+Sets the default statistics target for table columns without a column-specific target set via ALTER TABLE SET STATISTICS. Larger values increase the time needed to do ANALYZE, but might improve the quality of the planner's estimates. The default is 100. For more information on the use of statistics by the PostgreSQL query planner, refer to Section 14.2.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This controls whether CREATE TABLE and CREATE TABLE AS include an OID column in newly-created tables, if neither WITH OIDS nor WITHOUT OIDS is specified. It also determines whether OIDs will be included in tables created by SELECT INTO. In PostgreSQL 8.1 default_with_oids is off by default; in prior versions of PostgreSQL, it was on by default. The use of OIDs in user tables is considered deprecated, so most installations should leave this variable disabled. Applications that require OIDs for a particular table should specify WITH OIDS when creating the table. This variable can be enabled for compatibility with old applications that do not follow this behavior.
+This controls whether CREATE TABLE and CREATE TABLE AS include an OID column in newly-created tables, if neither WITH OIDS nor WITHOUT OIDS is specified. It also determines whether OIDs will be included in tables created by SELECT INTO. The parameter is off by default; in PostgreSQL 8.0 and earlier, it was on by default. The use of OIDs in user tables is considered deprecated, so most installations should leave this variable disabled. Applications that require OIDs for a particular table should specify WITH OIDS when creating the table. This variable can be enabled for compatibility with old applications that do not follow this behavior.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-If a dynamically loadable module needs to be opened and the file name specified in the CREATE FUNCTION or LOAD command does not have a directory component (i.e., the name does not contain a slash), the system will search this path for the required file. The value for dynamic_library_path has to be a list of absolute directory paths separated by colons (or semi-colons on Windows). If a list element starts with the special string $libdir, the compiled-in PostgreSQL package library directory is substituted for $libdir. This is where the modules provided by the standard PostgreSQL distribution are installed. (Use pg_config --pkglibdir to find out the name of this directory.) For example: dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir' or, in a Windows environment: dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir' The default value for this parameter is '$libdir'. If the value is set to an empty string, the automatic path search is turned off. This parameter can be changed at run time by superusers, but a setting done that way will only persist until the end of the client connection, so this method should be reserved for development purposes. The recommended way to set this parameter is in the postgresql.conf configuration file.
+If a dynamically loadable module needs to be opened and the file name specified in the CREATE FUNCTION or LOAD command does not have a directory component (i.e., the name does not contain a slash), the system will search this path for the required file. The value for dynamic_library_path must be a list of absolute directory paths separated by colons (or semi-colons on Windows). If a list element starts with the special string $libdir, the compiled-in PostgreSQL package library directory is substituted for $libdir; this is where the modules provided by the standard PostgreSQL distribution are installed. (Use pg_config --pkglibdir to find out the name of this directory.) For example: dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir' or, in a Windows environment: dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir' The default value for this parameter is '$libdir'. If the value is set to an empty string, the automatic path search is turned off. This parameter can be changed at run time by superusers, but a setting done that way will only persist until the end of the client connection, so this method should be reserved for development purposes. The recommended way to set this parameter is in the postgresql.conf configuration file.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: bool
+context: user
+boot_val: on
+unit: None
+enumvals: None
+
+Enables or disables the query planner's use of materialization. It is impossible to suppress materialization entirely, but turning this variable off prevents the planner from inserting materialize nodes except in cases where it is required for correctness. The default is on.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Enables or disables the query planner's use of nested-loop join plans. It's not possible to suppress nested-loop joins entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.
+Enables or disables the query planner's use of nested-loop join plans. It is impossible to suppress nested-loop joins entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Enables or disables the query planner's use of sequential scan plan types. It's not possible to suppress sequential scans entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.
+Enables or disables the query planner's use of sequential scan plan types. It is impossible to suppress sequential scans entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Enables or disables the query planner's use of explicit sort steps. It's not possible to suppress explicit sorts entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.
+Enables or disables the query planner's use of explicit sort steps. It is impossible to suppress explicit sorts entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This parameter adjusts the number of digits displayed for floating-point values, including float4, float8, and geometric data types. The parameter value is added to the standard number of digits (FLT_DIG or DBL_DIG as appropriate). The value can be set as high as 2, to include partially-significant digits; this is especially useful for dumping float data that needs to be restored exactly. Or it can be set negative to suppress unwanted digits. See also Section 8.1.3.
+This parameter adjusts the number of digits displayed for floating-point values, including float4, float8, and geometric data types. The parameter value is added to the standard number of digits (FLT_DIG or DBL_DIG as appropriate). The value can be set as high as 3, to include partially-significant digits; this is especially useful for dumping float data that needs to be restored exactly. Or it can be set negative to suppress unwanted digits. See also Section 8.1.3.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-The planner will merge sub-queries into upper queries if the resulting FROM list would have no more than this many items. Smaller values reduce planning time but might yield inferior query plans. The default is eight. For more information see Section 14.3. Setting this value to geqo_threshold or more may trigger use of the GEQO planner, resulting in nondeterministic plans. See Section 18.6.3.
+The planner will merge sub-queries into upper queries if the resulting FROM list would have no more than this many items. Smaller values reduce planning time but might yield inferior query plans. The default is eight. For more information see Section 14.3. Setting this value to geqo_threshold or more may trigger use of the GEQO planner, resulting in non-optimal plans. See Section 18.6.3.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-If this parameter is on, the PostgreSQL server will try to make sure that updates are physically written to disk, by issuing fsync() system calls or various equivalent methods (see wal_sync_method). This ensures that the database cluster can recover to a consistent state after an operating system or hardware crash. However, using fsync results in a performance penalty: when a transaction is committed, PostgreSQL must wait for the operating system to flush the write-ahead log to disk. When fsync is disabled, the operating system is allowed to do its best in buffering, ordering, and delaying writes. This can result in significantly improved performance. However, if the system crashes, the results of the last few committed transactions might be lost in part or whole. In the worst case, unrecoverable data corruption might occur. (Crashes of the database software itself are not a risk factor here. Only an operating-system-level crash creates a risk of corruption.) Due to the risks involved, there is no universally correct setting for fsync. Some administrators always disable fsync, while others only turn it off during initial bulk data loads, where there is a clear restart point if something goes wrong. Others always leave fsync enabled. The default is to enable fsync, for maximum reliability. If you trust your operating system, your hardware, and your utility company (or your battery backup), you can consider disabling fsync. In many situations, turning off synchronous_commit for noncritical transactions can provide much of the potential performance benefit of turning off fsync, without the attendant risks of data corruption. This parameter can only be set in the postgresql.conf file or on the server command line. If you turn this parameter off, also consider turning off full_page_writes.
+If this parameter is on, the PostgreSQL server will try to make sure that updates are physically written to disk, by issuing fsync() system calls or various equivalent methods (see wal_sync_method). This ensures that the database cluster can recover to a consistent state after an operating system or hardware crash. While turning off fsync is often a performance benefit, this can result in unrecoverable data corruption in the event of a power failure or system crash. Thus it is only advisable to turn off fsync if you can easily recreate your entire database from external data. Examples of safe circumstances for turning off fsync include the initial loading of a new database cluster from a backup file, using a database cluster for processing a batch of data after which the database will be thrown away and recreated, or for a read-only database clone which gets recreated frequently and is not used for failover. High quality hardware alone is not a sufficient justification for turning off fsync. In many situations, turning off synchronous_commit for noncritical transactions can provide much of the potential performance benefit of turning off fsync, without the attendant risks of data corruption. fsync can only be set in the postgresql.conf file or on the server command line. If you turn this parameter off, also consider turning off full_page_writes.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-When this parameter is on, the PostgreSQL server writes the entire content of each disk page to WAL during the first modification of that page after a checkpoint. This is needed because a page write that is in process during an operating system crash might be only partially completed, leading to an on-disk page that contains a mix of old and new data. The row-level change data normally stored in WAL will not be enough to completely restore such a page during post-crash recovery. Storing the full page image guarantees that the page can be correctly restored, but at a price in increasing the amount of data that must be written to WAL. (Because WAL replay always starts from a checkpoint, it is sufficient to do this during the first change of each page after a checkpoint. Therefore, one way to reduce the cost of full-page writes is to increase the checkpoint interval parameters.) Turning this parameter off speeds normal operation, but might lead to a corrupt database after an operating system crash or power failure. The risks are similar to turning off fsync, though smaller. It might be safe to turn off this parameter if you have hardware (such as a battery-backed disk controller) or file-system software that reduces the risk of partial page writes to an acceptably low level (e.g., ReiserFS 4). Turning off this parameter does not affect use of WAL archiving for point-in-time recovery (PITR) (see Section 24.3). This parameter can only be set in the postgresql.conf file or on the server command line. The default is on.
+When this parameter is on, the PostgreSQL server writes the entire content of each disk page to WAL during the first modification of that page after a checkpoint. This is needed because a page write that is in process during an operating system crash might be only partially completed, leading to an on-disk page that contains a mix of old and new data. The row-level change data normally stored in WAL will not be enough to completely restore such a page during post-crash recovery. Storing the full page image guarantees that the page can be correctly restored, but at the price of increasing the amount of data that must be written to WAL. (Because WAL replay always starts from a checkpoint, it is sufficient to do this during the first change of each page after a checkpoint. Therefore, one way to reduce the cost of full-page writes is to increase the checkpoint interval parameters.) Turning this parameter off speeds normal operation, but might lead to either unrecoverable data corruption, or silent data corruption, after a system failure. The risks are similar to turning off fsync, though smaller, and it should be turned off only based on the same circumstances recommended for that parameter. Turning off this parameter does not affect use of WAL archiving for point-in-time recovery (PITR) (see Section 24.3). This parameter can only be set in the postgresql.conf file or on the server command line. The default is on.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Enables or disables genetic query optimization. This is on by default. It is usually best not to turn it off in production; the geqo_threshold variable provides a more granular way to control use of GEQO.
