## E.2. Release 12

Release date: 2019-10-03

### E.2.1. Overview

Major enhancements in PostgreSQL 12 include:

- General performance improvements, including:

  - Optimizations to space utilization and read/write performance for B-tree indexes

  - Partitioning performance enhancements, including improved query performance on tables with thousands of partitions, improved insertion performance with INSERT and COPY, and the ability to execute `ALTER TABLE ATTACH PARTITION` without blocking queries

  - Automatic (but overridable) inlining of common table expressions (CTEs)

  - Reduction of WAL overhead for creation of GiST, GIN, and SP-GiST indexes

  - Support for covering GiST indexes, via the `INCLUDE` clause

  - Multi-column most-common-value (MCV) statistics can be defined via CREATE STATISTICS, to support better plans for queries that test several non-uniformly-distributed columns

- Enhancements to administrative functionality, including:

  - `REINDEX CONCURRENTLY` can rebuild an index without blocking writes to its table

  - pg_checksums can enable/disable page checksums (used for detecting data corruption) in an offline cluster

  - Progress reporting statistics for CREATE INDEX, REINDEX, CLUSTER, VACUUM FULL, and pg_checksums

- Support for the SQL/JSON path language

- Stored generated columns

- Nondeterministic ICU collations, enabling case-insensitive and accent-insensitive grouping and ordering

- New authentication features, including:

  - Encryption of TCP/IP connections when using GSSAPI authentication

  - Discovery of LDAP servers using DNS SRV records

  - Multi-factor authentication, using the `clientcert=verify-full` option combined with an additional authentication method in `pg_hba.conf`

The above items are explained in more detail in the sections below.

### E.2.2. Migration to Version 12

A dump/restore using pg_dumpall, or use of pg_upgrade, is required for those wishing to migrate data from any previous release.

Version 12 contains a number of changes that may affect compatibility with previous releases. Observe the following incompatibilities:

- Remove the special behavior of oid columns (Andres Freund, John Naylor)

  Previously, a normally-invisible `oid` column could be specified during table creation using `WITH OIDS`; that ability has been removed. Columns can still be explicitly declared as type `oid`. Operations on tables that have columns created using `WITH OIDS` will need adjustment.

  The system catalogs that previously had hidden `oid` columns now have ordinary `oid` columns. Hence, `SELECT *` will now output those columns, whereas previously they would be displayed only if selected explicitly.

- Remove data types `abstime`, `reltime`, and `tinterval` (Andres Freund)

  These are obsoleted by SQL-standard types such as `timestamp`.

- Remove the `timetravel` extension (Andres Freund)

- Move `recovery.conf` settings into `postgresql.conf` (Masao Fujii, Simon Riggs, Abhijit Menon-Sen, Sergei Kornilov)

  `recovery.conf` is no longer used, and the server will not start if that file exists. recovery.signal and `standby.signal` files are now used to switch into non-primary mode. The `trigger_file` setting has been renamed to promote_trigger_file. The `standby_mode` setting has been removed.

- Do not allow multiple conflicting `recovery_target`* specifications (Peter Eisentraut)

  Specifically, only allow one of recovery_target, recovery_target_lsn, recovery_target_name, recovery_target_time, and recovery_target_xid. Previously, multiple different instances of these parameters could be specified, and the last one was honored. Now, only one can be specified, though the same one can be specified multiple times and the last specification is honored.

- Cause recovery to advance to the latest timeline by default (Peter Eisentraut)

  Specifically, recovery_target_timeline now defaults to `latest`. Previously, it defaulted to `current`.

- Refactor code for geometric functions and operators (Emre Hasegeli)

  This could lead to more accurate, but slightly different, results compared to previous releases. Notably, cases involving NaN, underflow, overflow, and division by zero are handled more consistently than before.

- Improve performance by using a new algorithm for output of `real` and `double precision` values (Andrew Gierth)

  Previously, displayed floating-point values were rounded to 6 (for `real`) or 15 (for `double precision`) digits by default, adjusted by the value of extra_float_digits. Now, whenever `extra_float_digits` is more than zero (as it now is by default), only the minimum number of digits required to preserve the exact binary value are output. The behavior is the same as before when `extra_float_digits` is set to zero or less.

  Also, formatting of floating-point exponents is now uniform across platforms: two digits are used unless three are necessary. In previous releases, Windows builds always printed three digits.

- `random()` and `setseed()` now behave uniformly across platforms (Tom Lane)

  The sequence of `random()` values generated following a `setseed()` call with a particular seed value is likely to be different now than before. However, it will also be repeatable, which was not previously guaranteed because of interference from other uses of random numbers inside the server. The SQL `random()` function now has its own private per-session state to forestall that.

- Change SQL-style `substring()` to have standard-compliant greediness behavior (Tom Lane)

  In cases where the pattern can be matched in more than one way, the initial sub-pattern is now treated as matching the least possible amount of text rather than the greatest; for example, a pattern such as `%#"aa*#"%` now selects the first group of `a`'s from the input, not the last group.

- Do not pretty-print the result of `xpath()` or the `XMLTABLE` construct (Tom Lane)

  In some cases, these functions would insert extra whitespace (newlines and/or spaces) in nodeset values. This is undesirable since depending on usage, the whitespace might be considered semantically significant.

- Rename command-line tool pg_verify_checksums to pg_checksums (Michaël Paquier)

- In pg_restore, require specification of `-f -` to send the dump contents to standard output (Euler Taveira)

  Previously, this happened by default if no destination was specified, but that was deemed to be unfriendly.

- Disallow non-unique abbreviations in psql's `\pset format` command (Daniel Vérité)

  Previously, for example, `\pset format a` chose `aligned`; it will now fail since that could equally well mean `asciidoc`.

