Per today's -hackers discussion, add a GUC variable 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. This is a
pretty trivial patch, but seeing how late we are in the 8.3 release
cycle, I thought I'd better post it for comment anyway.
regards, tom lane
Index: doc/src/sgml/config.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/config.sgml,v
retrieving revision 1.162
diff -c -r1.162 config.sgml
*** doc/src/sgml/config.sgml 27 Jan 2008 19:12:28 -0000 1.162
--- doc/src/sgml/config.sgml 27 Jan 2008 20:00:16 -0000
***************
*** 4611,4616 ****
--- 4611,4638 ----
</listitem>
</varlistentry>
+ <varlistentry id="guc-synchronized-scanning"
xreflabel="synchronized_scanning">
+ <term><varname>synchronized_scanning</varname>
(<type>boolean</type>)</term>
+ <indexterm>
+ <primary><varname>synchronized_scanning</> 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>
Index: src/backend/access/heap/heapam.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/heap/heapam.c,v
retrieving revision 1.248
diff -c -r1.248 heapam.c
*** src/backend/access/heap/heapam.c 14 Jan 2008 01:39:09 -0000 1.248
--- src/backend/access/heap/heapam.c 27 Jan 2008 20:00:18 -0000
***************
*** 59,64 ****
--- 59,68 ----
#include "utils/syscache.h"
+ /* GUC variable */
+ bool synchronized_scanning = true;
+
+
static HeapScanDesc heap_beginscan_internal(Relation relation,
Snapshot snapshot,
int nkeys, ScanKey key,
***************
*** 104,110 ****
* 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.)
*
* During a rescan, don't make a new strategy object if we don't have
to.
*/
--- 108,115 ----
* 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; 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.
*/
***************
*** 129,135 ****
scan->rs_strategy = NULL;
}
! if (allow_sync)
{
scan->rs_syncscan = true;
scan->rs_startblock = ss_get_location(scan->rs_rd,
scan->rs_nblocks);
--- 134,140 ----
scan->rs_strategy = NULL;
}
! if (allow_sync && synchronized_scanning)
{
scan->rs_syncscan = true;
scan->rs_startblock = ss_get_location(scan->rs_rd,
scan->rs_nblocks);
Index: src/backend/utils/misc/guc.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/misc/guc.c,v
retrieving revision 1.431
diff -c -r1.431 guc.c
*** src/backend/utils/misc/guc.c 27 Jan 2008 19:12:28 -0000 1.431
--- src/backend/utils/misc/guc.c 27 Jan 2008 20:00:18 -0000
***************
*** 110,115 ****
--- 110,116 ----
extern int CommitSiblings;
extern char *default_tablespace;
extern char *temp_tablespaces;
+ extern bool synchronized_scanning;
extern bool fullPageWrites;
#ifdef TRACE_SORT
***************
*** 1053,1058 ****
--- 1054,1068 ----
},
{
+ {"synchronized_scanning", PGC_USERSET, COMPAT_OPTIONS_PREVIOUS,
+ gettext_noop("Enable synchronized scans."),
+ NULL
+ },
+ &synchronized_scanning,
+ true, NULL, NULL
+ },
+
+ {
{"archive_mode", PGC_POSTMASTER, WAL_SETTINGS,
gettext_noop("Allows archiving of WAL files using
archive_command."),
NULL
Index: src/backend/utils/misc/postgresql.conf.sample
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/misc/postgresql.conf.sample,v
retrieving revision 1.235
diff -c -r1.235 postgresql.conf.sample
*** src/backend/utils/misc/postgresql.conf.sample 27 Jan 2008 19:12:28
-0000 1.235
--- src/backend/utils/misc/postgresql.conf.sample 27 Jan 2008 20:00:18
-0000
***************
*** 476,484 ****
#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
# - Other Platforms and Clients -
--- 476,485 ----
#backslash_quote = safe_encoding # on, off, or safe_encoding
#default_with_oids = off
#escape_string_warning = on
#regex_flavor = advanced # advanced, extended, or basic
#sql_inheritance = on
+ #standard_conforming_strings = off
+ #synchronized_scanning = on
# - Other Platforms and Clients -
Index: src/bin/pg_dump/pg_dump.c
===================================================================
RCS file: /cvsroot/pgsql/src/bin/pg_dump/pg_dump.c,v
retrieving revision 1.481
diff -c -r1.481 pg_dump.c
*** src/bin/pg_dump/pg_dump.c 1 Jan 2008 19:45:55 -0000 1.481
--- src/bin/pg_dump/pg_dump.c 27 Jan 2008 20:00:18 -0000
***************
*** 553,558 ****
--- 553,572 ----
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 synchronized_scanning TO off");
+
+ /*
* Start serializable transaction to dump consistent data.
*/
do_sql_command(g_conn, "BEGIN");
***************
*** 567,579 ****
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)
{
--- 581,586 ----
---------------------------(end of broadcast)---------------------------
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