+Enables or disables genetic query optimization. This is on by default. It is usually best not to turn it off in production; the geqo_threshold variable provides more granular control of GEQO.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: real
+context: user
+boot_val: 0
+unit: None
+enumvals: None
+
+Controls the initial value of the random number generator used by GEQO to select random paths through the join order search space. The value can range from zero (the default) to one. Varying the value changes the set of join paths explored, and may result in a better or worse best path being found.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Use genetic query optimization to plan queries with at least this many FROM items involved. (Note that a FULL OUTER JOIN construct counts as only one FROM item.) The default is 12. For simpler queries it is usually best to use the deterministic, exhaustive planner, but for queries with many tables the deterministic planner takes too long.
+Use genetic query optimization to plan queries with at least this many FROM items involved. (Note that a FULL OUTER JOIN construct counts as only one FROM item.) The default is 12. For simpler queries it is usually best to use the regular, exhaustive-search planner, but for queries with many tables the exhaustive search takes too long, often longer than the penalty of executing a suboptimal plan. Thus, a threshold on the size of the query is a convenient way to manage use of GEQO.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Soft upper limit of the size of the set returned by GIN index. For more information see Section 52.4.
+Soft upper limit of the size of the set returned by GIN index scans. For more information see Section 53.4.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: bool
+context: postmaster
+boot_val: off
+unit: None
+enumvals: None
+
+Specifies whether or not you can connect and run queries during recovery, as described in Section 25.5. The default value is off. This parameter can only be set at server start. It only has effect during archive recovery or in standby mode.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the configuration file for Section 19.2 username mapping (customarily called pg_ident.conf). This parameter can only be set at server start.
+Specifies the configuration file for Section 19.2 user name mapping (customarily called pg_ident.conf). This parameter can only be set at server start.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-The planner will rewrite explicit JOIN constructs (except FULL JOINs) into lists of FROM items whenever a list of no more than this many items would result. Smaller values reduce planning time but might yield inferior query plans. By default, this variable is set the same as from_collapse_limit, which is appropriate for most uses. Setting it to 1 prevents any reordering of explicit JOINs. Thus, the explicit join order specified in the query will be the actual order in which the relations are joined. The query planner does not always choose the optimal join order; advanced users can elect to temporarily set this variable to 1, and then specify the join order they desire explicitly. For more information see Section 14.3. Setting this value to geqo_threshold or more may trigger use of the GEQO planner, resulting in nondeterministic plans. See Section 18.6.3.
+The planner will rewrite explicit JOIN constructs (except FULL JOINs) into lists of FROM items whenever a list of no more than this many items would result. Smaller values reduce planning time but might yield inferior query plans. By default, this variable is set the same as from_collapse_limit, which is appropriate for most uses. Setting it to 1 prevents any reordering of explicit JOINs. Thus, the explicit join order specified in the query will be the actual order in which the relations are joined. Because the query planner does not always choose the optimal join order, advanced users can elect to temporarily set this variable to 1, and then specify the join order they desire explicitly. For more information see Section 14.3. Setting this value to geqo_threshold or more may trigger use of the GEQO planner, resulting in non-optimal plans. See Section 18.6.3.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the language in which messages are displayed. Acceptable values are system-dependent; see Section 22.1 for more information. If this variable is set to the empty string (which is the default) then the value is inherited from the execution environment of the server in a system-dependent way. On some systems, this locale category does not exist. Setting this variable will still work, but there will be no effect. Also, there is a chance that no translated messages for the desired language exist. In that case you will continue to see the English messages. Only superusers can change this setting, because it affects the messages sent to the server log as well as to the client.
+Sets the language in which messages are displayed. Acceptable values are system-dependent; see Section 22.1 for more information. If this variable is set to the empty string (which is the default) then the value is inherited from the execution environment of the server in a system-dependent way. On some systems, this locale category does not exist. Setting this variable will still work, but there will be no effect. Also, there is a chance that no translated messages for the desired language exist. In that case you will continue to see the English messages. Only superusers can change this setting, because it affects the messages sent to the server log as well as to the client, and an improper value might obscure the readability of the server logs.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the TCP/IP address(es) on which the server is to listen for connections from client applications. The value takes the form of a comma-separated list of host names and/or numeric IP addresses. The special entry * corresponds to all available IP interfaces. If the list is empty, the server does not listen on any IP interface at all, in which case only Unix-domain sockets can be used to connect to it. The default value is localhost, which allows only local "loopback" connections to be made. This parameter can only be set at server start.
+Specifies the TCP/IP address(es) on which the server is to listen for connections from client applications. The value takes the form of a comma-separated list of host names and/or numeric IP addresses. The special entry * corresponds to all available IP interfaces. If the list is empty, the server does not listen on any IP interface at all, in which case only Unix-domain sockets can be used to connect to it. The default value is localhost, which allows only local TCP/IP "loopback" connections to be made. While client authentication (Chapter 19) allows fine-grained control over who can access the server, listen_addresses controls which interfaces accept connection attempts, which can help prevent repeated malicious connection requests on insecure network interfaces. This parameter can only be set at server start.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: bool
+context: superuser
+boot_val: off
+unit: None
+enumvals: None
+
+In PostgreSQL releases prior to 9.0, large objects did not have access privileges and were, in effect, readable and writable by all users. Setting this variable to on disables the new privilege checks, for compatibility with prior releases. The default is off. Setting this variable does not disable all security checks related to large objects — only those for which the default behavior has changed in PostgreSQL 9.0. For example, lo_import() and lo_export() need superuser privileges independent of this setting.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This variable specifies one or more shared libraries that are to be preloaded at connection start. If more than one library is to be loaded, separate their names with commas. This parameter cannot be changed after the start of a particular session. Because this is not a superuser-only option, the libraries that can be loaded are restricted to those appearing in the plugins subdirectory of the installation's standard library directory. (It is the database administrator's responsibility to ensure that only "safe" libraries are installed there.) Entries in local_preload_libraries can specify this directory explicitly, for example $libdir/plugins/mylib, or just specify the library name — mylib would have the same effect as $libdir/plugins/mylib. There is no performance advantage to loading a library at session start rather than when it is first used. Rather, the intent of this feature is to allow debugging or performance-measurement libraries to be loaded into specific sessions without an explicit LOAD command being given. For example, debugging could be enabled for all sessions under a given user name by setting this parameter with ALTER USER SET. If a specified library is not found, the connection attempt will fail. Every PostgreSQL-supported library has a "magic block" that is checked to guarantee compatibility. For this reason, non-PostgreSQL libraries cannot be loaded in this way.
+This variable specifies one or more shared libraries that are to be preloaded at connection start. If more than one library is to be loaded, separate their names with commas. All library names are converted to lower case unless double-quoted. This parameter cannot be changed after the start of a particular session. Because this is not a superuser-only option, the libraries that can be loaded are restricted to those appearing in the plugins subdirectory of the installation's standard library directory. (It is the database administrator's responsibility to ensure that only "safe" libraries are installed there.) Entries in local_preload_libraries can specify this directory explicitly, for example $libdir/plugins/mylib, or just specify the library name — mylib would have the same effect as $libdir/plugins/mylib. Unlike shared_preload_libraries, there is no performance advantage to loading a library at session start rather than when it is first used. Rather, the intent of this feature is to allow debugging or performance-measurement libraries to be loaded into specific sessions without an explicit LOAD command being given. For example, debugging could be enabled for a session by setting this parameter via the PGOPTIONS environment variable. If a specified library is not found, the connection attempt will fail. Every PostgreSQL-supported library has a "magic block" that is checked to guarantee compatibility. For this reason, non-PostgreSQL libraries cannot be loaded in this way.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-If set, logs system resource usage statistics (memory and CPU) on various btree operations. This parameter is only available if the BTREE_BUILD_STATS macro was defined when PostgreSQL was compiled.
+If set, logs system resource usage statistics (memory and CPU) on various B-tree operations. This parameter is only available if the BTREE_BUILD_STATS macro was defined when PostgreSQL was compiled.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-PostgreSQL supports several methods for logging server messages, including stderr, csvlog and syslog. On Windows, eventlog is also supported. Set this parameter to a list of desired log destinations separated by commas. The default is to log to stderr only. This parameter can only be set in the postgresql.conf file or on the server command line. If csvlog is included in log_destination, log entries are output in "comma separated value" format, which is convenient for loading them into programs. See Section 18.7.4 for details. logging_collector must be enabled to generate CSV-format log output. Note: On most Unix systems, you will need to alter the configuration of your system's syslog daemon in order to make use of the syslog option for log_destination. PostgreSQL can log to syslog facilities LOCAL0 through LOCAL7 (see syslog_facility), but the default syslog configuration on most platforms will discard all such messages. You will need to add something like local0.* /var/log/postgresql to the syslog daemon's configuration file to make it work.