- In new btree indexes, the maximum index entry length is reduced by eight bytes, to improve handling of duplicate entries (Peter Geoghegan)

  This means that a REINDEX operation on an index pg_upgrade'd from a previous release could potentially fail.

- Cause `DROP IF EXISTS FUNCTION`/`PROCEDURE`/`AGGREGATE`/`ROUTINE` to generate an error if no argument list is supplied and there are multiple matching objects (David Rowley)

  Also improve the error message in such cases.

- Split the `pg_statistic_ext` catalog into two catalogs, and add the `pg_stats_ext` view of it (Dean Rasheed, Tomas Vondra)

  This change supports hiding potentially-sensitive statistics data from unprivileged users.

- Remove obsolete `pg_constraint`.`consrc` column (Peter Eisentraut)

- Remove obsolete `pg_attrdef`.`adsrc` column (Peter Eisentraut)

- Mark table columns of type name as having “C” collation by default (Tom Lane, Daniel Vérité)

  The comparison operators for data type `name` can now use any collation, rather than always using “C” collation. To preserve the previous semantics of queries, columns of type `name` are now explicitly marked as having “C” collation. A side effect of this is that regular-expression operators on `name` columns will now use the “C” collation by default, not the database collation, to determine the behavior of locale-dependent regular expression patterns (such as `\w`). If you want non-C behavior for a regular expression on a `name` column, attach an explicit `COLLATE` clause. (For user-defined `name` columns, another possibility is to specify a different collation at table creation time; but that just moves the non-backwards-compatibility to the comparison operators.)

- Treat object-name columns in the `information_schema` views as being of type `name`, not `varchar` (Tom Lane)

  Per the SQL standard, object-name columns in the `information_schema` views are declared as being of domain type `sql_identifier`. In PostgreSQL, the underlying catalog columns are really of type `name`. This change makes `sql_identifier` be a domain over `name`, rather than `varchar` as before. This eliminates a semantic mismatch in comparison and sorting behavior, which can greatly improve the performance of queries on `information_schema` views that restrict an object-name column. Note however that inequality restrictions, for example

      SELECT ... FROM information_schema.tables WHERE table_name < 'foo';

  will now use “C”-locale comparison semantics by default, rather than the database's default collation as before. Sorting on these columns will also follow “C” ordering rules. The previous behavior (and inefficiency) can be enforced by adding a `COLLATE "default"` clause.

- Remove the ability to disable dynamic shared memory (Kyotaro Horiguchi)

  Specifically, dynamic_shared_memory_type can no longer be set to `none`.

- Parse libpq integer connection parameters more strictly (Fabien Coelho)

  In previous releases, using an incorrect integer value for connection parameters `connect_timeout`, `keepalives`, `keepalives_count`, `keepalives_idle`, `keepalives_interval` and `port` resulted in libpq either ignoring those values or failing with incorrect error messages.

### E.2.3. Changes

Below you will find a detailed account of the changes between PostgreSQL 12 and the previous major release.

#### E.2.3.1. Server

##### E.2.3.1.1. Partitioning

- Improve performance of many operations on partitioned tables (Amit Langote, David Rowley, Tom Lane, Álvaro Herrera)

  Allow tables with thousands of child partitions to be processed efficiently by operations that only affect a small number of partitions.

- Allow foreign keys to reference partitioned tables (Álvaro Herrera)

- Improve speed of `COPY` into partitioned tables (David Rowley)

- Allow partition bounds to be any expression (Kyotaro Horiguchi, Tom Lane, Amit Langote)

  Such expressions are evaluated at partitioned-table creation time. Previously, only simple constants were allowed as partition bounds.

- Allow `CREATE TABLE`'s tablespace specification for a partitioned table to affect the tablespace of its children (David Rowley, Álvaro Herrera)

- Avoid sorting when partitions are already being scanned in the necessary order (David Rowley)

- `ALTER TABLE ATTACH PARTITION` is now performed with reduced locking requirements (Robert Haas)

- Add partition introspection functions (Michaël Paquier, Álvaro Herrera, Amit Langote)

  The new function `pg_partition_root()` returns the top-most parent of a partition tree, `pg_partition_ancestors()` reports all ancestors of a partition, and `pg_partition_tree()` displays information about partitions.

- Include partitioned indexes in the system view `pg_indexes` (Suraj Kharage)

- Add psql command `\dP` to list partitioned tables and indexes (Pavel Stehule)

- Improve psql `\d` and `\z` display of partitioned tables (Pavel Stehule, Michaël Paquier, Álvaro Herrera)

- Fix bugs that could cause `ALTER TABLE DETACH PARTITION` to leave behind incorrect dependency state, allowing subsequent operations to misbehave, for example by not dropping a former partition child index when its table is dropped (Tom Lane)

##### E.2.3.1.2. Indexes

- Improve performance and space utilization of btree indexes with many duplicates (Peter Geoghegan, Heikki Linnakangas)

  Previously, duplicate index entries were stored unordered within their duplicate groups. This caused overhead during index inserts, wasted space due to excessive page splits, and it reduced `VACUUM`'s ability to recycle entire pages. Duplicate index entries are now sorted in heap-storage order.

  Indexes pg_upgrade'd from previous releases will not have these benefits.

- Allow multi-column btree indexes to be smaller (Peter Geoghegan, Heikki Linnakangas)

  Internal pages and min/max leaf page indicators now only store index keys until the change key, rather than all indexed keys. This also improves the locality of index access.

  Indexes pg_upgrade'd from previous releases will not have these benefits.

