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Commit47df4f6

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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 thatit will reliably preserve row ordering. Per recent discussions.
1 parent6dfa40d commit47df4f6

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5 files changed

+59
-14
lines changed

5 files changed

+59
-14
lines changed

‎doc/src/sgml/config.sgml‎

Lines changed: 23 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1,4 +1,4 @@
1-
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.162 2008/01/27 19:12:28 tgl Exp $ -->
1+
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.163 2008/01/30 18:35:55 tgl Exp $ -->
22

33
<chapter Id="runtime-config">
44
<title>Server Configuration</title>
@@ -4611,6 +4611,28 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
46114611
</listitem>
46124612
</varlistentry>
46134613

4614+
<varlistentry id="guc-synchronize-seqscans" xreflabel="synchronize_seqscans">
4615+
<term><varname>synchronize_seqscans</varname> (<type>boolean</type>)</term>
4616+
<indexterm>
4617+
<primary><varname>synchronize_seqscans</> configuration parameter</primary>
4618+
</indexterm>
4619+
<listitem>
4620+
<para>
4621+
This allows sequential scans of large tables to synchronize with each
4622+
other, so that concurrent scans read the same block at about the
4623+
same time and hence share the I/O workload. When this is enabled,
4624+
a scan might start in the middle of the table and then <quote>wrap
4625+
around</> the end to cover all rows, so as to synchronize with the
4626+
activity of scans already in progress. This can result in
4627+
unpredictable changes in the row ordering returned by queries that
4628+
have no <literal>ORDER BY</> clause. Setting this parameter to
4629+
<literal>off</> ensures the pre-8.3 behavior in which a sequential
4630+
scan always starts from the beginning of the table. The default
4631+
is <literal>on</>.
4632+
</para>
4633+
</listitem>
4634+
</varlistentry>
4635+
46144636
</variablelist>
46154637
</sect2>
46164638

‎src/backend/access/heap/heapam.c‎

Lines changed: 8 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.248 2008/01/14 01:39:09 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.249 2008/01/30 18:35:55 tgl Exp $
1212
*
1313
*
1414
* INTERFACE ROUTINES
@@ -59,6 +59,10 @@
5959
#include"utils/syscache.h"
6060

6161

62+
/* GUC variable */
63+
boolsynchronize_seqscans= true;
64+
65+
6266
staticHeapScanDescheap_beginscan_internal(Relationrelation,
6367
Snapshotsnapshot,
6468
intnkeys,ScanKeykey,
@@ -104,7 +108,8 @@ initscan(HeapScanDesc scan, ScanKey key)
104108
* the thresholds for these features could be different, we make them the
105109
* same so that there are only two behaviors to tune rather than four.
106110
* (However, some callers need to be able to disable one or both of
107-
* these behaviors, independently of the size of the table.)
111+
* these behaviors, independently of the size of the table; also there
112+
* is a GUC variable that can disable synchronized scanning.)
108113
*
109114
* During a rescan, don't make a new strategy object if we don't have to.
110115
*/
@@ -129,7 +134,7 @@ initscan(HeapScanDesc scan, ScanKey key)
129134
scan->rs_strategy=NULL;
130135
}
131136

132-
if (allow_sync)
137+
if (allow_sync&&synchronize_seqscans)
133138
{
134139
scan->rs_syncscan= true;
135140
scan->rs_startblock=ss_get_location(scan->rs_rd,scan->rs_nblocks);

‎src/backend/utils/misc/guc.c‎

Lines changed: 11 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -10,7 +10,7 @@
1010
* Written by Peter Eisentraut <peter_e@gmx.net>.
1111
*
1212
* IDENTIFICATION
13-
* $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.431 2008/01/27 19:12:28 tgl Exp $
13+
* $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.432 2008/01/30 18:35:55 tgl Exp $
1414
*
1515
*--------------------------------------------------------------------
1616
*/
@@ -110,6 +110,7 @@ extern intCommitDelay;
110110
externintCommitSiblings;
111111
externchar*default_tablespace;
112112
externchar*temp_tablespaces;
113+
externboolsynchronize_seqscans;
113114
externboolfullPageWrites;
114115

115116
#ifdefTRACE_SORT
@@ -1052,6 +1053,15 @@ static struct config_bool ConfigureNamesBool[] =
10521053
false,NULL,NULL
10531054
},
10541055

1056+
{
1057+
{"synchronize_seqscans",PGC_USERSET,COMPAT_OPTIONS_PREVIOUS,
1058+
gettext_noop("Enable synchronized sequential scans."),
1059+
NULL
1060+
},
1061+
&synchronize_seqscans,
1062+
true,NULL,NULL
1063+
},
1064+
10551065
{
10561066
{"archive_mode",PGC_POSTMASTER,WAL_SETTINGS,
10571067
gettext_noop("Allows archiving of WAL files using archive_command."),

‎src/backend/utils/misc/postgresql.conf.sample‎

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -476,9 +476,10 @@
476476
#backslash_quote = safe_encoding# on, off, or safe_encoding
477477
#default_with_oids = off
478478
#escape_string_warning = on
479-
#standard_conforming_strings = off
480479
#regex_flavor = advanced# advanced, extended, or basic
481480
#sql_inheritance = on
481+
#standard_conforming_strings = off
482+
#synchronize_seqscans = on
482483

483484
# - Other Platforms and Clients -
484485

‎src/bin/pg_dump/pg_dump.c‎

Lines changed: 15 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -12,7 +12,7 @@
1212
*by PostgreSQL
1313
*
1414
* IDENTIFICATION
15-
* $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.481 2008/01/01 19:45:55momjian Exp $
15+
* $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.482 2008/01/30 18:35:55tgl Exp $
1616
*
1717
*-------------------------------------------------------------------------
1818
*/
@@ -552,6 +552,20 @@ main(int argc, char **argv)
552552
/* Set the datestyle to ISO to ensure the dump's portability */
553553
do_sql_command(g_conn,"SET DATESTYLE = ISO");
554554

555+
/*
556+
* If supported, set extra_float_digits so that we can dump float data
557+
* exactly (given correctly implemented float I/O code, anyway)
558+
*/
559+
if (g_fout->remoteVersion >=70400)
560+
do_sql_command(g_conn,"SET extra_float_digits TO 2");
561+
562+
/*
563+
* If synchronized scanning is supported, disable it, to prevent
564+
* unpredictable changes in row ordering across a dump and reload.
565+
*/
566+
if (g_fout->remoteVersion >=80300)
567+
do_sql_command(g_conn,"SET synchronize_seqscans TO off");
568+
555569
/*
556570
* Start serializable transaction to dump consistent data.
557571
*/
@@ -567,13 +581,6 @@ main(int argc, char **argv)
567581
else
568582
username_subquery="SELECT usename FROM pg_user WHERE usesysid =";
569583

570-
/*
571-
* If supported, set extra_float_digits so that we can dump float data
572-
* exactly (given correctly implemented float I/O code, anyway)
573-
*/
574-
if (g_fout->remoteVersion >=70400)
575-
do_sql_command(g_conn,"SET extra_float_digits TO 2");
576-
577584
/* Find the last built-in OID, if needed */
578585
if (g_fout->remoteVersion<70300)
579586
{

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