+PostgreSQL supports several methods for logging server messages, including stderr, csvlog and syslog. On Windows, eventlog is also supported. Set this parameter to a list of desired log destinations separated by commas. The default is to log to stderr only. This parameter can only be set in the postgresql.conf file or on the server command line. If csvlog is included in log_destination, log entries are output in "comma separated value" (CSV) format, which is convenient for loading logs into programs. See Section 18.7.4 for details. logging_collector must be enabled to generate CSV-format log output. Note: On most Unix systems, you will need to alter the configuration of your system's syslog daemon in order to make use of the syslog option for log_destination. PostgreSQL can log to syslog facilities LOCAL0 through LOCAL7 (see syslog_facility), but the default syslog configuration on most platforms will discard all such messages. You will need to add something like: local0.* /var/log/postgresql to the syslog daemon's configuration file to make it work.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Controls the amount of detail written in the server log for each message that is logged. Valid values are TERSE, DEFAULT, and VERBOSE, each adding more fields to displayed messages. Only superusers can change this setting.
+Controls the amount of detail written in the server log for each message that is logged. Valid values are TERSE, DEFAULT, and VERBOSE, each adding more fields to displayed messages. TERSE excludes the logging of DETAIL, HINT, QUERY, and CONTEXT error information. VERBOSE output includes the SQLSTATE error code (see also Appendix A) and the source code file name, function name, and line number that generated the error. Only superusers can change this setting.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-For each query, write performance statistics of the respective module to the server log. This is a crude profiling instrument. log_statement_stats reports total statement statistics, while the others report per-module statistics. log_statement_stats cannot be enabled together with any of the per-module options. All of these options are disabled by default. Only superusers can change these settings.
+For each query, output performance statistics of the respective module to the server log. This is a crude profiling instrument, similar to the Unix getrusage() operating system facility. log_statement_stats reports total statement statistics, while the others report per-module statistics. log_statement_stats cannot be enabled together with any of the per-module options. All of these options are disabled by default. Only superusers can change these settings.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-By default, connection log messages only show the IP address of the connecting host. Turning on this parameter causes logging of the host name as well. Note that depending on your host name resolution setup this might impose a non-negligible performance penalty. This parameter can only be set in the postgresql.conf file or on the server command line.
+By default, connection log messages only show the IP address of the connecting host. Turning this parameter on causes logging of the host name as well. Note that depending on your host name resolution setup this might impose a non-negligible performance penalty. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This is a printf-style string that is output at the beginning of each log line. % characters begin "escape sequences" that are replaced with status information as outlined below. Unrecognized escapes are ignored. Other characters are copied straight to the log line. Some escapes are only recognized by session processes, and do not apply to background processes such as the main server process. This parameter can only be set in the postgresql.conf file or on the server command line. The default is an empty string. Escape Effect Session only %u User name yes %d Database name yes %r Remote host name or IP address, and remote port yes %h Remote host name or IP address yes %p Process ID no %t Time stamp without milliseconds no %m Time stamp with milliseconds no %i Command tag: type of session's current command yes %c Session ID: see below no %l Number of the log line for each session or process, starting at 1 no %s Process start time stamp no %v Virtual transaction ID (backendID/localXID) no %x Transaction ID (0 if none is assigned) no %q Produces no output, but tells non-session processes to stop at this point in the string; ignored by session processes no %% Literal % no The %c escape prints a quasi-unique session identifier, consisting of two 4-byte hexadecimal numbers (without leading zeros) separated by a dot. The numbers are the process start time and the process ID, so %c can also be used as a space saving way of printing those items. For example, to generate the session identifier from pg_stat_activity, use this query: SELECT to_hex(EXTRACT(EPOCH FROM backend_start)::integer) || '.' || to_hex(procpid) FROM pg_stat_activity; Tip: If you set a nonempty value for log_line_prefix, you should usually make its last character be a space, to provide visual separation from the rest of the log line. A punctuation character could be used too. Tip: Syslog produces its own time stamp and process ID information, so you probably do not want to use those escapes if you are logging to syslog.
+This is a printf-style string that is output at the beginning of each log line. % characters begin "escape sequences" that are replaced with status information as outlined below. Unrecognized escapes are ignored. Other characters are copied straight to the log line. Some escapes are only recognized by session processes, and are ignored by background processes such as the main server process. This parameter can only be set in the postgresql.conf file or on the server command line. The default is an empty string. Escape Effect Session only %a Application name yes %u User name yes %d Database name yes %r Remote host name or IP address, and remote port yes %h Remote host name or IP address yes %p Process ID no %t Time stamp without milliseconds no %m Time stamp with milliseconds no %i Command tag: type of session's current command yes %e SQLSTATE error code no %c Session ID: see below no %l Number of the log line for each session or process, starting at 1 no %s Process start time stamp no %v Virtual transaction ID (backendID/localXID) no %x Transaction ID (0 if none is assigned) no %q Produces no output, but tells non-session processes to stop at this point in the string; ignored by session processes no %% Literal % no The %c escape prints a quasi-unique session identifier, consisting of two 4-byte hexadecimal numbers (without leading zeros) separated by a dot. The numbers are the process start time and the process ID, so %c can also be used as a space saving way of printing those items. For example, to generate the session identifier from pg_stat_activity, use this query: SELECT to_hex(EXTRACT(EPOCH FROM backend_start)::integer) || '.' || to_hex(procpid) FROM pg_stat_activity; Tip: If you set a nonempty value for log_line_prefix, you should usually make its last character be a space, to provide visual separation from the rest of the log line. A punctuation character can be used too. Tip: Syslog produces its own time stamp and process ID information, so you probably do not want to include those escapes if you are logging to syslog.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Controls whether or not the SQL statement that causes an error condition will be recorded in the server log. The current SQL statement is included in the log entry for any message of the specified severity or higher. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, INFO, NOTICE, WARNING, ERROR, LOG, FATAL, and PANIC. The default is ERROR, which means statements causing errors, log messages, fatal errors, or panics will be logged. To effectively turn off logging of failing statements, set this parameter to PANIC. Only superusers can change this setting.
+Controls which SQL statements that cause an error condition are recorded in the server log. The current SQL statement is included in the log entry for any message of the specified severity or higher. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, INFO, NOTICE, WARNING, ERROR, LOG, FATAL, and PANIC. The default is ERROR, which means statements causing errors, log messages, fatal errors, or panics will be logged. To effectively turn off logging of failing statements, set this parameter to PANIC. Only superusers can change this setting.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-For each query, write performance statistics of the respective module to the server log. This is a crude profiling instrument. log_statement_stats reports total statement statistics, while the others report per-module statistics. log_statement_stats cannot be enabled together with any of the per-module options. All of these options are disabled by default. Only superusers can change these settings.
+For each query, output performance statistics of the respective module to the server log. This is a crude profiling instrument, similar to the Unix getrusage() operating system facility. log_statement_stats reports total statement statistics, while the others report per-module statistics. log_statement_stats cannot be enabled together with any of the per-module options. All of these options are disabled by default. Only superusers can change these settings.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-For each query, write performance statistics of the respective module to the server log. This is a crude profiling instrument. log_statement_stats reports total statement statistics, while the others report per-module statistics. log_statement_stats cannot be enabled together with any of the per-module options. All of these options are disabled by default. Only superusers can change these settings.
+For each query, output performance statistics of the respective module to the server log. This is a crude profiling instrument, similar to the Unix getrusage() operating system facility. log_statement_stats reports total statement statistics, while the others report per-module statistics. log_statement_stats cannot be enabled together with any of the per-module options. All of these options are disabled by default. Only superusers can change these settings.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Controls which SQL statements are logged. Valid values are none, ddl, mod, and all. ddl logs all data definition statements, such as CREATE, ALTER, and DROP statements. mod logs all ddl statements, plus data-modifying statements such as INSERT, UPDATE, DELETE, TRUNCATE, and COPY FROM. PREPARE, EXECUTE, and EXPLAIN ANALYZE statements are also logged if their contained command is of an appropriate type. For clients using extended query protocol, logging occurs when an Execute message is received, and values of the Bind parameters are included (with any embedded single-quote marks doubled). The default is none. Only superusers can change this setting. Note: Statements that contain simple syntax errors are not logged even by the log_statement = all setting, because the log message is emitted only after basic parsing has been done to determine the statement type. In the case of extended query protocol, this setting likewise does not log statements that fail before the Execute phase (i.e., during parse analysis or planning). Set log_min_error_statement to ERROR (or lower) to log such statements.
+Controls which SQL statements are logged. Valid values are none (off), ddl, mod, and all (all statements). ddl logs all data definition statements, such as CREATE, ALTER, and DROP statements. mod logs all ddl statements, plus data-modifying statements such as INSERT, UPDATE, DELETE, TRUNCATE, and COPY FROM. PREPARE, EXECUTE, and EXPLAIN ANALYZE statements are also logged if their contained command is of an appropriate type. For clients using extended query protocol, logging occurs when an Execute message is received, and values of the Bind parameters are included (with any embedded single-quote marks doubled). The default is none. Only superusers can change this setting. Note: Statements that contain simple syntax errors are not logged even by the log_statement = all setting, because the log message is emitted only after basic parsing has been done to determine the statement type. In the case of extended query protocol, this setting likewise does not log statements that fail before the Execute phase (i.e., during parse analysis or planning). Set log_min_error_statement to ERROR (or lower) to log such statements.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-For each query, write performance statistics of the respective module to the server log. This is a crude profiling instrument. log_statement_stats reports total statement statistics, while the others report per-module statistics. log_statement_stats cannot be enabled together with any of the per-module options. All of these options are disabled by default. Only superusers can change these settings.
