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Revert: Implement pg_wal_replay_wait() stored procedure
This commit reverts 06c418e, e37662f, bf1e650, 25f4242, ee79928, and 74eaf66 per review by Heikki Linnakangas. Discussion: https://postgr.es/m/b155606b-e744-4218-bda5-29379779da1a%40iki.fi
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‎doc/src/sgml/func.sgml

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@@ -28770,119 +28770,6 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
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the pause, the rate of WAL generation and available disk space.
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</para>
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<para>
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There are also procedures to control the progress of recovery.
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They are shown in <xref linkend="procedures-recovery-control-table"/>.
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These procedures may be executed only during recovery.
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</para>
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<table id="procedures-recovery-control-table">
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<title>Recovery Control Procedures</title>
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<tgroup cols="1">
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<thead>
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<row>
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<entry role="func_table_entry"><para role="func_signature">
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Procedure
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</para>
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<para>
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Description
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</para></entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry role="func_table_entry"><para role="func_signature">
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<indexterm>
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<primary>pg_wal_replay_wait</primary>
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</indexterm>
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<function>pg_wal_replay_wait</function> (
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<parameter>target_lsn</parameter> <type>pg_lsn</type>,
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<parameter>timeout</parameter> <type>bigint</type> <literal>DEFAULT</literal> <literal>0</literal>)
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<returnvalue>void</returnvalue>
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</para>
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<para>
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If <parameter>timeout</parameter> is not specified or zero, this
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procedure returns once WAL is replayed upto
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<literal>target_lsn</literal>.
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If the <parameter>timeout</parameter> is specified (in
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milliseconds) and greater than zero, the procedure waits until the
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server actually replays the WAL upto <literal>target_lsn</literal> or
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until the given time has passed. On timeout, an error is emitted.
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</para></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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<function>pg_wal_replay_wait</function> waits till
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<parameter>target_lsn</parameter> to be replayed on standby.
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That is, after this function execution, the value returned by
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<function>pg_last_wal_replay_lsn</function> should be greater or equal
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to the <parameter>target_lsn</parameter> value. This is useful to achieve
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read-your-writes-consistency, while using async replica for reads and
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primary for writes. In that case <acronym>lsn</acronym> of the last
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modification should be stored on the client application side or the
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connection pooler side.
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</para>
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<para>
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You can use <function>pg_wal_replay_wait</function> to wait for
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the <type>pg_lsn</type> value. For example, an application could update
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the <literal>movie</literal> table and get the <acronym>lsn</acronym> after
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changes just made. This example uses <function>pg_current_wal_insert_lsn</function>
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on primary server to get the <acronym>lsn</acronym> given that
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<varname>synchronous_commit</varname> could be set to
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<literal>off</literal>.
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<programlisting>
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postgres=# UPDATE movie SET genre = 'Dramatic' WHERE genre = 'Drama';
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UPDATE 100
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postgres=# SELECT pg_current_wal_insert_lsn();
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pg_current_wal_insert_lsn
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--------------------
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0/306EE20
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(1 row)
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</programlisting>
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Then an application could run <function>pg_wal_replay_wait</function>
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with the <acronym>lsn</acronym> obtained from primary. After that the
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changes made of primary should be guaranteed to be visible on replica.
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<programlisting>
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postgres=# CALL pg_wal_replay_wait('0/306EE20');
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CALL
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postgres=# SELECT * FROM movie WHERE genre = 'Drama';
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genre
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-------
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(0 rows)
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</programlisting>
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It may also happen that target <acronym>lsn</acronym> is not achieved
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within the timeout. In that case the error is thrown.
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<programlisting>
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postgres=# CALL pg_wal_replay_wait('0/306EE20', 100);
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ERROR: timed out while waiting for target LSN 0/306EE20 to be replayed; current replay LSN 0/306EA60
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</programlisting>
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</para>
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<para>
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<function>pg_wal_replay_wait</function> can't be used within
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the transaction, another procedure or function. Any of them holds a
28875-
snapshot, which could prevent the replay of WAL records.
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28877-
<programlisting>
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postgres=# BEGIN;
28879-
BEGIN
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postgres=*# CALL pg_wal_replay_wait('0/306EE20');
28881-
ERROR: pg_wal_replay_wait() must be only called in non-atomic context
28882-
DETAIL: Make sure pg_wal_replay_wait() isn't called within a transaction, another procedure, or a function.
28883-
</programlisting>
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28885-
</para>
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</sect2>
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<sect2 id="functions-snapshot-synchronization">

