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.

doc/src/sgml/config.sgml

-<!-- $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>
 

src/backend/access/heap/heapam.c

@@ -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);

src/backend/utils/misc/guc.c

@@ -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."),

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

@@ -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 -
 

src/bin/pg_dump/pg_dump.c

@@ -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)
 	{