- Improve speed of btree index insertions by reducing locking overhead (Alexander Korotkov)

- Support `INCLUDE` columns in GiST indexes (Andrey Borodin)

- Add support for nearest-neighbor (KNN) searches of SP-GiST indexes (Nikita Glukhov, Alexander Korotkov, Vlad Sterzhanov)

- Reduce the WAL write overhead of GiST, GIN, and SP-GiST index creation (Anastasia Lubennikova, Andrey V. Lepikhov)

- Allow index-only scans to be more efficient on indexes with many columns (Konstantin Knizhnik)

- Improve the performance of vacuum scans of GiST indexes (Andrey Borodin, Konstantin Kuznetsov, Heikki Linnakangas)

- Delete empty leaf pages during GiST `VACUUM` (Andrey Borodin)

- Reduce locking requirements for index renaming (Peter Eisentraut)

##### E.2.3.1.3. Optimizer

- Allow CREATE STATISTICS to create most-common-value statistics for multiple columns (Tomas Vondra)

  This improves optimization for queries that test several columns, requiring an estimate of the combined effect of several `WHERE` clauses. If the columns are correlated and have non-uniform distributions then multi-column statistics will allow much better estimates.

- Allow common table expressions (CTEs) to be inlined into the outer query (Andreas Karlsson, Andrew Gierth, David Fetter, Tom Lane)

  Specifically, CTEs are automatically inlined if they have no side-effects, are not recursive, and are referenced only once in the query. Inlining can be prevented by specifying `MATERIALIZED`, or forced for multiply-referenced CTEs by specifying `NOT MATERIALIZED`. Previously, CTEs were never inlined and were always evaluated before the rest of the query.

- Allow control over when generic plans are used for prepared statements (Pavel Stehule)

  This is controlled by the plan_cache_mode server parameter.

- Improve optimization of partition and `UNION ALL` queries that have only a single child (David Rowley)

- Improve processing of domains that have no check constraints (Tom Lane)

  Domains that are being used purely as type aliases no longer cause optimization difficulties.

- Pre-evaluate calls of `LEAST` and `GREATEST` when their arguments are constants (Vik Fearing)

- Improve optimizer's ability to verify that partial indexes with `IS NOT NULL` conditions are usable in queries (Tom Lane, James Coleman)

  Usability can now be recognized in more cases where the calling query involves casts or large ``x` IN (`array`)` clauses.

- Compute `ANALYZE` statistics using the collation defined for each column (Tom Lane)

  Previously, the database's default collation was used for all statistics. This potentially gives better optimizer behavior for columns with non-default collations.

- Improve selectivity estimates for inequality comparisons on `ctid` columns (Edmund Horner)

- Improve optimization of joins on columns of type `tid` (Tom Lane)

  These changes primarily improve the efficiency of self-joins on `ctid` columns.

- Fix the leakproofness designations of some btree comparison operators and support functions (Tom Lane)

  This allows some optimizations that previously would not have been applied in the presence of security barrier views or row-level security.

##### E.2.3.1.4. General Performance

- Enable Just-in-Time (JIT) compilation by default, if the server has been built with support for it (Andres Freund)

  Note that this support is not built by default, but has to be selected explicitly while configuring the build.

- Speed up keyword lookup (John Naylor, Joerg Sonnenberger, Tom Lane)

- Improve search performance for multi-byte characters in `position()` and related functions (Heikki Linnakangas)

- Allow toasted values to be minimally decompressed (Paul Ramsey)

  This is useful for routines that only need to examine the initial portion of a toasted field.

- Allow `ALTER TABLE ... SET NOT NULL` to avoid unnecessary table scans (Sergei Kornilov)

  This can be optimized when the table's column constraints can be recognized as disallowing nulls.

- Allow `ALTER TABLE ... SET DATA TYPE` changing between `timestamp` and `timestamptz` to avoid a table rewrite when the session time zone is UTC (Noah Misch)

  In the UTC time zone, these two data types are binary compatible.

- Improve speed in converting strings to `int2` or `int4` integers (Andres Freund)

- Allow parallelized queries when in `SERIALIZABLE` isolation mode (Thomas Munro)

  Previously, parallelism was disabled when in this mode.

- Use `pread()` and `pwrite()` for random I/O (Oskari Saarenmaa, Thomas Munro)

  This reduces the number of system calls required for I/O.

- Improve the speed of setting the process title on FreeBSD (Thomas Munro)

##### E.2.3.1.5. Monitoring

- Allow logging of statements from only a percentage of transactions (Adrien Nayrat)

  The parameter log_transaction_sample_rate controls this.

- Add progress reporting to `CREATE INDEX` and `REINDEX` operations (Álvaro Herrera, Peter Eisentraut)

  Progress is reported in the `pg_stat_progress_create_index` system view.

- Add progress reporting to `CLUSTER` and `VACUUM FULL` (Tatsuro Yamada)

  Progress is reported in the `pg_stat_progress_cluster` system view.

- Add progress reporting to pg_checksums (Michael Banck, Bernd Helmle)

  This is enabled with the option `--progress`.

- Add counter of checksum failures to `pg_stat_database` (Magnus Hagander)

- Add tracking of global objects in system view `pg_stat_database` (Julien Rouhaud)

  Global objects are shown with a `pg_stat_database`.`datid` value of zero.

- Add the ability to list the contents of the archive directory (Christoph Moench-Tegeder)

  The function is `pg_ls_archive_statusdir()`.

- Add the ability to list the contents of temporary directories (Nathan Bossart)

  The function, `pg_ls_tmpdir()`, optionally allows specification of a tablespace.

- Add information about the client certificate to the system view `pg_stat_ssl` (Peter Eisentraut)

  The new columns are `client_serial` and `issuer_dn`. Column `clientdn` has been renamed to `client_dn` for clarity.

