Reduce default value of max_prepared_transactions from 50 to 5. This

saves nearly 700kB in the default shared memory segment size, which seems
worthwhile, and it is a feature that many users won't use anyway.  Per
Heikki's argument, there is no point in a compromise value --- those who
are using 2PC at all will probably want it at least equal to max_connections.
But we can't set it to zero by default without breaking the prepared_xacts
regression test.

doc/src/sgml/runtime.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.348 2005/08/24 18:56:07 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.349 2005/08/29 21:38:17 tgl Exp $
 -->
 
 <chapter Id="runtime">
@@ -1179,7 +1179,7 @@ SET ENABLE_SEQSCAN TO OFF;
         endterm="sql-prepare-transaction-title">).
         Setting this parameter to zero disables the prepared-transaction
         feature.
-        The default is 50.
+        The default is 5.
         This option can only be set at server start.
        </para>
 

src/backend/access/transam/twophase.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.10 2005/08/20 23:26:10 tgl Exp $
+ *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.11 2005/08/29 21:38:18 tgl Exp $
  *
  * NOTES
  *		Each global transaction is associated with a global transaction
@@ -64,7 +64,7 @@
 #define TWOPHASE_DIR "pg_twophase"
 
 /* GUC variable, can't be changed after startup */
-int max_prepared_xacts = 50;
+int max_prepared_xacts = 5;
 
 /*
  * This struct describes one global transaction that is in prepared state

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 1.286 2005/08/22 17:34:59 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.287 2005/08/29 21:38:18 tgl Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -1203,7 +1203,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&max_prepared_xacts,
-		50, 0, INT_MAX, NULL, NULL
+		5, 0, INT_MAX, NULL, NULL
 	},
 
 #ifdef LOCK_DEBUG

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

@@ -90,7 +90,7 @@
 
 #shared_buffers = 1000			# min 16 or max_connections*2, 8KB each
 #temp_buffers = 1000			# min 100, 8KB each
-#max_prepared_transactions = 50		# can be 0 or more
+#max_prepared_transactions = 5		# can be 0 or more
 #work_mem = 1024			# min 64, size in KB
 #maintenance_work_mem = 16384		# min 1024, size in KB
 #max_stack_depth = 2048			# min 100, size in KB