8000 Revert: Implement pg_wal_replay_wait() stored procedure · postgrespro/postgres@772faaf · GitHub
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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>
    2877228772

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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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    28779-
    <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>
    28783-
    <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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    28793-
    <tbody>
    28794-
    <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>.
    28808-
    If the <parameter>timeout</parameter> is specified (in
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    milliseconds) and greater than zero, the procedure waits until the
    28810-
    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.
    28812-
    </para></entry>
    28813-
    </row>
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    </tbody>
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    </tgroup>
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    </table>
    28817-
    28818-
    <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
    28823-
    to the <parameter>target_lsn</parameter> value. This is useful to achieve
    28824-
    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
    28826-
    modification should be stored on the client application side or the
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    connection pooler side.
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    </para>
    28829-
    28830-
    <para>
    28831-
    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
    28833-
    the <literal>movie</literal> table and get the <acronym>lsn</acronym> after
    28834-
    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
    28837-
    <literal>off</literal>.
    28838-
    28839-
    <programlisting>
    28840-
    postgres=# UPDATE movie SET genre = 'Dramatic' WHERE genre = 'Drama';
    28841-
    UPDATE 100
    28842-
    postgres=# SELECT pg_current_wal_insert_lsn();
    28843-
    pg_current_wal_insert_lsn
    28844-
    --------------------
    28845-
    0/306EE20
    28846-
    (1 row)
    28847-
    </programlisting>
    28848-
    28849-
    Then an application could run <function>pg_wal_replay_wait</function>
    28850-
    with the <acronym>lsn</acronym> obtained from primary. After that the
    28851-
    changes made of primary should be guaranteed to be visible on replica.
    28852-
    28853-
    <programlisting>
    28854-
    postgres=# CALL pg_wal_replay_wait('0/306EE20');
    28855-
    CALL
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    postgres=# SELECT * FROM movie WHERE genre = 'Drama';
    28857-
    genre
    28858-
    -------
    28859-
    (0 rows)
    28860-
    </programlisting>
    28861-
    28862-
    It may also happen that target <acronym>lsn</acronym> is not achieved
    28863-
    within the timeout. In that case the error is thrown.
    28864-
    28865-
    <programlisting>
    28866-
    postgres=# CALL pg_wal_replay_wait('0/306EE20', 100);
    28867-
    ERROR: timed out while waiting for target LSN 0/306EE20 to be replayed; current replay LSN 0/306EA60
    28868-
    </programlisting>
    28869-
    28870-
    </para>
    28871-
    28872-
    <para>
    28873-
    <function>pg_wal_replay_wait</function> can't be used within
    28874-
    the transaction, another procedure or function. Any of them holds a
    28875-
    snapshot, which could prevent the replay of WAL records.
    28876-
    28877-
    <programlisting>
    28878-
    postgres=# BEGIN;
    28879-
    BEGIN
    28880-
    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>
    28884-
    28885-
    </para>
    2888628773
    </sect2>
    2888728774

    2888828775
    <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"
    4342
    #include "executor/spi.h"
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    #include "libpq/be-fsstubs.h"
    @@ -2772,11 +2771,6 @@ AbortTransaction(void)
    27722771
    */
    27732772
    LWLockReleaseAll();
    27742773

    2775-
    /*
    2776-
    * Cleanup waiting for LSN if any.
    2777-
    */
    2778-
    WaitLSNCleanup();
    2779-
    27802774
    /* Clear wait information and command progress indicator */
    27812775
    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"
    7271
    #include "executor/instrument.h"
    @@ -6130,12 +6129,6 @@ StartupXLOG(void)
    61306129
    UpdateControlFile();
    61316130
    LWLockRelease(ControlFileLock);
    61326131

    6133-
    /*
    6134-
    * Wake up all waiters for replay LSN. They need to report an error that
    6135-
    * recovery was ended before achieving the target LSN.
    6136-
    */
    6137-
    WaitLSNSetLatches(InvalidXLogRecPtr);
    6138-
    61396132
    /*
    61406133
    * Shutdown the recovery environment. This must occur after
    61416134
    * RecoverPreparedTransactions() (see notes in lock_twophase_recover())

    ‎src/backend/access/transam/xlogrecovery.c

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    @@ -43,7 +43,6 @@
    4343
    #include "backup/basebackup.h"
    4444
    #include "catalog/pg_control.h"
    4545
    #include "commands/tablespace.h"
    46-
    #include "commands/waitlsn.h"
    4746
    #include "common/file_utils.h"
    4847
    #include "miscadmin.h"
    4948
    #include "pgstat.h"
    @@ -1829,16 +1828,6 @@ PerformWalRecovery(void)
    18291828
    break;
    18301829
    }
    18311830

    1832-
    /*
    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 */
    184318 10000 32
    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)
    415415
    RETURNS SETOF anyelement LANGUAGE internal STABLE ROWS 100 AS 'json_populate_recordset' PARALLEL SAFE;
    416416

    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-
    420417
    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 '{}',
    422419
    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 \
    64-
    view.o \
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    waitlsn.o
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    view.o
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    6766
    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',
    53-
    'waitlsn.c',
    5453
    )

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