- Restrict visibility of rows in `pg_stat_ssl` for unprivileged users (Peter Eisentraut)

- At server start, emit a log message including the server version number (Christoph Berg)

- Prevent logging “incomplete startup packet” if a new connection is immediately closed (Tom Lane)

  This avoids log spam from certain forms of monitoring.

- Include the application_name, if set, in log_connections log messages (Don Seiler)

- Make the walreceiver set its application name to the cluster name, if set (Peter Eisentraut)

- Add the timestamp of the last received standby message to `pg_stat_replication` (Lim Myungkyu)

- Add a wait event for fsync of WAL segments (Konstantin Knizhnik)

##### E.2.3.1.6. Authentication

- Add GSSAPI encryption support (Robbie Harwood, Stephen Frost)

  This feature allows TCP/IP connections to be encrypted when using GSSAPI authentication, without having to set up a separate encryption facility such as SSL. In support of this, add `hostgssenc` and `hostnogssenc` record types in `pg_hba.conf` for selecting connections that do or do not use GSSAPI encryption, corresponding to the existing `hostssl` and `hostnossl` record types. There is also a new gssencmode libpq option, and a pg_stat_gssapi system view.

- Allow the `clientcert` `pg_hba.conf` option to check that the database user name matches the client certificate's common name (Julian Markwort, Marius Timmer)

  This new check is enabled with `clientcert=verify-full`.

- Allow discovery of an LDAP server using DNS SRV records (Thomas Munro)

  This avoids the requirement of specifying `ldapserver`. It is only supported if PostgreSQL is compiled with OpenLDAP.

##### E.2.3.1.7. Server Configuration

- Add ability to enable/disable cluster checksums using pg_checksums (Michael Banck, Michaël Paquier)

  The cluster must be shut down for these operations.

- Reduce the default value of autovacuum_vacuum_cost_delay to 2ms (Tom Lane)

  This allows autovacuum operations to proceed faster by default.

- Allow vacuum_cost_delay to specify sub-millisecond delays, by accepting fractional values (Tom Lane)

- Allow time-based server parameters to use units of microseconds (`us`) (Tom Lane)

- Allow fractional input for integer server parameters (Tom Lane)

  For example, `SET work_mem = '30.1GB'` is now allowed, even though `work_mem` is an integer parameter. The value will be rounded to an integer after any required units conversion.

- Allow units to be defined for floating-point server parameters (Tom Lane)

- Add wal_recycle and wal_init_zero server parameters to control WAL file recycling (Jerry Jelinek)

  Avoiding file recycling can be beneficial on copy-on-write file systems like ZFS.

- Add server parameter tcp_user_timeout to control the server's TCP timeout (Ryohei Nagaura)

- Allow control of the minimum and maximum SSL protocol versions (Peter Eisentraut)

  The server parameters are ssl_min_protocol_version and ssl_max_protocol_version.

- Add server parameter ssl_library to report the SSL library version used by the server (Peter Eisentraut)

- Add server parameter shared_memory_type to control the type of shared memory to use (Andres Freund)

  This allows selection of System V shared memory, if desired.

#### E.2.3.2. Streaming Replication and Recovery

- Allow some recovery parameters to be changed with reload (Peter Eisentraut)

  These parameters are archive_cleanup_command, promote_trigger_file, recovery_end_command, and recovery_min_apply_delay.

- Allow the streaming replication timeout (wal_sender_timeout) to be set per connection (Takayuki Tsunakawa)

  Previously, this could only be set cluster-wide.

- Add function `pg_promote()` to promote standbys to primaries (Laurenz Albe, Michaël Paquier)

  Previously, this operation was only possible by using pg_ctl or creating a trigger file.

- Allow replication slots to be copied (Masahiko Sawada)

  The functions for this are `pg_copy_physical_replication_slot()` and `pg_copy_logical_replication_slot()`.

- Make max_wal_senders not count as part of max_connections (Alexander Kukushkin)

- Add an explicit value of `current` for recovery_target_timeline (Peter Eisentraut)

- Make recovery fail if a two-phase transaction status file is corrupt (Michaël Paquier)

  Previously, a warning was logged and recovery continued, allowing the transaction to be lost.

#### E.2.3.3. Utility Commands

- Add REINDEX `CONCURRENTLY` option to allow reindexing without locking out writes (Michaël Paquier, Andreas Karlsson, Peter Eisentraut)

  This is also controlled by the reindexdb application's `--concurrently` option.

- Add support for generated columns (Peter Eisentraut)

  The content of generated columns are computed from expressions (including references to other columns in the same table) rather than being specified by `INSERT` or `UPDATE` commands.

- Add a `WHERE` clause to `COPY FROM` to control which rows are accepted (Surafel Temesgen)

  This provides a simple way to filter incoming data.

- Allow enumerated values to be added more flexibly (Andrew Dunstan, Tom Lane, Thomas Munro)

  Previously, `ALTER TYPE ... ADD VALUE` could not be called in a transaction block, unless it was part of the same transaction that created the enumerated type. Now it can be called in a later transaction, so long as the new enumerated value is not referenced until after it is committed.

- Add commands to end a transaction and start a new one (Peter Eisentraut)

  The commands are `COMMIT AND CHAIN` and `ROLLBACK AND CHAIN`.

- Add VACUUM and `CREATE TABLE` options to prevent `VACUUM` from truncating trailing empty pages (Takayuki Tsunakawa)

  These options are `vacuum_truncate` and `toast.vacuum_truncate`. Use of these options reduces `VACUUM`'s locking requirements, but prevents returning disk space to the operating system.

- Allow `VACUUM` to skip index cleanup (Masahiko Sawada)

  This change adds a `VACUUM` command option `INDEX_CLEANUP` as well as a table storage option `vacuum_index_cleanup`. Use of this option reduces the ability to reclaim space and can lead to index bloat, but it is helpful when the main goal is to freeze old tuples.