+For each query, output performance statistics of the respective module to the server log. This is a crude profiling instrument, similar to the Unix getrusage() operating system facility. log_statement_stats reports total statement statistics, while the others report per-module statistics. log_statement_stats cannot be enabled together with any of the per-module options. All of these options are disabled by default. Only superusers can change these settings.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Controls logging of use of temporary files. Temporary files can be created for sorts, hashes, and temporary query results. A log entry is made for each temporary file when it is deleted. A value of zero logs all temporary files, while positive values log only files whose size is greater than or equal to the specified number of kilobytes. The default setting is -1, which disables such logging. Only superusers can change this setting.
+Controls logging of temporary file names and sizes. Temporary files can be created for sorts, hashes, and temporary query results. A log entry is made for each temporary file when it is deleted. A value of zero logs all temporary file information, while positive values log only files whose size is greater than or equal to the specified number of kilobytes. The default setting is -1, which disables such logging. Only superusers can change this setting.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the time zone used for timestamps written in the log. Unlike timezone, this value is cluster-wide, so that all sessions will report timestamps consistently. The default is unknown, which means to use whatever the system environment specifies as the time zone. See Section 8.5.3 for more information. This parameter can only be set in the postgresql.conf file or on the server command line.
+Sets the time zone used for timestamps written in the log. Unlike timezone, this value is cluster-wide, so that all sessions will report timestamps consistently. The default is unknown, which means use whatever the system environment specifies as the time zone. See Section 8.5.3 for more information. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This parameter allows messages sent to stderr, and CSV-format log output, to be captured and redirected into log files. This approach is often more useful than logging to syslog, since some types of messages might not appear in syslog output (a common example is dynamic-linker failure messages). This parameter can only be set at server start.
+This parameter captures plain and CSV-format log messages sent to stderr and redirects them into log files. This approach is often more useful than logging to syslog, since some types of messages might not appear in syslog output (a common example is dynamic-linker failure messages). This parameter can only be set at server start. Note: The logging collector is designed to never lose messages. This means that in case of extremely high load, server processes could be blocked due to trying to send additional log messages when the collector has fallen behind. In contrast, syslog prefers to drop messages if it cannot write them, which means it's less reliable in those cases but it will not block the rest of the system.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the maximum amount of memory to be used in maintenance operations, such as VACUUM, CREATE INDEX, and ALTER TABLE ADD FOREIGN KEY. It defaults to 16 megabytes (16MB). Since only one of these operations can be executed at a time by a database session, and an installation normally doesn't have many of them running concurrently, it's safe to set this value significantly larger than work_mem. Larger settings might improve performance for vacuuming and for restoring database dumps. Note that when autovacuum runs, up to autovacuum_max_workers times this memory may be allocated, so be careful not to set the default value too high.
+Specifies the maximum amount of memory to be used by maintenance operations, such as VACUUM, CREATE INDEX, and ALTER TABLE ADD FOREIGN KEY. It defaults to 16 megabytes (16MB). Since only one of these operations can be executed at a time by a database session, and an installation normally doesn't have many of them running concurrently, it's safe to set this value significantly larger than work_mem. Larger settings might improve performance for vacuuming and for restoring database dumps. Note that when autovacuum runs, up to autovacuum_max_workers times this memory may be allocated, so be careful not to set the default value too high.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Determines the maximum number of concurrent connections to the database server. The default is typically 100 connections, but might be less if your kernel settings will not support it (as determined during initdb). This parameter can only be set at server start. Increasing this parameter might cause PostgreSQL to request more System V shared memory or semaphores than your operating system's default configuration allows. See Section 17.4.1 for information on how to adjust those parameters, if necessary.
+Determines the maximum number of concurrent connections to the database server. The default is typically 100 connections, but might be less if your kernel settings will not support it (as determined during initdb). This parameter can only be set at server start. Increasing this parameter might cause PostgreSQL to request more System V shared memory or semaphores than your operating system's default configuration allows. See Section 17.4.1 for information on how to adjust those parameters, if necessary. When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the maximum number of simultaneously open files allowed to each server subprocess. The default is one thousand files. If the kernel is enforcing a safe per-process limit, you don't need to worry about this setting. But on some platforms (notably, most BSD systems), the kernel will allow individual processes to open many more files than the system can really support when a large number of processes all try to open that many files. If you find yourself seeing "Too many open files" failures, try reducing this setting. This parameter can only be set at server start.
+Sets the maximum number of simultaneously open files allowed to each server subprocess. The default is one thousand files. If the kernel is enforcing a safe per-process limit, you don't need to worry about this setting. But on some platforms (notably, most BSD systems), the kernel will allow individual processes to open many more files than the system can actually support if many processes all try to open that many files. If you find yourself seeing "Too many open files" failures, try reducing this setting. This parameter can only be set at server start.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Reports the maximum identifier length. It is determined as one less than the value of NAMEDATALEN when building the server. The default value of NAMEDATALEN is 64; therefore the default max_identifier_length is 63 bytes.
+Reports the maximum identifier length. It is determined as one less than the value of NAMEDATALEN when building the server. The default value of NAMEDATALEN is 64; therefore the default max_identifier_length is 63 bytes, which can be less than 63 characters when using multibyte encodings.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-The shared lock table is created to track locks on max_locks_per_transaction * (max_connections + max_prepared_transactions) objects (e.g., tables); hence, no more than this many distinct objects can be locked at any one time. This parameter controls the average number of object locks allocated for each transaction; individual transactions can lock more objects as long as the locks of all transactions fit in the lock table. This is not the number of rows that can be locked; that value is unlimited. The default, 64, has historically proven sufficient, but you might need to raise this value if you have clients that touch many different tables in a single transaction. This parameter can only be set at server start. Increasing this parameter might cause PostgreSQL to request more System V shared memory than your operating system's default configuration allows. See Section 17.4.1 for information on how to adjust those parameters, if necessary.
+The shared lock table tracks locks on max_locks_per_transaction * (max_connections + max_prepared_transactions) objects (e.g., tables); hence, no more than this many distinct objects can be locked at any one time. This parameter controls the average number of object locks allocated for each transaction; individual transactions can lock more objects as long as the locks of all transactions fit in the lock table. This is not the number of rows that can be locked; that value is unlimited. The default, 64, has historically proven sufficient, but you might need to raise this value if you have clients that touch many different tables in a single transaction. This parameter can only be set at server start. Increasing this parameter might cause PostgreSQL to request more System V shared memory than your operating system's default configuration allows. See Section 17.4.1 for information on how to adjust those parameters, if necessary. When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the maximum number of transactions that can be in the "prepared" state simultaneously (see PREPARE TRANSACTION). Setting this parameter to zero (which is the default) disables the prepared-transaction feature. This parameter can only be set at server start. If you are not planning to use prepared transactions, this parameter should be set to zero to prevent accidental creation of prepared transactions. If you are using prepared transactions, you will probably want max_prepared_transactions to be at least as large as max_connections, so that every session can have a prepared transaction pending. Increasing this parameter might cause PostgreSQL to request more System V shared memory than your operating system's default configuration allows. See Section 17.4.1 for information on how to adjust those parameters, if necessary.
+Sets the maximum number of transactions that can be in the "prepared" state simultaneously (see PREPARE TRANSACTION). Setting this parameter to zero (which is the default) disables the prepared-transaction feature. This parameter can only be set at server start. If you are not planning to use prepared transactions, this parameter should be set to zero to prevent accidental creation of prepared transactions. If you are using prepared transactions, you will probably want max_prepared_transactions to be at least as large as max_connections, so that every session can have a prepared transaction pending. Increasing this parameter might cause PostgreSQL to request more System V shared memory than your operating system's default configuration allows. See Section 17.4.1 for information on how to adjust those parameters, if necessary. When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the maximum safe depth of the server's execution stack. The ideal setting for this parameter is the actual stack size limit enforced by the kernel (as set by ulimit -s or local equivalent), less a safety margin of a megabyte or so. The safety margin is needed because the stack depth is not checked in every routine in the server, but only in key potentially-recursive routines such as expression evaluation. The default setting is two megabytes (2MB), which is conservatively small and unlikely to risk crashes. However, it might be too small to allow execution of complex functions. Only superusers can change this setting. Setting max_stack_depth higher than the actual kernel limit will mean that a runaway recursive function can crash an individual backend process. On platforms where PostgreSQL can determine the kernel limit, it will not let you set this variable to an unsafe value. However, not all platforms provide the information, so caution is recommended in selecting a value.
