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WAL ConfigurationManual ChaptersPG1828.5. WAL Configuration There are several WAL-related configuration parameters that affect database performance. This section explains their use. Consult Chapter 19 for general information a…WAL Configuration WAL InternalsManual ChaptersPG1828.6. WAL Internals WAL is automatically enabled; no action is required from the administrator except ensuring that the disk-space requirements for the WAL files are met, and that any necess…WAL Internals WAL SummarizationManual ChaptersPG1819.5.7. WAL Summarization These settings control WAL summarization, a feature which must be enabled in order to perform an incremental backup. summarize_wal (boolean) # Enables the WAL summa…WAL Summarization WAL Summarization Information FunctionsManual ChaptersPG189.27.12. WAL Summarization Information Functions The functions shown in Table 9.94 print information about the status of WAL summarization. See summarize_wal. Table 9.94. WAL Summarization I…WAL Summarization Information Functions WarningsManual ChaptersPG18Warnings If pg_createsubscriber fails after the target server was promoted, then the data directory is likely not in a state that can be recovered. In such case, creating a new standby serve…Warnings What Is a Document?Manual ChaptersPG1812.1.1. What Is a Document? A document is the unit of searching in a full text search system; for example, a magazine article or email message. The text search engine must be able to parse d…What Is a Document? What Is JIT compilation?Manual ChaptersPG1830.1. What Is JIT compilation? Just-in-Time (JIT) compilation is the process of turning some form of interpreted program evaluation into a native program, and doing so at run time. For examp…What Is JIT compilation? What Is PostgreSQL?Manual ChaptersPG181. What Is PostgreSQL? PostgreSQL is an object-relational database management system (ORDBMS) based on POSTGRES, Version 4.2, developed at the University of California at Berkeley Computer S…What Is PostgreSQL? What Is the “Transaction” Actually Performed in pgbench?Manual ChaptersPG18What Is the “Transaction” Actually Performed in pgbench? pgbench executes test scripts chosen randomly from a specified list. The scripts may include built-in scripts specified with -b and u…What Is the “Transaction” Actually Performed in pgbench? What to LogManual ChaptersPG1819.8.3. What to Log Note What you choose to log can have security implications; see Section 24.3. application_name (string) The application_name can be any string of less than NAMEDATALEN ch…What to Log What to ReportManual ChaptersPG185.2. What to Report The most important thing to remember about bug reporting is to state all the facts and only facts. Do not speculate what you think went wrong, what “it seemed to do”, or …What to Report When Can Parallel Query Be Used?Manual ChaptersPG1815.2. When Can Parallel Query Be Used? There are several settings that can cause the query planner not to generate a parallel query plan under any circumstances. In order for any parallel qu…When Can Parallel Query Be Used? When Color is UsedManual ChaptersPG18N.1. When Color is Used To use colorized output, set the environment variable PG_COLOR as follows: If the value is always, then color is used. If the value is auto and the standard error str…When Color is Used When to JIT?Manual ChaptersPG1830.2. When to JIT? JIT compilation is beneficial primarily for long-running CPU-bound queries. Frequently these will be analytical queries. For short queries the added overhead of performing…When to JIT? When to LogManual ChaptersPG1819.8.2. When to Log log_min_messages (enum) Controls which message levels are written to the server log. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, INFO, NOTICE, WARNING, ERROR…When to Log When to Use Pipeline ModeManual ChaptersPG1832.5.3. When to Use Pipeline Mode Much like asynchronous query mode, there is no meaningful performance overhead when using pipeline mode. It increases client application complexity, and ext…When to Use Pipeline Mode WHENEVERManual ChaptersPG18WHENEVER WHENEVER — specify the action to be taken when an SQL statement causes a specific class condition to be raised Synopsis WHENEVER { NOT FOUND | SQLERROR | SQLWARNING } actionWHENEVER WHERE ClauseManual ChaptersPG18WHERE Clause The optional WHERE clause has the general form WHERE condition where condition is any expression that evaluates to a result of type boolean. Any row that does not satisfy this c…WHERE Clause Where to LogManual ChaptersPG1819.8.1. Where to Log log_destination (string) PostgreSQL supports several methods for logging server messages, including stderr, csvlog, jsonlog, and syslog. On Windows, eventlog is also sup…Where to Log Where to Report BugsManual ChaptersPG185.3. Where to Report Bugs In general, send bug reports to the bug report mailing list at <pgsql-bugs@lists.postgresql.org>. You are requested to use a descriptive