- Add the ability to skip `VACUUM` and `ANALYZE` operations on tables that cannot be locked immediately (Nathan Bossart)

  This option is called `SKIP_LOCKED`.

- Allow `VACUUM` and `ANALYZE` to take optional Boolean argument specifications (Masahiko Sawada)

- Prevent TRUNCATE, `VACUUM` and `ANALYZE` from requesting a lock on tables for which the user lacks permission (Michaël Paquier)

  This prevents unauthorized locking, which could interfere with user queries.

- Add EXPLAIN option `SETTINGS` to output non-default optimizer settings (Tomas Vondra)

  This output can also be obtained when using auto_explain by setting `auto_explain.log_settings`.

- Add `OR REPLACE` option to CREATE AGGREGATE (Andrew Gierth)

- Allow modifications of system catalogs' options using ALTER TABLE (Peter Eisentraut)

  Modifications of catalogs' `reloptions` and autovacuum settings are now supported. (Setting allow_system_table_mods is still required.)

- Use all key columns' names when selecting default constraint names for foreign keys (Peter Eisentraut)

  Previously, only the first column name was included in the constraint name, resulting in ambiguity for multi-column foreign keys.

#### E.2.3.4. Data Types

- Update assorted knowledge about Unicode to match Unicode 12.1.0 (Peter Eisentraut)

  This fixes, for example, cases where psql would misformat output involving combining characters.

- Update Snowball stemmer dictionaries with support for new languages (Arthur Zakirov)

  This adds word stemming support for Arabic, Indonesian, Irish, Lithuanian, Nepali, and Tamil to full text search.

- Allow creation of collations that report string equality for strings that are not bit-wise equal (Peter Eisentraut)

  This feature supports “nondeterministic” collations that can define case- and accent-agnostic equality comparisons. Thus, for example, a case-insensitive uniqueness constraint on a text column can be made more easily than before. This is only supported for ICU collations.

- Add support for ICU collation attributes on older ICU versions (Peter Eisentraut)

  This allows customization of the collation rules in a consistent way across all ICU versions.

- Allow data type name to more seamlessly be compared to other text types (Tom Lane)

  Type `name` now behaves much like a domain over type `text` that has default collation “C”. This allows cross-type comparisons to be processed more efficiently.

#### E.2.3.5. Functions

- Add support for the SQL/JSON path language (Nikita Glukhov, Teodor Sigaev, Alexander Korotkov, Oleg Bartunov, Liudmila Mantrova)

  This allows execution of complex queries on `JSON` values using an SQL-standard language.

- Add support for hyperbolic functions (Lætitia Avrot)

  Also add `log10()` as an alias for `log()`, for standards compliance.

- Improve the accuracy of statistical aggregates like `variance()` by using more precise algorithms (Dean Rasheed)

- Allow `date_trunc()` to have an additional argument to control the time zone (Vik Fearing, Tom Lane)

  This is faster and simpler than using the `AT TIME ZONE` clause.

- Adjust `to_timestamp()`/`to_date()` functions to be more forgiving of template mismatches (Artur Zakirov, Alexander Korotkov, Liudmila Mantrova)

  This new behavior more closely matches the Oracle functions of the same name.

- Fix assorted bugs in XML functions (Pavel Stehule, Markus Winand, Chapman Flack)

  Specifically, in `XMLTABLE`, `xpath()`, and `xmlexists()`, fix some cases where nothing was output for a node, or an unexpected error was thrown, or necessary escaping of XML special characters was omitted.

- Allow the `BY VALUE` clause in `XMLEXISTS` and `XMLTABLE` (Chapman Flack)

  This SQL-standard clause has no effect in PostgreSQL's implementation, but it was unnecessarily being rejected.

- Prevent `current_schema()` and `current_schemas()` from being run by parallel workers, as they are not parallel-safe (Michaël Paquier)

- Allow `RECORD` and `RECORD[]` to be used as column types in a query's column definition list for a table function that is declared to return `RECORD` (Elvis Pranskevichus)

#### E.2.3.6. PL/pgSQL

- Allow SQL commands and variables with the same names as those commands to be used in the same PL/pgSQL function (Tom Lane)

  For example, allow a variable called `comment` to exist in a function that calls the `COMMENT` SQL command. Previously this combination caused a parse error.

- Add new optional warning and error checks to PL/pgSQL (Pavel Stehule)

  The new checks allow for run-time validation of `INTO` column counts and single-row results.

#### E.2.3.7. Client Interfaces

- Add connection parameter tcp_user_timeout to control libpq's TCP timeout (Ryohei Nagaura)

- Allow libpq (and thus psql) to report only the `SQLSTATE` value in error messages (Didier Gautheron)

- Add libpq function `PQresultMemorySize()` to report the memory used by a query result (Lars Kanis, Tom Lane)

- Remove the no-display/debug flag from libpq's `options` connection parameter (Peter Eisentraut)

  This allows this parameter to be set by postgres_fdw.

- Allow ecpg to create variables of data type `bytea` (Ryo Matsumura)

  This allows ECPG clients to interact with `bytea` data directly, rather than using an encoded form.

- Add `PREPARE AS` support to ECPG (Ryo Matsumura)

#### E.2.3.8. Client Applications

- Allow vacuumdb to select tables for vacuum based on their wraparound horizon (Nathan Bossart)

  The options are `--min-xid-age` and `--min-mxid-age`.

- Allow vacuumdb to disable waiting for locks or skipping all-visible pages (Nathan Bossart)

  The options are `--skip-locked` and `--disable-page-skipping`.