+Specifies the maximum safe depth of the server's execution stack. The ideal setting for this parameter is the actual stack size limit enforced by the kernel (as set by ulimit -s or local equivalent), less a safety margin of a megabyte or so. The safety margin is needed because the stack depth is not checked in every routine in the server, but only in key potentially-recursive routines such as expression evaluation. The default setting is two megabytes (2MB), which is conservatively small and unlikely to risk crashes. However, it might be too small to allow execution of complex functions. Only superusers can change this setting. Setting max_stack_depth higher than the actual kernel limit will mean that a runaway recursive function can crash an individual backend process. On platforms where PostgreSQL can determine the kernel limit, the server will not allow this variable to be set to an unsafe value. However, not all platforms provide the information, so caution is recommended in selecting a value.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: integer
+context: sighup
+boot_val: 30000
+unit: ms
+enumvals: None
+
+When Hot Standby is active, this parameter determines how long the standby server should wait before canceling standby queries that conflict with about-to-be-applied WAL entries, as described in Section 25.5.2. max_standby_archive_delay applies when WAL data is being read from WAL archive (and is therefore not current). The default is 30 seconds. Units are milliseconds if not specified. A value of -1 allows the standby to wait forever for conflicting queries to complete. This parameter can only be set in the postgresql.conf file or on the server command line. Note that max_standby_archive_delay is not the same as the maximum length of time a query can run before cancellation; rather it is the maximum total time allowed to apply any one WAL segment's data. Thus, if one query has resulted in significant delay earlier in the WAL segment, subsequent conflicting queries will have much less grace time.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: integer
+context: sighup
+boot_val: 30000
+unit: ms
+enumvals: None
+
+When Hot Standby is active, this parameter determines how long the standby server should wait before canceling standby queries that conflict with about-to-be-applied WAL entries, as described in Section 25.5.2. max_standby_streaming_delay applies when WAL data is being received via streaming replication. The default is 30 seconds. Units are milliseconds if not specified. A value of -1 allows the standby to wait forever for conflicting queries to complete. This parameter can only be set in the postgresql.conf file or on the server command line. Note that max_standby_streaming_delay is not the same as the maximum length of time a query can run before cancellation; rather it is the maximum total time allowed to apply WAL data once it has been received from the primary server. Thus, if one query has resulted in significant delay, subsequent conflicting queries will have much less grace time until the standby server has caught up again.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: integer
+context: postmaster
+boot_val: 0
+unit: 
+enumvals: None
+
+Specifies the maximum number of concurrent connections from standby servers (i.e., the maximum number of simultaneously running WAL sender processes). The default is zero, meaning replication is disabled. WAL sender processes count towards the total number of connections, so the parameter cannot be set higher than max_connections. This parameter can only be set at server start. wal_level must be set to archive or hot_standby to allow connections from standby servers.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-If nonzero, a delay of this many seconds occurs when a new server process is started, after it conducts the authentication procedure. This is intended to give an opportunity to attach to the server process with a debugger. This parameter cannot be changed after session start.
+If nonzero, a delay of this many seconds occurs when a new server process is started, after it conducts the authentication procedure. This is intended to give developers an opportunity to attach to the server process with a debugger. This parameter cannot be changed after session start.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-If nonzero, a delay of this many seconds occurs just after a new server process is forked, before it conducts the authentication procedure. This is intended to give an opportunity to attach to the server process with a debugger to trace down misbehavior in authentication. This parameter can only be set in the postgresql.conf file or on the server command line.
+If nonzero, a delay of this many seconds occurs just after a new server process is forked, before it conducts the authentication procedure. This is intended to give developers an opportunity to attach to the server process with a debugger to trace down misbehavior in authentication. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the planner's estimate of the cost of a non-sequentially-fetched disk page. The default is 4.0. Reducing this value relative to seq_page_cost will cause the system to prefer index scans; raising it will make index scans look relatively more expensive. You can raise or lower both values together to change the importance of disk I/O costs relative to CPU costs, which are described by the following parameters. Tip: Although the system will let you set random_page_cost to less than seq_page_cost, it is not physically sensible to do so. However, setting them equal makes sense if the database is entirely cached in RAM, since in that case there is no penalty for touching pages out of sequence. Also, in a heavily-cached database you should lower both values relative to the CPU parameters, since the cost of fetching a page already in RAM is much smaller than it would normally be.
+Sets the planner's estimate of the cost of a non-sequentially-fetched disk page. The default is 4.0. This value can be overridden for a particular tablespace by setting the tablespace parameter of the same name (see ALTER TABLESPACE). Reducing this value relative to seq_page_cost will cause the system to prefer index scans; raising it will make index scans look relatively more expensive. You can raise or lower both values together to change the importance of disk I/O costs relative to CPU costs, which are described by the following parameters. Tip: Although the system will let you set random_page_cost to less than seq_page_cost, it is not physically sensible to do so. However, setting them equal makes sense if the database is entirely cached in RAM, since in that case there is no penalty for touching pages out of sequence. Also, in a heavily-cached database you should lower both values relative to the CPU parameters, since the cost of fetching a page already in RAM is much smaller than it would normally be.

--- 8.4
+++ 9.0
@@ -1,3 +0,0 @@
-
-
-The regular expression "flavor" can be set to advanced, extended, or basic. The default is advanced. The extended setting might be useful for exact backwards compatibility with pre-7.4 releases of PostgreSQL. See Section 9.7.3.1 for details.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This variable specifies the order in which schemas are searched when an object (table, data type, function, etc.) is referenced by a simple name with no schema component. When there are objects of identical names in different schemas, the one found first in the search path is used. An object that is not in any of the schemas in the search path can only be referenced by specifying its containing schema with a qualified (dotted) name. The value for search_path has to be a comma-separated list of schema names. If one of the list items is the special value $user, then the schema having the name returned by SESSION_USER is substituted, if there is such a schema. (If not, $user is ignored.) The system catalog schema, pg_catalog, is always searched, whether it is mentioned in the path or not. If it is mentioned in the path then it will be searched in the specified order. If pg_catalog is not in the path then it will be searched before searching any of the path items. Likewise, the current session's temporary-table schema, pg_temp_nnn, is always searched if it exists. It can be explicitly listed in the path by using the alias pg_temp. If it is not listed in the path then it is searched first (before even pg_catalog). However, the temporary schema is only searched for relation (table, view, sequence, etc) and data type names. It will never be searched for function or operator names. When objects are created without specifying a particular target schema, they will be placed in the first schema listed in the search path. An error is reported if the search path is empty. The default value for this parameter is '"$user", public' (where the second part will be ignored if there is no schema named public). This supports shared use of a database (where no users have private schemas, and all share use of public), private per-user schemas, and combinations of these. Other effects can be obtained by altering the default search path setting, either globally or per-user. The current effective value of the search path can be examined via the SQL function current_schemas(). This is not quite the same as examining the value of search_path, since current_schemas() shows how the requests appearing in search_path were resolved. For more information on schema handling, see Section 5.7.
+This variable specifies the order in which schemas are searched when an object (table, data type, function, etc.) is referenced by a simple name with no schema specified. When there are objects of identical names in different schemas, the one found first in the search path is used. An object that is not in any of the schemas in the search path can only be referenced by specifying its containing schema with a qualified (dotted) name. The value for search_path must be a comma-separated list of schema names. If one of the list items is the special value $user, then the schema having the name returned by SESSION_USER is substituted, if there is such a schema. (If not, $user is ignored.) The system catalog schema, pg_catalog, is always searched, whether it is mentioned in the path or not. If it is mentioned in the path then it will be searched in the specified order. If pg_catalog is not in the path then it will be searched before searching any of the path items. Likewise, the current session's temporary-table schema, pg_temp_nnn, is always searched if it exists. It can be explicitly listed in the path by using the alias pg_temp. If it is not listed in the path then it is searched first (even before pg_catalog). However, the temporary schema is only searched for relation (table, view, sequence, etc) and data type names. It is never searched for function or operator names. When objects are created without specifying a particular target schema, they will be placed in the first schema listed in the search path. An error is reported if the search path is empty. The default value for this parameter is '"$user", public' (where the second part will be ignored if there is no schema named public). This supports shared use of a database (where no users have private schemas, and all share use of public), private per-user schemas, and combinations of these. Other effects can be obtained by altering the default search path setting, either globally or per-user. The current effective value of the search path can be examined via the SQL function current_schemas (see Section 9.23). This is not quite the same as examining the value of search_path, since current_schemas shows how the items appearing in search_path were resolved. For more information on schema handling, see Section 5.7.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the planner's estimate of the cost of a disk page fetch that is part of a series of sequential fetches. The default is 1.0.
+Sets the planner's estimate of the cost of a disk page fetch that is part of a series of sequential fetches. The default is 1.0. This value can be overridden for a particular tablespace by setting the tablespace parameter of the same name (see ALTER TABLESPACE).

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This variable specifies one or more shared libraries that are to be preloaded at server start. If more than one library is to be loaded, separate their names with commas. For example, '$libdir/mylib' would cause mylib.so (or on some platforms, mylib.sl) to be preloaded from the installation's standard library directory. This parameter can only be set at server start. PostgreSQL procedural language libraries can be preloaded in this way, typically by using the syntax '$libdir/plXXX' where XXX is pgsql, perl, tcl, or python. By preloading a shared library, the library startup time is avoided when the library is first used. However, the time to start each new server process might increase slightly, even if that process never uses the library. So this parameter is recommended only for libraries that will be used in most sessions. Note: On Windows hosts, preloading a library at server start will not reduce the time required to start each new server process; each server process will re-load all preload libraries. However, shared_preload_libraries is still useful on Windows hosts because some shared libraries may need to perform certain operations that only take place at postmaster start (for example, a shared library may need to reserve lightweight locks or shared memory and you can't do that after the postmaster has started). If a specified library is not found, the server will fail to start. Every PostgreSQL-supported library has a "magic block" that is checked to guarantee compatibility. For this reason, non-PostgreSQL libraries cannot be loaded in this way.