subject for your email mess…Where to Report Bugs WHILEManual ChaptersPG1841.6.5.4. WHILE [ <<label>> ] WHILE boolean-expression LOOP statements END LOOP [ label ]; The WHILE statement repeats a sequence of statements so long as the boolean-expression evaluates to…WHILE WINDOW ClauseManual ChaptersPG18WINDOW Clause The optional WINDOW clause has the general form WINDOW window_name AS ( window_definition ) [, ...] where window_name is a name that can be referenced from OVER clauses or subs…WINDOW Clause Window Function CallsManual ChaptersPG184.2.8. Window Function Calls A window function call represents the application of an aggregate-like function over some portion of the rows selected by a query. Unlike non-window aggregate ca…Window Function Calls Window Function ProcessingManual ChaptersPG187.2.5. Window Function Processing If the query contains any window functions (see Section 3.5, Section 9.22 and Section 4.2.8), these functions are evaluated after any grouping, aggregation,…Window Function Processing Window FunctionsManual ChaptersPG189.22. Window Functions Window functions provide the ability to perform calculations across sets of rows that are related to the current query row. See Section 3.5 for an introduction to this…Window Functions Window FunctionsManual ChaptersPG183.5. Window Functions A window function performs a calculation across a set of table rows that are somehow related to the current row. This is comparable to the type of calculation that can …Window Functions WITH ClauseManual ChaptersPG18WITH Clause The WITH clause is a PostgreSQL extension; storage parameters are not in the standard.WITH Clause WITH ClauseManual ChaptersPG18WITH Clause The WITH clause allows you to specify one or more subqueries that can be referenced by name in the primary query. The subqueries effectively act as temporary tables or views for …WITH Clause WITH Queries (Common Table Expressions)Manual ChaptersPG187.8. WITH Queries (Common Table Expressions) WITH provides a way to write auxiliary statements for use in a larger query. These statements, which are often referred to as Common Table Expres…WITH Queries (Common Table Expressions) Worker ProcessesManual ChaptersPG1819.4.6. Worker Processes max_worker_processes (integer) # Sets the maximum number of background processes that the cluster can support. This parameter can only be set at server start. The de…Worker Processes Write Ahead LogManual ChaptersPG1819.5. Write Ahead Log For additional information on tuning these settings, see Section 28.5.Write Ahead Log Write Ahead Logging for ExtensionsManual ChaptersPG18Chapter 64. Write Ahead Logging for Extensions Certain extensions, principally extensions that implement custom access methods, may need to perform write-ahead logging in order to ensure cra…Write Ahead Logging for Extensions Write-Ahead Logging (WAL)Manual ChaptersPG1828.3. Write-Ahead Logging (WAL) Write-Ahead Logging (WAL) is a standard method for ensuring data integrity. A detailed description can be found in most (if not all) books about transaction p…Write-Ahead Logging (WAL) Writing a Custom Scan ProviderManual ChaptersPG18Chapter 60. Writing a Custom Scan Provider PostgreSQL supports a set of experimental facilities which are intended to allow extension modules to add new scan types to the system. Unlike a fo…Writing a Custom Scan Provider Writing a Foreign Data WrapperManual ChaptersPG18Chapter 58. Writing a Foreign Data Wrapper All operations on a foreign table are handled through its foreign data wrapper, which consists of a set of functions that the core server calls. Th…Writing a Foreign Data Wrapper Writing a Procedural Language HandlerManual ChaptersPG18Chapter 57. Writing a Procedural Language Handler All calls to functions that are written in a language other than the current “version 1” interface for compiled languages (this includes fun…Writing a Procedural Language Handler Writing a Table Sampling MethodManual ChaptersPG18Chapter 59. Writing a Table Sampling Method PostgreSQL's implementation of the TABLESAMPLE clause supports custom table sampling methods, in addition to the BERNOULLI and SYSTEM methods that…Writing a Table Sampling Method Writing CodeManual ChaptersPG1836.10.4. Writing Code Before we turn to the more advanced topics, we should discuss some coding rules for PostgreSQL C-language functions. While it might be possible to load functions writte…Writing Code Writing Data to a Large ObjectManual ChaptersPG1833.3.5. Writing Data to a Large Object The function int lo_write(PGconn *conn, int fd, const char *buf, size_t len); writes len bytes from buf (which must be of size len) to large object des…Writing Data to a Large Object Writing Event Trigger Functions in CManual ChaptersPG1838.2. Writing Event Trigger Functions in C This section describes the low-level details of the interface to an event trigger function. This information is only needed when writing event trig…Writing Event Trigger Functions in C