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Commit3c5db1d

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Implement pg_wal_replay_wait() stored procedure
pg_wal_replay_wait() is to be used on standby and specifies waiting forthe specific WAL location to be replayed. This option is useful whenthe user makes some data changes on primary and needs a guarantee to seethese changes are on standby.The queue of waiters is stored in the shared memory as an LSN-ordered pairingheap, where the waiter with the nearest LSN stays on the top. Duringthe replay of WAL, waiters whose LSNs have already been replayed are deletedfrom the shared memory pairing heap and woken up by setting their latches.pg_wal_replay_wait() needs to wait without any snapshot held. Otherwise,the snapshot could prevent the replay of WAL records, implying a kind ofself-deadlock. This is why it is only possible to implementpg_wal_replay_wait() as a procedure working without an active snapshot,not a function.Catversion is bumped.Discussion:https://postgr.es/m/eb12f9b03851bb2583adab5df9579b4b%40postgrespro.ruAuthor: Kartyshov Ivan, Alexander KorotkovReviewed-by: Michael Paquier, Peter Eisentraut, Dilip Kumar, Amit KapilaReviewed-by: Alexander Lakhin, Bharath Rupireddy, Euler TaveiraReviewed-by: Heikki Linnakangas, Kyotaro Horiguchi
1 parenta83f308 commit3c5db1d

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21 files changed

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-8
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21 files changed

+786
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‎doc/src/sgml/func.sgml

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@@ -28911,6 +28911,123 @@ 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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The procedure shown in <xref linkend="recovery-synchronization-procedure-table"/>
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can be executed only during recovery.
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</para>
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<table id="recovery-synchronization-procedure-table">
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<title>Recovery Synchronization Procedure</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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Waits until recovery replays <literal>target_lsn</literal>.
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If no <parameter>timeout</parameter> is specified or it is set to
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zero, this procedure waits indefinitely for the
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<literal>target_lsn</literal>. If the <parameter>timeout</parameter>
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is specified (in milliseconds) and is greater than zero, the
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procedure waits until <literal>target_lsn</literal> is reached or
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the specified <parameter>timeout</parameter> has elapsed.
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On timeout, or if the server is promoted before
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<literal>target_lsn</literal> is reached, 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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a transaction with an isolation level higher than
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<literal>READ COMMITTED</literal>, another procedure, or a function.
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All the cases above imply holding a snapshot, which could prevent
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WAL records from replaying (see <xref linkend="hot-standby-conflict"/>)
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and cause an indirect deadlock.
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<programlisting>
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postgres=# BEGIN;
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BEGIN
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postgres=*# CALL pg_wal_replay_wait('0/306EE20');
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ERROR: pg_wal_replay_wait() must be only called without an active or registered snapshot
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DETAIL: Make sure pg_wal_replay_wait() isn't called within a transaction with an isolation level higher than READ COMMITTED, another procedure, or a function.
29028+
</programlisting>
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29030+
</para>
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</sect2>
2891529032

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<sect2 id="functions-snapshot-synchronization">

‎src/backend/access/transam/xact.c

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@@ -38,6 +38,7 @@
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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"
@@ -2809,6 +2810,11 @@ AbortTransaction(void)
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*/
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LWLockReleaseAll();
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/*
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* Cleanup waiting for LSN if any.
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*/
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WaitLSNCleanup();
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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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@@ -66,6 +66,7 @@
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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"
@@ -6143,6 +6144,12 @@ StartupXLOG(void)
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UpdateControlFile();
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LWLockRelease(ControlFileLock);
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/*
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* Wake up all waiters for replay LSN. They need to report an error that
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* recovery was ended before achieving the target LSN.
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*/
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WaitLSNSetLatches(InvalidXLogRecPtr);
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61466153
/*
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* 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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@@ -43,6 +43,7 @@
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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"
@@ -1828,6 +1829,16 @@ PerformWalRecovery(void)
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break;
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}
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/*
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* If we replayed an LSN that someone was waiting for then walk
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* over the shared memory array and set latches to notify the
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* waiters.
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*/
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if (waitLSNState&&
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(XLogRecoveryCtl->lastReplayedEndRecPtr >=
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pg_atomic_read_u64(&waitLSNState->minWaitedLSN)))
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WaitLSNSetLatches(XLogRecoveryCtl->lastReplayedEndRecPtr);
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/* Else, try to fetch the next WAL record */
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record=ReadRecord(xlogprefetcher,LOG, false,replayTLI);
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}while (record!=NULL);

‎src/backend/catalog/system_functions.sql

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@@ -414,6 +414,9 @@ CREATE OR REPLACE FUNCTION
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json_populate_recordset(base anyelement, from_json json, use_json_as_textboolean DEFAULT false)
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RETURNS SETOF anyelement LANGUAGE internal STABLE ROWS100AS'json_populate_recordset' PARALLEL SAFE;
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417+
CREATEOR REPLACE PROCEDURE pg_wal_replay_wait(target_lsn pg_lsn, timeout int8 DEFAULT0)
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LANGUAGE internalAS'pg_wal_replay_wait';
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CREATE OR REPLACEFUNCTIONpg_logical_slot_get_changes(
418421
IN slot_name name,IN upto_lsn pg_lsn,IN upto_nchangesint, VARIADIC optionstext[] DEFAULT'{}',
419422
OUT lsn pg_lsn, OUT xid xid, OUT datatext)

‎src/backend/commands/Makefile

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

‎src/backend/commands/meson.build

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@@ -50,4 +50,5 @@ 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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