+This variable specifies one or more shared libraries to be preloaded at server start. For example, '$libdir/mylib' would cause mylib.so (or on some platforms, mylib.sl) to be preloaded from the installation's standard library directory. All library names are converted to lower case unless double-quoted. If more than one library is to be loaded, separate their names with commas. This parameter can only be set at server start. PostgreSQL procedural language libraries can be preloaded in this way, typically by using the syntax '$libdir/plXXX' where XXX is pgsql, perl, tcl, or python. By preloading a shared library, the library startup time is avoided when the library is first used. However, the time to start each new server process might increase slightly, even if that process never uses the library. So this parameter is recommended only for libraries that will be used in most sessions. Note: On Windows hosts, preloading a library at server start will not reduce the time required to start each new server process; each server process will re-load all preload libraries. However, shared_preload_libraries is still useful on Windows hosts because some shared libraries may need to perform certain operations that only take place at postmaster start (for example, a shared library may need to reserve lightweight locks or shared memory and you can't do that after the postmaster has started). If a specified library is not found, the server will fail to start. Every PostgreSQL-supported library has a "magic block" that is checked to guarantee compatibility. For this reason, non-PostgreSQL libraries cannot be loaded in this way.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies how much data can flow over an SSL encrypted connection before renegotiation of the session will take place. Renegotiation of the session decreases the chance of doing cryptanalysis when large amounts of data are sent, but it also carries a large performance penalty. The sum of sent and received traffic is used to check the limit. If the parameter is set to 0, renegotiation is disabled. The default is 512MB. Note: SSL libraries from before November 2009 are insecure when using SSL renegotiation, due to a vulnerability in the SSL protocol. As a stop-gap fix for this vulnerability, some vendors also shipped SSL libraries incapable of doing renegotiation. If any of these libraries are in use on the client or server, SSL renegotiation should be disabled.
+Specifies how much data can flow over an SSL-encrypted connection before renegotiation of the session keys will take place. Renegotiation decreases an attacker's chances of doing cryptanalysis when large amounts of traffic can be examined, but it also carries a large performance penalty. The sum of sent and received traffic is used to check the limit. If this parameter is set to 0, renegotiation is disabled. The default is 0. Note: SSL libraries from before November 2009 are insecure when using SSL renegotiation, due to a vulnerability in the SSL protocol. As a stop-gap fix for this vulnerability, some vendors shipped SSL libraries incapable of doing renegotiation. If any such libraries are in use on the client or server, SSL renegotiation should be disabled. Warning Due to bugs in OpenSSL enabling ssl renegotiation, by configuring a non-zero ssl_renegotiation_limit, is likely to lead to problems like long-lived connections breaking.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This controls whether ordinary string literals ('...') treat backslashes literally, as specified in the SQL standard. The default is currently off, causing PostgreSQL to have its historical behavior of treating backslashes as escape characters. The default will change to on in a future release to improve compatibility with the standard. Applications can check this parameter to determine how string literals will be processed. The presence of this parameter can also be taken as an indication that the escape string syntax (E'...') is supported. Escape string syntax should be used if an application desires backslashes to be treated as escape characters.
+This controls whether ordinary string literals ('...') treat backslashes literally, as specified in the SQL standard. The default is currently off, causing PostgreSQL to have its historical behavior of treating backslashes as escape characters. The default will change to on in a future release to improve compatibility with the SQL standard. Applications can check this parameter to determine how string literals will be processed. The presence of this parameter can also be taken as an indication that the escape string syntax (E'...') is supported. Escape string syntax (Section 4.1.2.2) should be used if an application desires backslashes to be treated as escape characters.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Abort any statement that takes over the specified number of milliseconds, starting from the time the command arrives at the server from the client. If log_min_error_statement is set to ERROR or lower, the statement that timed out will also be logged. A value of zero (the default) turns off the limitation. Setting statement_timeout in postgresql.conf is not recommended because it affects all sessions.
+Abort any statement that takes over the specified number of milliseconds, starting from the time the command arrives at the server from the client. If log_min_error_statement is set to ERROR or lower, the statement that timed out will also be logged. A value of zero (the default) turns this off. Setting statement_timeout in postgresql.conf is not recommended because it affects all sessions.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the directory to store temporary statistics data in. This can be a path relative to the data directory or an absolute path. The default is pg_stat_tmp. Pointing this at a RAM based filesystem will decrease physical I/O requirements and can lead to improved performance. This parameter can only be set in the postgresql.conf file or on the server command line.
+Sets the directory to store temporary statistics data in. This can be a path relative to the data directory or an absolute path. The default is pg_stat_tmp. Pointing this at a RAM-based file system will decrease physical I/O requirements and can lead to improved performance. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Determines the number of connection "slots" that are reserved for connections by PostgreSQL superusers. At most max_connections connections can ever be active simultaneously. Whenever the number of active concurrent connections is at least max_connections minus superuser_reserved_connections, new connections will be accepted only for superusers. The default value is three connections. The value must be less than the value of max_connections. This parameter can only be set at server start.
+Determines the number of connection "slots" that are reserved for connections by PostgreSQL superusers. At most max_connections connections can ever be active simultaneously. Whenever the number of active concurrent connections is at least max_connections minus superuser_reserved_connections, new connections will be accepted only for superusers, and no new replication connections will be accepted. The default value is three connections. The value must be less than the value of max_connections. This parameter can only be set at server start.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies whether transaction commit will wait for WAL records to be written to disk before the command returns a "success" indication to the client. The default, and safe, setting is on. When off, there can be a delay between when success is reported to the client and when the transaction is really guaranteed to be safe against a server crash. (The maximum delay is three times wal_writer_delay.) Unlike fsync, setting this parameter to off does not create any risk of database inconsistency: a crash might result in some recent allegedly-committed transactions being lost, but the database state will be just the same as if those transactions had been aborted cleanly. So, turning synchronous_commit off can be a useful alternative when performance is more important than exact certainty about the durability of a transaction. For more discussion see Section 28.3. This parameter can be changed at any time; the behavior for any one transaction is determined by the setting in effect when it commits. It is therefore possible, and useful, to have some transactions commit synchronously and others asynchronously. For example, to make a single multi-statement transaction commit asynchronously when the default is the opposite, issue SET LOCAL synchronous_commit TO OFF within the transaction.
+Specifies whether transaction commit will wait for WAL records to be written to disk before the command returns a "success" indication to the client. The default, and safe, setting is on. When off, there can be a delay between when success is reported to the client and when the transaction is really guaranteed to be safe against a server crash. (The maximum delay is three times wal_writer_delay.) Unlike fsync, setting this parameter to off does not create any risk of database inconsistency: an operating system or database crash might result in some recent allegedly-committed transactions being lost, but the database state will be just the same as if those transactions had been aborted cleanly. So, turning synchronous_commit off can be a useful alternative when performance is more important than exact certainty about the durability of a transaction. For more discussion see Section 29.3. This parameter can be changed at any time; the behavior for any one transaction is determined by the setting in effect when it commits. It is therefore possible, and useful, to have some transactions commit synchronously and others asynchronously. For example, to make a single multistatement transaction commit asynchronously when the default is the opposite, issue SET LOCAL synchronous_commit TO OFF within the transaction.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-On systems that support the TCP_KEEPCNT socket option, specifies how many keepalives can be lost before the connection is considered dead. A value of zero uses the system default. If TCP_KEEPCNT is not supported, this parameter must be zero. This parameter is ignored for connections made via a Unix-domain socket.
+Specifies the number of keepalive packets to send on an otherwise idle connection. A value of 0 uses the system default. This parameter is supported only on systems that support the TCP_KEEPCNT symbol; on other systems, it must be zero. In sessions connected via a Unix-domain socket, this parameter is ignored and always reads as zero. Note: This parameter is not supported on Windows, and must be zero.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-On systems that support the TCP_KEEPIDLE socket option, specifies the number of seconds between sending keepalives on an otherwise idle connection. A value of zero uses the system default. If TCP_KEEPIDLE is not supported, this parameter must be zero. This parameter is ignored for connections made via a Unix-domain socket.
+Specifies the number of seconds before sending a keepalive packet on an otherwise idle connection. A value of 0 uses the system default. This parameter is supported only on systems that support the TCP_KEEPIDLE or TCP_KEEPALIVE symbols, and on Windows; on other systems, it must be zero. In sessions connected via a Unix-domain socket, this parameter is ignored and always reads as zero. Note: On Windows, a value of 0 will set this parameter to 2 hours, since Windows does not provide a way to read the system default value.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-On systems that support the TCP_KEEPINTVL socket option, specifies how long, in seconds, to wait for a response to a keepalive before retransmitting. A value of zero uses the system default. If TCP_KEEPINTVL is not supported, this parameter must be zero. This parameter is ignored for connections made via a Unix-domain socket.
+Specifies the number of seconds between sending keepalives on an otherwise idle connection. A value of 0 uses the system default. This parameter is supported only on systems that support the TCP_KEEPINTVL symbol, and on Windows; on other systems, it must be zero. In sessions connected via a Unix-domain socket, this parameter is ignored and always reads as zero. Note: On Windows, a value of 0 will set this parameter to 1 second, since Windows does not provide a way to read the system default value.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the maximum number of temporary buffers used by each database session. These are session-local buffers used only for access to temporary tables. The default is eight megabytes (8MB). The setting can be changed within individual sessions, but only up until the first use of temporary tables within a session; subsequent attempts to change the value will have no effect on that session. A session will allocate temporary buffers as needed up to the limit given by temp_buffers. The cost of setting a large value in sessions that do not actually need a lot of temporary buffers is only a buffer descriptor, or about 64 bytes, per increment in temp_buffers. However if a buffer is actually used an additional 8192 bytes will be consumed for it (or in general, BLCKSZ bytes).
