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Julian DatesManual ChaptersPG18B.7. Julian Dates The Julian Date system is a method for numbering days. It is unrelated to the Julian calendar, though it is confusingly named similarly to that calendar. The Julian Date sy…Julian Dates Just-in-Time Compilation (JIT)Manual ChaptersPG18Chapter 30. Just-in-Time Compilation (JIT) This chapter explains what just-in-time compilation is, and how it can be configured in PostgreSQL.Just-in-Time Compilation (JIT) Kernel Resource UsageManual ChaptersPG1819.4.3. Kernel Resource Usage max_files_per_process (integer) # Sets the maximum number of open files each server subprocess is allowed to open simultaneously; files already opened in the po…Kernel Resource Usage key_column_usageManual ChaptersPG1835.32. key_column_usage The view key_column_usage identifies all columns in the current database that are restricted by some unique, primary key, or foreign key constraint. Check constraints…key_column_usage Keyword/Value Connection StringsManual ChaptersPG1832.1.1.1. Keyword/Value Connection Strings In the keyword/value format, each parameter setting is in the form keyword = value, with space(s) between settings. Spaces around a setting's equal…Keyword/Value Connection Strings Language TagManual ChaptersPG1823.1.5.3. Language Tag A language tag, defined in BCP 47, is a standardized identifier used to identify languages, regions, and other information about a locale. Basic language tags are simp…Language Tag Large ObjectsManual ChaptersPG1834.12. Large Objects Large objects are not directly supported by ECPG, but ECPG application can manipulate large objects through the libpq large object functions, obtaining the necessary PGc…Large Objects Large ObjectsManual ChaptersPG18Chapter 33. Large Objects PostgreSQL has a large object facility, which provides stream-style access to user data that is stored in a special large-object structure. Streaming access is usef…Large Objects LATERAL SubqueriesManual ChaptersPG187.2.1.5. LATERAL Subqueries Subqueries appearing in FROM can be preceded by the key word LATERAL. This allows them to reference columns provided by preceding FROM items. (Without LATERAL, ea…LATERAL Subqueries LDAP AuthenticationManual ChaptersPG1820.10. LDAP Authentication This authentication method operates similarly to password except that it uses LDAP as the password verification method. LDAP is used only to validate the user name…LDAP Authentication LDAP Lookup of Connection ParametersManual ChaptersPG1832.18. LDAP Lookup of Connection Parameters If libpq has been compiled with LDAP support (option --with-ldap for configure) it is possible to retrieve connection options like host or dbname …LDAP Lookup of Connection Parameters Legal NoticeManual ChaptersPG18Legal Notice PostgreSQL Database Management System (also known as Postgres, formerly known as Postgres95) Portions Copyright © 1996-2026, PostgreSQL Global Development Group Portions Copyrig…Legal Notice LevenshteinManual ChaptersPG18F.16.3. Levenshtein This function calculates the Levenshtein distance between two strings: levenshtein(source text, target text, ins_cost int, del_cost int, sub_cost int) returns int levensh…Levenshtein Lexical StructureManual ChaptersPG184.1. Lexical Structure SQL input consists of a sequence of commands. A command is composed of a sequence of tokens, terminated by a semicolon (“;”). The end of the input stream also terminat…Lexical Structure libc CollationsManual ChaptersPG1823.2.2.2.1. libc Collations For example, the operating system might provide a locale named de_DE.utf8. initdb would then create a collation named de_DE.utf8 for encoding UTF8 that has both L…libc Collations libc CollationsManual ChaptersPG1823.2.2.3.1. libc Collations New libc collations can be created like this: CREATE COLLATION german (provider = libc, locale = 'de_DE'); The exact values that are acceptable for the locale cla…libc Collations LibpqManual ChaptersPG18E.6.3.5. Libpq Add function PQfullProtocolVersion() to report the full, including minor, protocol version number (Jacob Champion, Jelte Fennema-Nio) § Add libpq connection parameters and env…Libpq libpq — C LibraryManual ChaptersPG18Chapter 32. libpq — C Library libpq is the C application programmer's interface to PostgreSQL. libpq is a set of library functions that allow client programs to pass queries to the PostgreSQ…libpq — C Library Library FunctionsManual ChaptersPG1834.11. Library Functions The libecpg library primarily contains “hidden” functions that are used to implement the functionality expressed by the embedded SQL commands. But there are some fun…Library Functions LIKEManual ChaptersPG189.7.1. LIKE string LIKE pattern [ESCAPE escape-character] string NOT LIKE pattern [ESCAPE escape-character] The LIKE expression returns true if the string matches the supplied