Add a function pg_stat_clear_snapshot() that discards any statistics snapshot

already collected in the current transaction; this allows plpgsql functions to
watch for stats updates even though they are confined to a single transaction.
Use this instead of the previous kluge involving pg_stat_file() to wait for
the stats collector to update in the stats regression test.  Internally,
decouple storage of stats snapshots from transaction boundaries; they'll
now stick around until someone calls pgstat_clear_snapshot --- which xact.c
still does at transaction end, to maintain the previous behavior.  This makes
the logic a lot cleaner, at the price of a couple dozen cycles per transaction
exit.

8 files changed, 172 insertions, 162 deletions

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 4ee1cc711cd..26a3c5ddc2c 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.232 2007/02/01 19:10:25 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.233 2007/02/07 23:11:29 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1628,6 +1628,7 @@ CommitTransaction(void)
 	AtEOXact_Namespace(true);
 	/* smgrcommit already done */
 	AtEOXact_Files();
+	pgstat_clear_snapshot();
 	pgstat_count_xact_commit();
 	pgstat_report_txn_timestamp(0);
 
@@ -1844,6 +1845,7 @@ PrepareTransaction(void)
 	AtEOXact_Namespace(true);
 	/* smgrcommit already done */
 	AtEOXact_Files();
+	pgstat_clear_snapshot();
 
 	CurrentResourceOwner = NULL;
 	ResourceOwnerDelete(TopTransactionResourceOwner);
@@ -1995,6 +1997,7 @@ AbortTransaction(void)
 	AtEOXact_Namespace(false);
 	smgrabort();
 	AtEOXact_Files();
+	pgstat_clear_snapshot();
 	pgstat_count_xact_rollback();
 	pgstat_report_txn_timestamp(0);
 
diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index 6a5907c466e..f64e02020a8 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -13,7 +13,7 @@
  *
  *	Copyright (c) 2001-2007, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.144 2007/01/26 20:06:52 tgl Exp $
+ *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.145 2007/02/07 23:11:29 tgl Exp $
  * ----------
  */
 #include "postgres.h"
@@ -130,9 +130,8 @@ static TabStatArray SharedTabStat = {0, 0, NULL};
 static int	pgStatXactCommit = 0;
 static int	pgStatXactRollback = 0;
 
-static TransactionId pgStatDBHashXact = InvalidTransactionId;
+static MemoryContext pgStatLocalContext = NULL;
 static HTAB *pgStatDBHash = NULL;
-static TransactionId pgStatLocalStatusXact = InvalidTransactionId;
 static PgBackendStatus *localBackendStatusTable = NULL;
 static int	localNumBackends = 0;
 
@@ -156,11 +155,13 @@ static void pgstat_beshutdown_hook(int code, Datum arg);
 static PgStat_StatDBEntry *pgstat_get_db_entry(Oid databaseid, bool create);
 static void pgstat_drop_database(Oid databaseid);
 static void pgstat_write_statsfile(void);
-static void pgstat_read_statsfile(HTAB **dbhash, Oid onlydb);
+static HTAB *pgstat_read_statsfile(Oid onlydb);
 static void backend_read_statsfile(void);
 static void pgstat_read_current_status(void);
 static HTAB *pgstat_collect_oids(Oid catalogid);
 
+static void pgstat_setup_memcxt(void);
+
 static void pgstat_setheader(PgStat_MsgHdr *hdr, StatMsgType mtype);
 static void pgstat_send(void *msg, int len);
 
@@ -1535,22 +1536,24 @@ pgstat_report_waiting(bool waiting)
 static void
 pgstat_read_current_status(void)
 {
-	TransactionId topXid = GetTopTransactionId();
 	volatile PgBackendStatus *beentry;
+	PgBackendStatus *localtable;
 	PgBackendStatus *localentry;
 	int			i;
 
 	Assert(!pgStatRunningInCollector);
-	if (TransactionIdEquals(pgStatLocalStatusXact, topXid))
+	if (localBackendStatusTable)
 		return;					/* already done */
 
-	localBackendStatusTable = (PgBackendStatus *)
-		MemoryContextAlloc(TopTransactionContext,
+	pgstat_setup_memcxt();
+
+	localtable = (PgBackendStatus *)
+		MemoryContextAlloc(pgStatLocalContext,
 						   sizeof(PgBackendStatus) * MaxBackends);
 	localNumBackends = 0;
 
 	beentry = BackendStatusArray;
-	localentry = localBackendStatusTable;
+	localentry = localtable;
 	for (i = 1; i <= MaxBackends; i++)
 	{
 		/*
@@ -1587,7 +1590,8 @@ pgstat_read_current_status(void)
 		}
 	}
 
-	pgStatLocalStatusXact = topXid;
+	/* Set the pointer only after completion of a valid table */
+	localBackendStatusTable = localtable;
 }
 
 
@@ -1720,7 +1724,7 @@ PgstatCollectorMain(int argc, char *argv[])
 	 * zero.
 	 */
 	pgStatRunningInCollector = true;
-	pgstat_read_statsfile(&pgStatDBHash, InvalidOid);
+	pgStatDBHash = pgstat_read_statsfile(InvalidOid);
 