- Add colorization to the output of command-line utilities (Peter Eisentraut)

  This is enabled by setting the environment variable `PG_COLOR` to `always` or `auto`. The specific colors used can be adjusted by setting the environment variable `PG_COLORS`, using ANSI escape codes for colors. For example, the default behavior is equivalent to `PG_COLORS="error=01;31:warning=01;35:locus=01"`.

##### E.2.3.8.1. psql

- Add CSV table output mode in psql (Daniel Vérité)

  This is controlled by `\pset format csv` or the command-line `--csv` option.

- Show the manual page URL in psql's `\help` output for a SQL command (Peter Eisentraut)

- Display the IP address in psql's `\conninfo` (Fabien Coelho)

- Improve tab completion of `CREATE TABLE`, `CREATE TRIGGER`, `CREATE EVENT TRIGGER`, `ANALYZE`, `EXPLAIN`, `VACUUM`, `ALTER TABLE`, `ALTER INDEX`, `ALTER DATABASE`, and `ALTER INDEX ALTER COLUMN` (Dagfinn Ilmari Mannsåker, Tatsuro Yamada, Michaël Paquier, Tom Lane, Justin Pryzby)

##### E.2.3.8.2. pgbench

- Allow values produced by queries to be assigned to pgbench variables (Fabien Coelho, Álvaro Herrera)

  The command for this is `\gset`.

- Improve precision of pgbench's `--rate` option (Tom Lane)

- Improve pgbench's error reporting with clearer messages and return codes (Peter Eisentraut)

#### E.2.3.9. Server Applications

- Allow control of log file rotation via pg_ctl (Kyotaro Horiguchi, Alexander Kuzmenkov, Alexander Korotkov)

  Previously, this was only possible via an SQL function or a process signal.

- Properly detach the new server process during `pg_ctl start` (Paul Guo)

  This prevents the server from being shut down if the shell script that invoked pg_ctl is interrupted later.

- Allow pg_upgrade to use the file system's cloning feature, if there is one (Peter Eisentraut)

  The `--clone` option has the advantages of `--link`, while preventing the old cluster from being changed after the new cluster has started.

- Allow specification of the socket directory to use in pg_upgrade (Daniel Gustafsson)

  This is controlled by `--socketdir`; the default is the current directory.

- Allow pg_checksums to disable fsync operations (Michaël Paquier)

  This is controlled by the `--no-sync` option.

- Allow pg_rewind to disable fsync operations (Michaël Paquier)

- Fix pg_test_fsync to report accurate `open_datasync` durations on Windows (Laurenz Albe)

##### E.2.3.9.1. pg_dump, pg_dumpall, pg_restore

- When pg_dump emits data with `INSERT` commands rather than `COPY`, allow more than one data row to be included in each `INSERT` (Surafel Temesgen, David Rowley)

  The option controlling this is `--rows-per-insert`.

- Allow pg_dump to emit `INSERT ... ON CONFLICT DO NOTHING` (Surafel Temesgen)

  This avoids conflict failures during restore. The option is `--on-conflict-do-nothing`.

- Decouple the order of operations in a parallel pg_dump from the order used by a subsequent parallel pg_restore (Tom Lane)

  This allows pg_restore to perform more-fully-parallelized parallel restores, especially in cases where the original dump was not done in parallel. Scheduling of a parallel pg_dump is also somewhat improved.

- Allow the extra_float_digits setting to be specified for pg_dump and pg_dumpall (Andrew Dunstan)

  This is primarily useful for making dumps that are exactly comparable across different source server versions. It is not recommended for normal use, as it may result in loss of precision when the dump is restored.

- Add `--exclude-database` option to pg_dumpall (Andrew Dunstan)

#### E.2.3.10. Source Code

- Add CREATE ACCESS METHOD command to create new table types (Andres Freund, Haribabu Kommi, Álvaro Herrera, Alexander Korotkov, Dmitry Dolgov)

  This enables the development of new table access methods, which can optimize storage for different use cases. The existing `heap` access method remains the default.

- Add planner support function interfaces to improve optimizer estimates, inlining, and indexing for functions (Tom Lane)

  This allows extensions to create planner support functions that can provide function-specific selectivity, cost, and row-count estimates that can depend on the function's arguments. Support functions can also supply simplified representations and index conditions, greatly expanding optimization possibilities.

- Simplify renumbering manually-assigned OIDs, and establish a new project policy for management of such OIDs (John Naylor, Tom Lane)

  Patches that manually assign OIDs for new built-in objects (such as new functions) should now randomly choose OIDs in the range 8000—9999. At the end of a development cycle, the OIDs used by committed patches will be renumbered down to lower numbers, currently somewhere in the 4`xxx` range, using the new `renumber_oids.pl` script. This approach should greatly reduce the odds of OID collisions between different in-process patches.

  While there is no specific policy reserving any OIDs for external use, it is recommended that forks and other projects needing private manually-assigned OIDs use numbers in the high 7`xxx` range. This will avoid conflicts with recently-merged patches, and it should be a long time before the core project reaches that range.

- Build Cygwin binaries using dynamic instead of static libraries (Marco Atzeri)

- Remove configure switch `--disable-strong-random` (Michaël Paquier)

  A strong random-number source is now required.

- `printf`-family functions, as well as `strerror` and `strerror_r`, now behave uniformly across platforms within Postgres code (Tom Lane)

  Notably, `printf` understands `%m` everywhere; on Windows, `strerror` copes with Winsock error codes (it used to do so in backend but not frontend code); and `strerror_r` always follows the GNU return convention.

- Require a C99-compliant compiler, and MSVC 2013 or later on Windows (Andres Freund)

- Use pandoc, not lynx, for generating plain-text documentation output files (Peter Eisentraut)

  This affects only the `INSTALL` file generated during `make dist` and the seldom-used plain-text `postgres.txt` output file. Pandoc produces better output than lynx and avoids some locale/encoding issues. Pandoc version 1.13 or later is required.