pattern. (As e…LIKE LIKE ClauseManual ChaptersPG18LIKE Clause While a LIKE clause exists in the SQL standard, many of the options that PostgreSQL accepts for it are not in the standard, and some of the standard's options are not implemented…LIKE Clause LIMIT and OFFSETManual ChaptersPG187.6. LIMIT and OFFSET LIMIT and OFFSET allow you to retrieve just a portion of the rows that are generated by the rest of the query: SELECT select_list FROM table_expression [ ORDER BY ... ]…LIMIT and OFFSET LIMIT and OFFSETManual ChaptersPG18LIMIT and OFFSET The clauses LIMIT and OFFSET are PostgreSQL-specific syntax, also used by MySQL. The SQL:2008 standard has introduced the clauses OFFSET ... FETCH {FIRST|NEXT} ... for the s…LIMIT and OFFSET LIMIT ClauseManual ChaptersPG18LIMIT Clause The LIMIT clause consists of two independent sub-clauses: LIMIT { count | ALL } OFFSET start The parameter count specifies the maximum number of rows to return, while start spec…LIMIT Clause LimitationsManual ChaptersPG18Limitations pg_combinebackup does not recompute page checksums when writing the output directory. Therefore, if any of the backups used for reconstruction were taken with checksums disabled,…Limitations LimitationsManual ChaptersPG18F.6.4. Limitations Only operator classes for int4 and text are included with the module. Only the = operator is supported for search. But it is possible to add support for arrays with union …Limitations LimitationsManual ChaptersPG18F.9.4. Limitations citext's case-folding behavior depends on the LC_CTYPE setting of your database. How it compares values is therefore determined when the database is created. It is not tru…Limitations LimitationsManual ChaptersPG185.12.2.3. Limitations The following limitations apply to partitioned tables: To create a unique or primary key constraint on a partitioned table, the partition keys must not include any expr…Limitations LimitationsManual ChaptersPG1865.4.6. Limitations GIN assumes that indexable operators are strict. This means that extractValue will not be called at all on a null item value (instead, a placeholder index entry is create…Limitations LimitationsManual ChaptersPG18F.21.3. Limitations Dropping a table will still orphan any objects it contains, as the trigger is not executed. You can avoid this by preceding the DROP TABLE with DELETE FROM table. TRUNCAT…Limitations LimitationsManual ChaptersPG18F.40.7. Limitations Data Definition Language (DDL) Permissions Due to implementation restrictions, some DDL operations do not check permissions. Data Control Language (DCL) Permissions Due t…Limitations LimitationsManual ChaptersPG1812.11. Limitations The current limitations of PostgreSQL's text search features are: The length of each lexeme must be less than 2 kilobytes The length of a tsvector (lexemes + positions) mu…Limitations Limitations and Missing FeaturesManual ChaptersPG1843.8.2. Limitations and Missing Features The following features are currently missing from PL/Perl, but they would make welcome contributions. PL/Perl functions cannot call each other direct…Limitations and Missing Features Limitations of Functional DependenciesManual ChaptersPG1814.2.2.1.1. Limitations of Functional Dependencies Functional dependencies are currently only applied when considering simple equality conditions that compare columns to constant values, and…Limitations of Functional Dependencies Limitations of PGP CodeManual ChaptersPG18F.26.3.10. Limitations of PGP Code No support for signing. That also means that it is not checked whether the encryption subkey belongs to the master key. No support for encryption key as ma…Limitations of PGP Code Limits and CompatibilityManual ChaptersPG189.7.3.6. Limits and Compatibility No particular limit is imposed on the length of REs in this implementation. However, programs intended to be highly portable should not employ REs longer th…Limits and Compatibility Line SegmentsManual ChaptersPG188.8.3. Line Segments Line segments are represented by pairs of points that are the endpoints of the segment. Values of type lseg are specified using any of the following syntaxes: [ ( x1 , y…Line Segments LinesManual ChaptersPG188.8.2. Lines Lines are represented by the linear equation Ax + By + C = 0, where A and B are not both zero. Values of type line are input and output in the following form: { A, B, C } Altern…Lines Linux Huge PagesManual ChaptersPG1818.4.5. Linux Huge Pages Using huge pages reduces overhead when using large contiguous chunks of memory, as PostgreSQL does, particularly when using large values of shared_buffers. To use th…Linux Huge Pages Linux Memory OvercommitManual ChaptersPG1818.4.4. Linux Memory Overcommit The default virtual memory behavior on Linux is not optimal for PostgreSQL. Because of the way that the kernel implements memory overcommit, the kernel might …Linux Memory Overcommit