Set a default autovacuum vacuum_cost_delay value of 20ms, to avoid excessive

I/O utilization, per discussion.

While at it, lower the autovacuum vacuum and analyze threshold values to 50
tuples.  It is a bit higher (i.e. more conservative) than what I originally
proposed but much better than the old values for small tables.

doc/src/sgml/config.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.131 2007/07/18 12:00:47 mha Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.132 2007/07/24 01:53:55 alvherre Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -3247,7 +3247,7 @@ SELECT * FROM parent WHERE key = 2400;
        <para>
         Specifies the minimum number of updated or deleted tuples needed
         to trigger a <command>VACUUM</> in any one table.
-        The default is 500 tuples.
+        The default is 50 tuples.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
         This setting can be overridden for individual tables by entries in
@@ -3265,7 +3265,7 @@ SELECT * FROM parent WHERE key = 2400;
        <para>
         Specifies the minimum number of inserted, updated or deleted tuples
         needed to trigger an <command>ANALYZE</> in any one table.
-        The default is 250 tuples.
+        The default is 50 tuples.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
         This setting can be overridden for individual tables by entries in
@@ -3343,8 +3343,9 @@ SELECT * FROM parent WHERE key = 2400;
        <para>
         Specifies the cost delay value that will be used in automatic
         <command>VACUUM</> operations.  If <literal>-1</> is
-        specified (which is the default), the regular
+        specified, the regular
         <xref linkend="guc-vacuum-cost-delay"> value will be used.
+        The default value is 20 milliseconds.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
         This setting can be overridden for individual tables by entries in

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 1.405 2007/07/10 13:14:21 mha Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.406 2007/07/24 01:53:56 alvherre Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -1348,7 +1348,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_MS
 		},
 		&autovacuum_vac_cost_delay,
-		-1, -1, 1000, NULL, NULL
+		20, -1, 1000, NULL, NULL
 	},
 
 	{
@@ -1656,7 +1656,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&autovacuum_vac_thresh,
-		500, 0, INT_MAX, NULL, NULL
+		50, 0, INT_MAX, NULL, NULL
 	},
 	{
 		{"autovacuum_analyze_threshold", PGC_SIGHUP, AUTOVACUUM,
@@ -1664,7 +1664,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&autovacuum_anl_thresh,
-		250, 0, INT_MAX, NULL, NULL
+		50, 0, INT_MAX, NULL, NULL
 	},
 	{
 		/* see varsup.c for why this is PGC_POSTMASTER not PGC_SIGHUP */

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

@@ -381,9 +381,9 @@
 #log_autovacuum = -1			# -1 is disabled, 0 logs all actions
 					# and their durations, > 0 logs only
 					# actions running at least N msec.
-#autovacuum_vacuum_threshold = 500	# min # of tuple updates before
+#autovacuum_vacuum_threshold = 50	# min # of tuple updates before
 					# vacuum
-#autovacuum_analyze_threshold = 250	# min # of tuple updates before 
+#autovacuum_analyze_threshold = 50	# min # of tuple updates before 
 					# analyze
 #autovacuum_vacuum_scale_factor = 0.2	# fraction of rel size before 
 					# vacuum
@@ -391,7 +391,7 @@
 					# analyze
 #autovacuum_freeze_max_age = 200000000	# maximum XID age before forced vacuum
 					# (change requires restart)
-#autovacuum_vacuum_cost_delay = -1	# default vacuum cost delay for 
+#autovacuum_vacuum_cost_delay = 20	# default vacuum cost delay for 
 					# autovacuum, -1 means use 
 					# vacuum_cost_delay
 #autovacuum_vacuum_cost_limit = -1	# default vacuum cost limit for