+Sets the maximum number of temporary buffers used by each database session. These are session-local buffers used only for access to temporary tables. The default is eight megabytes (8MB). The setting can be changed within individual sessions, but only before the first use of temporary tables within the session; subsequent attempts to change the value will have no effect on that session. A session will allocate temporary buffers as needed up to the limit given by temp_buffers. The cost of setting a large value in sessions that do not actually need many temporary buffers is only a buffer descriptor, or about 64 bytes, per increment in temp_buffers. However if a buffer is actually used an additional 8192 bytes will be consumed for it (or in general, BLCKSZ bytes).

--- 8.4
+++ 9.0
@@ -1 +1 @@
-This variable specifies tablespace(s) in which to create temporary objects (temp tables and indexes on temp tables) when a CREATE command does not explicitly specify a tablespace. Temporary files for purposes such as sorting large data sets are also created in these tablespace(s). The value is a list of names of tablespaces. When there is more than one name in the list, PostgreSQL chooses a random member of the list each time a temporary object is to be created; except that within a transaction, successively created temporary objects are placed in successive tablespaces from the list. If the selected element of the list is an empty string, PostgreSQL will automatically use the default tablespace of the current database instead. When temp_tablespaces is set interactively, specifying a nonexistent tablespace is an error, as is specifying a tablespace for which the user does not have CREATE privilege. However, when using a previously set value, nonexistent tablespaces are ignored, as are tablespaces for which the user lacks CREATE privilege. In particular, this rule applies when using a value set in postgresql.conf. The default value is an empty string, which results in all temporary objects being created in the default tablespace of the current database. See also default_tablespace.
+This variable specifies tablespaces in which to create temporary objects (temp tables and indexes on temp tables) when a CREATE command does not explicitly specify a tablespace. Temporary files for purposes such as sorting large data sets are also created in these tablespaces. The value is a list of names of tablespaces. When there is more than one name in the list, PostgreSQL chooses a random member of the list each time a temporary object is to be created; except that within a transaction, successively created temporary objects are placed in successive tablespaces from the list. If the selected element of the list is an empty string, PostgreSQL will automatically use the default tablespace of the current database instead. When temp_tablespaces is set interactively, specifying a nonexistent tablespace is an error, as is specifying a tablespace for which the user does not have CREATE privilege. However, when using a previously set value, nonexistent tablespaces are ignored, as are tablespaces for which the user lacks CREATE privilege. In particular, this rule applies when using a value set in postgresql.conf. The default value is an empty string, which results in all temporary objects being created in the default tablespace of the current database. See also default_tablespace.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the collection of time zone abbreviations that will be accepted by the server for datetime input. The default is 'Default', which is a collection that works in most of the world; there are also 'Australia' and 'India', and other collections can be defined for a particular installation. See Appendix B for more information.
+Sets the collection of time zone abbreviations that will be accepted by the server for datetime input. The default is 'Default', which is a collection that works in most of the world; there are also 'Australia' and 'India', and other collections can be defined for a particular installation. See Section B.3 for more information.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-If on, emit information about lock usage. Information dumped includes the type of lock operation, the type of lock and the unique identifier of the object being locked or unlocked. Also included are bitmasks for the lock types already granted on this object as well as for the lock types awaited on this object. For each lock type a count of the number of granted locks and waiting locks is also dumped as well as the totals. An example of the log file output is shown here: LOG: LockAcquire: new: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0 wait(0) type(AccessShareLock) LOG: GrantLock: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(2) req(1,0,0,0,0,0,0)=1 grant(1,0,0,0,0,0,0)=1 wait(0) type(AccessShareLock) LOG: UnGrantLock: updated: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0 wait(0) type(AccessShareLock) LOG: CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0 wait(0) type(INVALID) Details of the structure being dumped may be found in src/include/storage/lock.h This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled.
+If on, emit information about lock usage. Information dumped includes the type of lock operation, the type of lock and the unique identifier of the object being locked or unlocked. Also included are bit masks for the lock types already granted on this object as well as for the lock types awaited on this object. For each lock type a count of the number of granted locks and waiting locks is also dumped as well as the totals. An example of the log file output is shown here: LOG: LockAcquire: new: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0 wait(0) type(AccessShareLock) LOG: GrantLock: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(2) req(1,0,0,0,0,0,0)=1 grant(1,0,0,0,0,0,0)=1 wait(0) type(AccessShareLock) LOG: UnGrantLock: updated: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0 wait(0) type(AccessShareLock) LOG: CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0 wait(0) type(INVALID) Details of the structure being dumped may be found in src/include/storage/lock.h. This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Generates a great amount of debugging output for the LISTEN and NOTIFY commands. client_min_messages or log_min_messages must be DEBUG1 or lower to send this output to the client or server log, respectively.
+Generates a great amount of debugging output for the LISTEN and NOTIFY commands. client_min_messages or log_min_messages must be DEBUG1 or lower to send this output to the client or server logs, respectively.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: enum
+context: sighup
+boot_val: log
+unit: None
+enumvals: ['debug5', 'debug4', 'debug3', 'debug2', 'debug1', 'log', 'notice', 'warning', 'error']
+
+Enables logging of recovery-related debugging output that otherwise would not be logged. This parameter allows the user to override the normal setting of log_min_messages, but only for specific messages. This is intended for use in debugging Hot Standby. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, and LOG. The default, LOG, does not affect logging decisions at all. The other values cause recovery-related debug messages of that priority or higher to be logged as though they had LOG priority; for common settings of log_min_messages this results in unconditionally sending them to the server log. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Enables the collection of information on the currently executing command of each session, along with the time at which that command began execution. This parameter is on by default. Note that even when enabled, this information is not visible to all users, only to superusers and the user owning the session being reported on; so it should not represent a security risk. Only superusers can change this setting.
+Enables the collection of information on the currently executing command of each session, along with the time when that command began execution. This parameter is on by default. Note that even when enabled, this information is not visible to all users, only to superusers and the user owning the session being reported on, so it should not represent a security risk. Only superusers can change this setting.

--- 8.4
+++ 9.0
@@ -0,0 +1,6 @@
+vartype: string
+context: user
+boot_val: None
+unit: None
+enumvals: None
+

--- 8.4
+++ 9.0
@@ -0,0 +1,6 @@
+vartype: bool
+context: user
+boot_val: off
+unit: None
+enumvals: None
+

--- 8.4
+++ 9.0
@@ -1 +1 @@
-When on, expressions of the form expr = NULL (or NULL = expr) are treated as expr IS NULL, that is, they return true if expr evaluates to the null value, and false otherwise. The correct SQL-spec-compliant behavior of expr = NULL is to always return null (unknown). Therefore this parameter defaults to off. However, filtered forms in Microsoft Access generate queries that appear to use expr = NULL to test for null values, so if you use that interface to access the database you might want to turn this option on. Since expressions of the form expr = NULL always return the null value (using the correct interpretation) they are not very useful and do not appear often in normal applications, so this option does little harm in practice. But new users are frequently confused about the semantics of expressions involving null values, so this option is not on by default. Note that this option only affects the exact form = NULL, not other comparison operators or other expressions that are computationally equivalent to some expression involving the equals operator (such as IN). Thus, this option is not a general fix for bad programming. Refer to Section 9.2 for related information.
+When on, expressions of the form expr = NULL (or NULL = expr) are treated as expr IS NULL, that is, they return true if expr evaluates to the null value, and false otherwise. The correct SQL-spec-compliant behavior of expr = NULL is to always return null (unknown). Therefore this parameter defaults to off. However, filtered forms in Microsoft Access generate queries that appear to use expr = NULL to test for null values, so if you use that interface to access the database you might want to turn this option on. Since expressions of the form expr = NULL always return the null value (using the SQL standard interpretation), they are not very useful and do not appear often in normal applications so this option does little harm in practice. But new users are frequently confused about the semantics of expressions involving null values, so this option is off by default. Note that this option only affects the exact form = NULL, not other comparison operators or other expressions that are computationally equivalent to some expression involving the equals operator (such as IN). Thus, this option is not a general fix for bad programming. Refer to Section 9.2 for related information.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the directory of the Unix-domain socket on which the server is to listen for connections from client applications. The default is normally /tmp, but can be changed at build time. This parameter can only be set at server start.
+Specifies the directory of the Unix-domain socket on which the server is to listen for connections from client applications. The default is normally /tmp, but can be changed at build time. This parameter can only be set at server start. In addition to the socket file itself, which is named .s.PGSQL.nnnn where nnnn is the server's port number, an ordinary file named .s.PGSQL.nnnn.lock will be created in the unix_socket_directory directory. Neither file should ever be removed manually. This parameter is irrelevant on Windows, which does not have Unix-domain sockets.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the owning group of the Unix-domain socket. (The owning user of the socket is always the user that starts the server.) In combination with the parameter unix_socket_permissions this can be used as an additional access control mechanism for Unix-domain connections. By default this is the empty string, which selects the default group for the current user. This parameter can only be set at server start.