‎src/backend/access/transam/xact.c

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@@ -38,7 +38,6 @@
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#include "commands/async.h"
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#include "commands/tablecmds.h"
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#include "commands/trigger.h"
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#include "commands/waitlsn.h"
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#include "common/pg_prng.h"
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#include "executor/spi.h"
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#include "libpq/be-fsstubs.h"
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*/
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LWLockReleaseAll();
27742773

2775-
/*
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* Cleanup waiting for LSN if any.
2777-
*/
2778-
WaitLSNCleanup();
2779-
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/* Clear wait information and command progress indicator */
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pgstat_report_wait_end();
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pgstat_progress_end_command();

‎src/backend/access/transam/xlog.c

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@@ -66,7 +66,6 @@
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#include "catalog/catversion.h"
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#include "catalog/pg_control.h"
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#include "catalog/pg_database.h"
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#include "commands/waitlsn.h"
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#include "common/controldata_utils.h"
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#include "common/file_utils.h"
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#include "executor/instrument.h"
@@ -6130,12 +6129,6 @@ StartupXLOG(void)
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UpdateControlFile();
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LWLockRelease(ControlFileLock);
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6133-
/*
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* Wake up all waiters for replay LSN. They need to report an error that
6135-
* recovery was ended before achieving the target LSN.
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*/
6137-
WaitLSNSetLatches(InvalidXLogRecPtr);
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61396132
/*
61406133
* Shutdown the recovery environment. This must occur after
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* RecoverPreparedTransactions() (see notes in lock_twophase_recover())

‎src/backend/access/transam/xlogrecovery.c

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@@ -43,7 +43,6 @@
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#include "backup/basebackup.h"
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#include "catalog/pg_control.h"
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#include "commands/tablespace.h"
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#include "commands/waitlsn.h"
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#include "common/file_utils.h"
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#include "miscadmin.h"
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#include "pgstat.h"
@@ -1829,16 +1828,6 @@ PerformWalRecovery(void)
18291828
break;
18301829
}
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/*
1833-
* If we replayed an LSN that someone was waiting for then walk
1834-
* over the shared memory array and set latches to notify the
1835-
* waiters.
1836-
*/
1837-
if (waitLSN &&
1838-
(XLogRecoveryCtl->lastReplayedEndRecPtr >=
1839-
pg_atomic_read_u64(&waitLSN->minWaitedLSN)))
1840-
WaitLSNSetLatches(XLogRecoveryCtl->lastReplayedEndRecPtr);
1841-
18421831
/* Else, try to fetch the next WAL record */
18431832
record = ReadRecord(xlogprefetcher, LOG, false, replayTLI);
18441833
} while (record != NULL);

‎src/backend/catalog/system_functions.sql

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@@ -414,9 +414,6 @@ CREATE OR REPLACE FUNCTION
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json_populate_recordset(base anyelement, from_json json, use_json_as_text boolean DEFAULT false)
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RETURNS SETOF anyelement LANGUAGE internal STABLE ROWS 100 AS 'json_populate_recordset' PARALLEL SAFE;
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417-
CREATE OR REPLACE PROCEDURE pg_wal_replay_wait(target_lsn pg_lsn, timeout int8 DEFAULT 0)
418-
LANGUAGE internal AS 'pg_wal_replay_wait';
419-
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CREATE OR REPLACE FUNCTION pg_logical_slot_get_changes(
421418
IN slot_name name, IN upto_lsn pg_lsn, IN upto_nchanges int, VARIADIC options text[] DEFAULT '{}',
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OUT lsn pg_lsn, OUT xid xid, OUT data text)

‎src/backend/commands/Makefile

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@@ -61,7 +61,6 @@ OBJS = \
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vacuum.o \
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vacuumparallel.o \
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variable.o \
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view.o \
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waitlsn.o
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view.o
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include $(top_srcdir)/src/backend/common.mk

‎src/backend/commands/meson.build

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@@ -50,5 +50,4 @@ backend_sources += files(
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'vacuumparallel.c',
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'variable.c',
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'view.c',
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'waitlsn.c',
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)

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