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Add a GUC variable "synchronize_seqscans" to allow clients to disable the new
synchronized-scanning behavior, and make pg_dump disable sync scans so that it will reliably preserve row ordering. Per recent discussions.
This commit is contained in:
Tom Lane
parent
6dfa40d69f
commit
47df4f6688
@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.162 2008/01/27 19:12:28 tgl Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.163 2008/01/30 18:35:55 tgl Exp $ -->
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<chapter Id="runtime-config">
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<title>Server Configuration</title>
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@ -4611,6 +4611,28 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
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</listitem>
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</varlistentry>
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<varlistentry id="guc-synchronize-seqscans" xreflabel="synchronize_seqscans">
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<term><varname>synchronize_seqscans</varname> (<type>boolean</type>)</term>
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<indexterm>
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<primary><varname>synchronize_seqscans</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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This allows sequential scans of large tables to synchronize with each
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other, so that concurrent scans read the same block at about the
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same time and hence share the I/O workload. When this is enabled,
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a scan might start in the middle of the table and then <quote>wrap
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around</> the end to cover all rows, so as to synchronize with the
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activity of scans already in progress. This can result in
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unpredictable changes in the row ordering returned by queries that
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have no <literal>ORDER BY</> clause. Setting this parameter to
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<literal>off</> ensures the pre-8.3 behavior in which a sequential
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scan always starts from the beginning of the table. The default
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is <literal>on</>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.248 2008/01/14 01:39:09 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.249 2008/01/30 18:35:55 tgl Exp $
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*
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*
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* INTERFACE ROUTINES
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@ -59,6 +59,10 @@
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#include "utils/syscache.h"
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/* GUC variable */
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bool synchronize_seqscans = true;
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static HeapScanDesc heap_beginscan_internal(Relation relation,
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Snapshot snapshot,
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int nkeys, ScanKey key,
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@ -104,7 +108,8 @@ initscan(HeapScanDesc scan, ScanKey key)
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* the thresholds for these features could be different, we make them the
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* same so that there are only two behaviors to tune rather than four.
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* (However, some callers need to be able to disable one or both of
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* these behaviors, independently of the size of the table.)
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* these behaviors, independently of the size of the table; also there
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* is a GUC variable that can disable synchronized scanning.)
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*
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* During a rescan, don't make a new strategy object if we don't have to.
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*/
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@ -129,7 +134,7 @@ initscan(HeapScanDesc scan, ScanKey key)
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scan->rs_strategy = NULL;
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}
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if (allow_sync)
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if (allow_sync && synchronize_seqscans)
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{
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scan->rs_syncscan = true;
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scan->rs_startblock = ss_get_location(scan->rs_rd, scan->rs_nblocks);
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@ -10,7 +10,7 @@
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* Written by Peter Eisentraut <peter_e@gmx.net>.
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.431 2008/01/27 19:12:28 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.432 2008/01/30 18:35:55 tgl Exp $
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*
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*--------------------------------------------------------------------
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*/
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@ -110,6 +110,7 @@ extern int CommitDelay;
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extern int CommitSiblings;
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extern char *default_tablespace;
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extern char *temp_tablespaces;
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extern bool synchronize_seqscans;
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extern bool fullPageWrites;
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#ifdef TRACE_SORT
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@ -1052,6 +1053,15 @@ static struct config_bool ConfigureNamesBool[] =
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false, NULL, NULL
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},
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{
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{"synchronize_seqscans", PGC_USERSET, COMPAT_OPTIONS_PREVIOUS,
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gettext_noop("Enable synchronized sequential scans."),
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NULL
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},
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&synchronize_seqscans,
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true, NULL, NULL
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},
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{
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{"archive_mode", PGC_POSTMASTER, WAL_SETTINGS,
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gettext_noop("Allows archiving of WAL files using archive_command."),
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@ -476,9 +476,10 @@
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#backslash_quote = safe_encoding # on, off, or safe_encoding
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#default_with_oids = off
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#escape_string_warning = on
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#standard_conforming_strings = off
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#regex_flavor = advanced # advanced, extended, or basic
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#sql_inheritance = on
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#standard_conforming_strings = off
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#synchronize_seqscans = on
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# - Other Platforms and Clients -
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@ -12,7 +12,7 @@
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* by PostgreSQL
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.481 2008/01/01 19:45:55 momjian Exp $
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* $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.482 2008/01/30 18:35:55 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -552,6 +552,20 @@ main(int argc, char **argv)
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/* Set the datestyle to ISO to ensure the dump's portability */
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do_sql_command(g_conn, "SET DATESTYLE = ISO");
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/*
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* If supported, set extra_float_digits so that we can dump float data
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* exactly (given correctly implemented float I/O code, anyway)
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*/
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if (g_fout->remoteVersion >= 70400)
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do_sql_command(g_conn, "SET extra_float_digits TO 2");
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/*
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* If synchronized scanning is supported, disable it, to prevent
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* unpredictable changes in row ordering across a dump and reload.
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*/
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if (g_fout->remoteVersion >= 80300)
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do_sql_command(g_conn, "SET synchronize_seqscans TO off");
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/*
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* Start serializable transaction to dump consistent data.
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*/
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@ -567,13 +581,6 @@ main(int argc, char **argv)
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else
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username_subquery = "SELECT usename FROM pg_user WHERE usesysid =";
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/*
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* If supported, set extra_float_digits so that we can dump float data
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* exactly (given correctly implemented float I/O code, anyway)
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*/
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if (g_fout->remoteVersion >= 70400)
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do_sql_command(g_conn, "SET extra_float_digits TO 2");
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/* Find the last built-in OID, if needed */
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if (g_fout->remoteVersion < 70300)
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{
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