- Support use of images in the PostgreSQL documentation (Jürgen Purtz)

#### E.2.3.11. Additional Modules

- Allow `ORDER BY` sorts and `LIMIT` clauses to be pushed to postgres_fdw foreign servers in more cases (Etsuro Fujita)

- Improve optimizer cost accounting for postgres_fdw queries (Etsuro Fujita)

- Properly honor `WITH CHECK OPTION` on views that reference postgres_fdw tables (Etsuro Fujita)

  While `CHECK OPTION`s on postgres_fdw tables are ignored (because the reference is foreign), views on such tables are considered local, so this change enforces `CHECK OPTION`s on them. Previously, only `INSERT`s and `UPDATE`s with `RETURNING` clauses that returned `CHECK OPTION` values were validated.

- Allow `pg_stat_statements_reset()` to be more granular (Haribabu Kommi, Amit Kapila)

  The function now allows reset of statistics for specific databases, users, and queries.

- Allow control of the auto_explain log level (Tom Dunstan, Andrew Dunstan)

  The default is `LOG`.

- Update unaccent rules with new punctuation and symbols (Hugh Ranalli, Michaël Paquier)

- Allow unaccent to handle some accents encoded as combining characters (Hugh Ranalli)

- Allow unaccent to remove accents from Greek characters (Tasos Maschalidis)

- Add a parameter to amcheck's `bt_index_parent_check()` function to check each index tuple from the root of the tree (Peter Geoghegan)

- Improve oid2name and vacuumlo option handling to match other commands (Tatsuro Yamada)

### E.2.4. Acknowledgments

The following individuals (in alphabetical order) have contributed to this release as patch authors, committers, reviewers, testers, or reporters of issues.

