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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 2008-01-30 18:35:55 +00:00
parent 6dfa40d69f
commit 47df4f6688
5 changed files with 59 additions and 14 deletions
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24
doc/src/sgml/config.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.162 2008/01/27 19:12:28 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.163 2008/01/30 18:35:55 tgl Exp $ -->
<chapter Id="runtime-config">
<title>Server Configuration</title>
@ -4611,6 +4611,28 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
</listitem>
</varlistentry>
<varlistentry id="guc-synchronize-seqscans" xreflabel="synchronize_seqscans">
<term><varname>synchronize_seqscans</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>synchronize_seqscans</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
This allows sequential scans of large tables to synchronize with each
other, so that concurrent scans read the same block at about the
same time and hence share the I/O workload. When this is enabled,
a scan might start in the middle of the table and then <quote>wrap
around</> the end to cover all rows, so as to synchronize with the
activity of scans already in progress. This can result in
unpredictable changes in the row ordering returned by queries that
have no <literal>ORDER BY</> clause. Setting this parameter to
<literal>off</> ensures the pre-8.3 behavior in which a sequential
scan always starts from the beginning of the table. The default
is <literal>on</>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>

11
src/backend/access/heap/heapam.c
View File

@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.248 2008/01/14 01:39:09 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.249 2008/01/30 18:35:55 tgl Exp $
*
*
* INTERFACE ROUTINES
@ -59,6 +59,10 @@
#include "utils/syscache.h"
/* GUC variable */
bool synchronize_seqscans = true;
static HeapScanDesc heap_beginscan_internal(Relation relation,
Snapshot snapshot,
int nkeys, ScanKey key,
@ -104,7 +108,8 @@ initscan(HeapScanDesc scan, ScanKey key)
* the thresholds for these features could be different, we make them the
* same so that there are only two behaviors to tune rather than four.
* (However, some callers need to be able to disable one or both of
* these behaviors, independently of the size of the table.)
* these behaviors, independently of the size of the table; also there
* is a GUC variable that can disable synchronized scanning.)
*
* During a rescan, don't make a new strategy object if we don't have to.
*/
@ -129,7 +134,7 @@ initscan(HeapScanDesc scan, ScanKey key)
scan->rs_strategy = NULL;
}
if (allow_sync)
if (allow_sync && synchronize_seqscans)
{
scan->rs_syncscan = true;
scan->rs_startblock = ss_get_location(scan->rs_rd, scan->rs_nblocks);

12
src/backend/utils/misc/guc.c
View File

@ -10,7 +10,7 @@
* Written by Peter Eisentraut <peter_e@gmx.net>.
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.431 2008/01/27 19:12:28 tgl Exp $
* $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.432 2008/01/30 18:35:55 tgl Exp $
*
*--------------------------------------------------------------------
*/
@ -110,6 +110,7 @@ extern int CommitDelay;
extern int CommitSiblings;
extern char *default_tablespace;
extern char *temp_tablespaces;
extern bool synchronize_seqscans;
extern bool fullPageWrites;
#ifdef TRACE_SORT
@ -1052,6 +1053,15 @@ static struct config_bool ConfigureNamesBool[] =
false, NULL, NULL
},
{
{"synchronize_seqscans", PGC_USERSET, COMPAT_OPTIONS_PREVIOUS,
gettext_noop("Enable synchronized sequential scans."),
NULL
},
&synchronize_seqscans,
true, NULL, NULL
},
{
{"archive_mode", PGC_POSTMASTER, WAL_SETTINGS,
gettext_noop("Allows archiving of WAL files using archive_command."),

3
src/backend/utils/misc/postgresql.conf.sample
View File

@ -476,9 +476,10 @@
#backslash_quote = safe_encoding # on, off, or safe_encoding
#default_with_oids = off
#escape_string_warning = on
#standard_conforming_strings = off
#regex_flavor = advanced # advanced, extended, or basic
#sql_inheritance = on
#standard_conforming_strings = off
#synchronize_seqscans = on
# - Other Platforms and Clients -

23
src/bin/pg_dump/pg_dump.c
View File

@ -12,7 +12,7 @@
* by PostgreSQL
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.481 2008/01/01 19:45:55 momjian Exp $
* $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.482 2008/01/30 18:35:55 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -552,6 +552,20 @@ main(int argc, char **argv)
/* Set the datestyle to ISO to ensure the dump's portability */
do_sql_command(g_conn, "SET DATESTYLE = ISO");
/*
* If supported, set extra_float_digits so that we can dump float data
* exactly (given correctly implemented float I/O code, anyway)
*/
if (g_fout->remoteVersion >= 70400)
do_sql_command(g_conn, "SET extra_float_digits TO 2");
/*
* If synchronized scanning is supported, disable it, to prevent
* unpredictable changes in row ordering across a dump and reload.
*/
if (g_fout->remoteVersion >= 80300)
do_sql_command(g_conn, "SET synchronize_seqscans TO off");
/*
* Start serializable transaction to dump consistent data.
*/
@ -567,13 +581,6 @@ main(int argc, char **argv)
else
username_subquery = "SELECT usename FROM pg_user WHERE usesysid =";
/*
* If supported, set extra_float_digits so that we can dump float data
* exactly (given correctly implemented float I/O code, anyway)
*/
if (g_fout->remoteVersion >= 70400)
do_sql_command(g_conn, "SET extra_float_digits TO 2");
/* Find the last built-in OID, if needed */
if (g_fout->remoteVersion < 70300)
{