+Sets the owning group of the Unix-domain socket. (The owning user of the socket is always the user that starts the server.) In combination with the parameter unix_socket_permissions this can be used as an additional access control mechanism for Unix-domain connections. By default this is the empty string, which uses the default group of the server user. This parameter can only be set at server start. This parameter is irrelevant on Windows, which does not have Unix-domain sockets.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Sets the access permissions of the Unix-domain socket. Unix-domain sockets use the usual Unix file system permission set. The parameter value is expected to be a numeric mode specification in the form accepted by the chmod and umask system calls. (To use the customary octal format the number must start with a 0 (zero).) The default permissions are 0777, meaning anyone can connect. Reasonable alternatives are 0770 (only user and group, see also unix_socket_group) and 0700 (only user). (Note that for a Unix-domain socket, only write permission matters and so there is no point in setting or revoking read or execute permissions.) This access control mechanism is independent of the one described in Chapter 19. This parameter can only be set at server start. This parameter is irrelevant on systems, notably Solaris as of Solaris 10, that ignore socket permissions entirely. There, one can achieve a similar effect by pointing unix_socket_directories to a directory having search permission limited to the desired audience. This parameter is also irrelevant on Windows, which does not have Unix-domain sockets.
+Sets the access permissions of the Unix-domain socket. Unix-domain sockets use the usual Unix file system permission set. The parameter value is expected to be a numeric mode specified in the format accepted by the chmod and umask system calls. (To use the customary octal format the number must start with a 0 (zero).) The default permissions are 0777, meaning anyone can connect. Reasonable alternatives are 0770 (only user and group, see also unix_socket_group) and 0700 (only user). (Note that for a Unix-domain socket, only write permission matters, so there is no point in setting or revoking read or execute permissions.) This access control mechanism is independent of the one described in Chapter 19. This parameter can only be set at server start. This parameter is irrelevant on systems, notably Solaris as of Solaris 10, that ignore socket permissions entirely. There, one can achieve a similar effect by pointing unix_socket_directory to a directory having search permission limited to the desired audience. This parameter is also irrelevant on Windows, which does not have Unix-domain sockets.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: integer
+context: sighup
+boot_val: 0
+unit: 
+enumvals: None
+
+Specifies the number of transactions by which VACUUM and HOT updates will defer cleanup of dead row versions. The default is zero transactions, meaning that dead row versions can be removed as soon as possible, that is, as soon as they are no longer visible to any open transaction. You may wish to set this to a non-zero value on a primary server that is supporting hot standby servers, as described in Section 25.5. This allows more time for queries on the standby to complete without incurring conflicts due to early cleanup of rows. However, since the value is measured in terms of number of write transactions occurring on the primary server, it is difficult to predict just how much additional grace time will be made available to standby queries. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: integer
+context: sighup
+boot_val: 0
+unit: 
+enumvals: None
+
+Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is normally 16 megabytes. If a standby server connected to the primary falls behind by more than wal_keep_segments segments, the primary might remove a WAL segment still needed by the standby, in which case the replication connection will be terminated. (However, the standby server can recover by fetching the segment from archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint. If wal_keep_segments is zero (the default), the system doesn't keep any extra segments for standby purposes, and the number of old WAL segments available to standby servers is a function of the location of the previous checkpoint and status of WAL archiving. This parameter has no effect on restartpoints. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: enum
+context: postmaster
+boot_val: minimal
+unit: None
+enumvals: ['minimal', 'archive', 'hot_standby']
+
+wal_level determines how much information is written to the WAL. The default value is minimal, which writes only the information needed to recover from a crash or immediate shutdown. archive adds logging required for WAL archiving, and hot_standby further adds information required to run read-only queries on a standby server. This parameter can only be set at server start. In minimal level, WAL-logging of some bulk operations can be safely skipped, which can make those operations much faster (see Section 14.4.7). Operations in which this optimization can be applied include: CREATE TABLE AS CREATE INDEX CLUSTER COPY into tables that were created or truncated in the same transaction But minimal WAL does not contain enough information to reconstruct the data from a base backup and the WAL logs, so either archive or hot_standby level must be used to enable WAL archiving (archive_mode) and streaming replication. In hot_standby level, the same information is logged as with archive, plus information needed to reconstruct the status of running transactions from the WAL. To enable read-only queries on a standby server, wal_level must be set to hot_standby on the primary, and hot_standby must be enabled in the standby. It is thought that there is little measurable difference in performance between using hot_standby and archive levels, so feedback is welcome if any production impacts are noticeable.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Reports the number of blocks (pages) in a WAL segment file. The total size of a WAL segment file in bytes is equal to wal_segment_size multiplied by wal_block_size; by default this is 16MB. See Section 28.4 for more information.
+Reports the number of blocks (pages) in a WAL segment file. The total size of a WAL segment file in bytes is equal to wal_segment_size multiplied by wal_block_size; by default this is 16MB. See Section 29.4 for more information.

--- 8.4
+++ 9.0
@@ -0,0 +1,7 @@
+vartype: integer
+context: sighup
+boot_val: 200
+unit: ms
+enumvals: None
+
+Specifies the delay between activity rounds for WAL sender processes. In each round the WAL sender sends any WAL accumulated since the last round to the standby server. It then sleeps for wal_sender_delay milliseconds, and repeats. The default value is 200 milliseconds (200ms). Note that on many systems, the effective resolution of sleep delays is 10 milliseconds; setting wal_sender_delay to a value that is not a multiple of 10 might have the same results as setting it to the next higher multiple of 10. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Method used for forcing WAL updates out to disk. If fsync is off then this setting is irrelevant, since updates will not be forced out at all. Possible values are: open_datasync (write WAL files with open() option O_DSYNC) fdatasync (call fdatasync() at each commit) fsync (call fsync() at each commit) fsync_writethrough (call fsync() at each commit, forcing write-through of any disk write cache) open_sync (write WAL files with open() option O_SYNC) The open_* options also use O_DIRECT if available. Not all of these choices are available on all platforms. The default is the first method in the above list that is supported by the platform, except that fdatasync is the default on Linux. The default is not necessarily ideal; it might be necessary to change this setting or other aspects of your system configuration in order to create a crash-safe configuration or achieve optimal performance. These aspects are discussed in Section 28.1. This parameter can only be set in the postgresql.conf file or on the server command line.
+Method used for forcing WAL updates out to disk. If fsync is off then this setting is irrelevant, since WAL file updates will not be forced out at all. Possible values are: open_datasync (write WAL files with open() option O_DSYNC) fdatasync (call fdatasync() at each commit) fsync (call fsync() at each commit) fsync_writethrough (call fsync() at each commit, forcing write-through of any disk write cache) open_sync (write WAL files with open() option O_SYNC) The open_* options also use O_DIRECT if available. Not all of these choices are available on all platforms. The default is the first method in the above list that is supported by the platform, except that fdatasync is the default on Linux. The default is not necessarily ideal; it might be necessary to change this setting or other aspects of your system configuration in order to create a crash-safe configuration or achieve optimal performance. These aspects are discussed in Section 29.1. This parameter can only be set in the postgresql.conf file or on the server command line.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Specifies the amount of memory to be used by internal sort operations and hash tables before switching to temporary disk files. The value defaults to one megabyte (1MB). Note that for a complex query, several sort or hash operations might be running in parallel; each one will be allowed to use as much memory as this value specifies before it starts to put data into temporary files. Also, several running sessions could be doing such operations concurrently. So the total memory used could be many times the value of work_mem; it is necessary to keep this fact in mind when choosing the value. Sort operations are used for ORDER BY, DISTINCT, and merge joins. Hash tables are used in hash joins, hash-based aggregation, and hash-based processing of IN subqueries.
+Specifies the amount of memory to be used by internal sort operations and hash tables before writing to temporary disk files. The value defaults to one megabyte (1MB). Note that for a complex query, several sort or hash operations might be running in parallel; each operation will be allowed to use as much memory as this value specifies before it starts to write data into temporary files. Also, several running sessions could be doing such operations concurrently. Therefore, the total memory used could be many times the value of work_mem; it is necessary to keep this fact in mind when choosing the value. Sort operations are used for ORDER BY, DISTINCT, and merge joins. Hash tables are used in hash joins, hash-based aggregation, and hash-based processing of IN subqueries.

--- 8.4
+++ 9.0
@@ -1 +1 @@
-Detection of a damaged page header normally causes PostgreSQL to report an error, aborting the current command. Setting zero_damaged_pages to on causes the system to instead report a warning, zero out the damaged page, and continue processing. This behavior will destroy data, namely all the rows on the damaged page. But it allows you to get past the error and retrieve rows from any undamaged pages that might be present in the table. So it is useful for recovering data if corruption has occurred due to hardware or software error. You should generally not set this on until you have given up hope of recovering data from the damaged page(s) of a table. The default setting is off, and it can only be changed by a superuser.
+Detection of a damaged page header normally causes PostgreSQL to report an error, aborting the current command. Setting zero_damaged_pages to on causes the system to instead report a warning, zero out the damaged page, and continue processing. This behavior will destroy data, namely all the rows on the damaged page. But it allows you to get past the error and retrieve rows from any undamaged pages that might be present in the table. So it is useful for recovering data if corruption has occurred due to a hardware or software error. You should generally not set this on until you have given up hope of recovering data from the damaged pages of a table. The default setting is off, and it can only be changed by a superuser.