Abhijit Menon-Sen
Achilleas Mantzios
Adam Berlin
Adam Bielanski
Aditya Toshniwal
Adrien Nayrat
Alan Jackson
Albert Schabhuetl
Aleksander Alekseev
Alex Aktsipetrov
Alex Kliukin
Alex Macy
Alexander Korotkov
Alexander Kukushkin
Alexander Kuzmenkov
Alexander Lakhin
Alexandra Ryzhevich
Alexey Bashtanov
Alexey Ermakov
Alexey Kondratov
Alexey Kryuchkov
Alexey Stepanov
Allison Kaptur
Álvaro Herrera
Alyssa Ross
Amit Kapila
Amit Khandekar
Amit Langote
Amul Sul
Anastasia Lubennikova
André Hänsel
Andrea Gelmini
Andreas Joseph Krogh
Andreas Karlsson
Andreas Kunert
Andreas Scherbaum
Andreas Seltenreich
Andrei Yahorau
Andres Freund
Andrew Dunstan
Andrew Fletcher
Andrew Gierth
Andrew Krasichkov
Andrey Borodin
Andrey Klychkov
Andrey Lepikhov
Andy Abelisto
Anthony Greene
Anthony Skorski
Antonin Houska
Arne Roland
Arseny Sher
Arthur Zakirov
Ash Marath
Ashutosh Bapat
Ashutosh Sharma
Ashwin Agrawal
Aya Iwata
Bartosz Polnik
Basil Bourque
Bernd Helmle
Brad DeJong
Brigitte Blanc-Lafay
Bruce Klein
Bruce Momjian
Bruno Wolff
Chapman Flack
Chen Huajun
Chris Travers
Chris Wilson
Christian Hofstaedtler
Christoph Berg
Christoph Moench-Tegeder
Clemens Ladisch
Colm McHugh
Corey Huinker
Craig Ringer
Dagfinn Ilmari Mannsåker
Daisuke Higuchi
Daniel Fiori
Daniel Gustafsson
Daniel Vérité
Daniel Westermann
Daniel Wilches
Darafei Praliaskouski
Daryl Waycott
Dave Cramer
David Binderman
David Fetter
David G. Johnston
David Rowley
David Steele
Davy Machado
Dean Rasheed
Derek Hans
Derek Nelson
Devrim Gündüz
Dian Fay
Didier Gautheron
Dilip Kumar
Dmitry Dolgov
Dmitry Marakasov
Dmitry Molotkov
Dmitry Shalashov
Don Seiler
Donald Dong
Doug Rady
Edmund Horner
Eduards Bezverhijs
Elvis Pranskevichus
Emanuel Araújo
Emre Hasegeli
Eric Cyr
Erik Rijkers
Ertugrul Kahveci
Etsuro Fujita
Eugen Konkov
Euler Taveira
Fabien Coelho
Fabrízio de Royes Mello
Feike Steenbergen
Filip Rembialkowski
Gaby Schilders
Geert Lobbestael
George Tarasov
Georgios Kokolatos
Gianni Ciolli
Gilles Darold
Greg Stark
Grigory Smolkin
Guillaume Lelarge
Gunnlaugur Thor Briem
Gurjeet Singh
Hadi Moshayedi
Hailong Li
Hans Buschmann
Haribabu Kommi
Haruka Takatsuka
Hayato Kuroda
Heikki Linnakangas
Hironobu Suzuki
Hubert Lubaczewski
Hugh Ranalli
Ian Barwick
Ibrar Ahmed
Ildar Musin
Insung Moon
Ioseph Kim
Isaac Morland
Ivan Panchenko
Jack Kelly
Jacob Champion
Jaime Casanova
Jakob Egger
Jakub Glapa
Jakub Janecek
James Coleman
James Inform
James Robinson
James Sewell
James Tomson
Jan Chochol
Jaroslav Sivy
Jean-Christophe Arnu
Jean-Marc Voillequin
Jean-Pierre Pelletier
Jeevan Chalke
Jeevan Ladhe
Jeff Davis
Jeff Janes
Jeremy Evans
Jeremy Schneider
Jeremy Smith
Jerry Jelinek
Jesper Pedersen
Jianing Yang
Jie Zhang
Jim Nasby
Jimmy Yih
Joe Conway
Joe Wildish
Joerg Sonnenberger
John Klann
John Naylor
Jonah Harris
Jonathan S. Katz
Jorge Gustavo Rocha
José Arthur Benetasso Villanova
Joshua D. Drake
Juan José Santamaría Flecha
Julian Hsiao
Julian Markwort
Julian Schauder
Julien Rouhaud
Jürgen Purtz
Jürgen Strobel
Justin Pryzby
Kaiting Chen
Karen Huddleston
Karl Czajkowski
Karl O. Pinc
Keiichi Hirobe
Keith Fiske
Ken Tanzer
Kenji Uno
Kevin Grittner
Kevin Hale Boyes
Kieran McCusker
Kirk Jamison
Kohei KaiGai
Konstantin Knizhnik
Konstantin Kuznetsov
Kristjan Tammekivi
Kuntal Ghosh
Kyle Samson
Kyotaro Horiguchi
Lætitia Avrot
Lars Kanis
Laurenz Albe
Lim Myungkyu
Liu Huailing
Liudmila Mantrova
Lloyd Albin
Luca Ferrari
Luis M. Carril
Lukas Eder
Lukas Fittl
Madelaine Thibaut
Madeleine Thompson
Magnus Hagander
Mahendra Singh
Mai Peng
Maksim Milyutin
Maksym Boguk
Malthe Borch
Manuel Rigger
Marco Atzeri
Marco Slot
Marina Polyakova
Mario De Frutos Dieguez
Marius Timmer
Mark Chambers
Mark Dilger
Marko Tiikkaja
Markus Winand
Martín Marqués
Masahiko Sawada
Masao Fujii
Mateusz Guzik
Mathias Brossard
Matt Williams
Matthias Otterbach
Matvey Arye
Melanie Plageman
Mi Tar
Michael Banck
Michael Davidson
Michael Meskes
Michael Paquier
Michael Vitale
Michel Pelletier
Mikalai Keida
Mike Palmiotto
Mithun Cy
Morgan Owens
Murat Kabilov
Nathan Bossart
Nawaz Ahmed
Neeraj Kumar
Nick Barnes
Nico Williams
Nikita Glukhov
Nikolay Shaplov
Ning Yu
Nishant Fnu
Noah Misch
Norbert Benkocs
Noriyoshi Shinoda
Oleg Bartunov
Oleg Samoilov
Oleksii Kliukin
Ondrej Bouda
Oskari Saarenmaa
Pan Bian
Patrick Francelle
Patrick McHardy
Paul A. Jungwirth
Paul Bonaud
Paul Guo
Paul Martinez
Paul Ramsey
Paul Schaap
Paul van der Linden
Pavan Deolasee
Pavel Oskin
Pavel Raiskup
Pavel Stehule
Peifeng Qiu
Peter Billen
Peter Eisentraut
Peter Geoghegan
Peter Neave
Petr Fedorov
Petr Jelínek
Petr Slavov
Petru-Florin Mihancea
Phil Bayer
Phil Florent
Philip Dubé
Pierre Ducroquet
Piotr Gabriel Kosinski
Piotr Stefaniak
Piotr Wlodarczyk
Prabhat Sahu
Quentin Rameau
Rafael Castro
Rafia Sabih
Rahila Syed
Rajkumar Raghuwanshi
Rares Salcudean
Raúl Marín Rodríguez
Regina Obe
Renaud Navarro
Richard Guo
Rick Otten
Rikard Falkeborn
RK Korlapati
Robbie Harwood
Robert Haas
Robert Treat
Robert Vollmert
Roger Curley
Roman Zharkov
Ronan Dunklau
Rui Hai Jiang
Rushabh Lathia
Ryan Lambert
Ryo Matsumura
Ryohei Nagaura
Ryohei Takahashi
Samuel Williams
Sand Stone
Sanyo Capobiango
Satoru Koizumi
Sean Johnston
Serge Latyntsev
Sergei Kornilov
Sergey Pashkov
Sergio Conde Gómez
Shawn Debnath
Shay Rojansky
Sho Kato
Shohei Mochizuki
Shouyu Luo
Simon Riggs
Sivasubramanian Ramasubramanian
Slawomir Chodnicki
Stas Kelvish
Stefan Kadow
Stepan Yankevych
Stephen Amell
Stephen Frost
Steve Rogerson
Steve Singer
Steven Winfield
Surafel Temesgen
Suraj Kharage
Suresh Kumar R
Takayuki Tsunakawa
Takeshi Ideriha
Takuma Hoshiai
Tasos Maschalidis
Tatsuo Ishii
Tatsuro Yamada
Teodor Sigaev
Thom Brown
Thomas Munro
Thomas Poty
Tillmann Schulz
Tim Möhlmann
Timur Birsh
Tobias Bussmann
Tom Cassidy
Tom Dunstan
Tom Gottfried
Tom Lane
Tomas Vondra
Tushar Ahuja
Ulf Adams
Vaishnavi Prabakaran
Victor Petrovykh
Victor Wagner
Victor Yegorov
Vijaykumar Jain
Vik Fearing
Vlad Sterzhanov
Vladimir Baranoff
Vladimir Kriukov
Wu Fei
Yaroslav Schekin
Yi Huang
Yoshikazu Imai
Yugo Nagata
Yulian Khodorkovskiy
Yuming Wang
YunQiang Su
Yuri Kurenkov
Yusuke Egashira
Yuzuko Hosoya
Zhou Digoal