 	/*
 	 * Setup the descriptor set for select(2).	Since only one bit in the set
@@ -2090,38 +2094,24 @@ pgstat_write_statsfile(void)
  *	databases' hash table (whose entries point to the tables' hash tables).
  * ----------
  */
-static void
-pgstat_read_statsfile(HTAB **dbhash, Oid onlydb)
+static HTAB *
+pgstat_read_statsfile(Oid onlydb)
 {
 	PgStat_StatDBEntry *dbentry;
 	PgStat_StatDBEntry dbbuf;
 	PgStat_StatTabEntry *tabentry;
 	PgStat_StatTabEntry tabbuf;
 	HASHCTL		hash_ctl;
+	HTAB	   *dbhash;
 	HTAB	   *tabhash = NULL;
 	FILE	   *fpin;
 	int32		format_id;
 	bool		found;
-	MemoryContext use_mcxt;
-	int			mcxt_flags;
 
 	/*
-	 * If running in the collector or the autovacuum process, we use the
-	 * DynaHashCxt memory context.	If running in a backend, we use the
-	 * TopTransactionContext instead, so the caller must only know the last
-	 * XactId when this call happened to know if his tables are still valid or
-	 * already gone!
+	 * The tables will live in pgStatLocalContext.
 	 */
-	if (pgStatRunningInCollector || IsAutoVacuumProcess())
-	{
-		use_mcxt = NULL;
-		mcxt_flags = 0;
-	}
-	else
-	{
-		use_mcxt = TopTransactionContext;
-		mcxt_flags = HASH_CONTEXT;
-	}
+	pgstat_setup_memcxt();
 
 	/*
 	 * Create the DB hashtable
@@ -2130,9 +2120,9 @@ pgstat_read_statsfile(HTAB **dbhash, Oid onlydb)
 	hash_ctl.keysize = sizeof(Oid);
 	hash_ctl.entrysize = sizeof(PgStat_StatDBEntry);
 	hash_ctl.hash = oid_hash;
-	hash_ctl.hcxt = use_mcxt;
-	*dbhash = hash_create("Databases hash", PGSTAT_DB_HASH_SIZE, &hash_ctl,
-						  HASH_ELEM | HASH_FUNCTION | mcxt_flags);
+	hash_ctl.hcxt = pgStatLocalContext;
+	dbhash = hash_create("Databases hash", PGSTAT_DB_HASH_SIZE, &hash_ctl,
+						 HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT);
 
 	/*
 	 * Try to open the status file. If it doesn't exist, the backends simply
@@ -2140,7 +2130,7 @@ pgstat_read_statsfile(HTAB **dbhash, Oid onlydb)
 	 * with empty counters.
 	 */
 	if ((fpin = AllocateFile(PGSTAT_STAT_FILENAME, PG_BINARY_R)) == NULL)
-		return;
+		return dbhash;
 
 	/*
 	 * Verify it's of the expected format.
@@ -2178,7 +2168,7 @@ pgstat_read_statsfile(HTAB **dbhash, Oid onlydb)
 				/*
 				 * Add to the DB hash
 				 */
-				dbentry = (PgStat_StatDBEntry *) hash_search(*dbhash,
+				dbentry = (PgStat_StatDBEntry *) hash_search(dbhash,
 												  (void *) &dbbuf.databaseid,
 															 HASH_ENTER,
 															 &found);
@@ -2207,11 +2197,11 @@ pgstat_read_statsfile(HTAB **dbhash, Oid onlydb)
 				hash_ctl.keysize = sizeof(Oid);
 				hash_ctl.entrysize = sizeof(PgStat_StatTabEntry);
 				hash_ctl.hash = oid_hash;
-				hash_ctl.hcxt = use_mcxt;
+				hash_ctl.hcxt = pgStatLocalContext;
 				dbentry->tables = hash_create("Per-database table",
 											  PGSTAT_TAB_HASH_SIZE,
 											  &hash_ctl,
-									 HASH_ELEM | HASH_FUNCTION | mcxt_flags);
+									 HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT);
 
 				/*
 				 * Arrange that following 'T's add entries to this database's
@@ -2274,44 +2264,78 @@ pgstat_read_statsfile(HTAB **dbhash, Oid onlydb)
 
 done:
 	FreeFile(fpin);
+
+	return dbhash;
 }
 
 /*
- * If not done for this transaction, read the statistics collector
- * stats file into some hash tables.
- *
- * Because we store the tables in TopTransactionContext, the result
- * is good for the entire current main transaction.
- *
- * Inside the autovacuum process, the statfile is assumed to be valid
- * "forever", that is one iteration, within one database.  This means
- * we only consider the statistics as they were when the autovacuum
- * iteration started.
+ * If not already done, read the statistics collector stats file into
+ * some hash tables.  The results will be kept until pgstat_clear_snapshot()
+ * is called (typically, at end of transaction).
  */
 static void
 backend_read_statsfile(void)
 {
+	/* already read it? */
+	if (pgStatDBHash)
+		return;
+	Assert(!pgStatRunningInCollector);
+
+	/* Autovacuum wants stats about all databases */
 	if (IsAutoVacuumProcess())
-	{
-		/* already read it? */
-		if (pgStatDBHash)
-			return;
-		Assert(!pgStatRunningInCollector);
-		pgstat_read_statsfile(&pgStatDBHash, InvalidOid);
-	}
+		pgStatDBHash = pgstat_read_statsfile(InvalidOid);
 	else
-	{
-		TransactionId topXid = GetTopTransactionId();
+		pgStatDBHash = pgstat_read_statsfile(MyDatabaseId);
+}
 
-		if (!TransactionIdEquals(pgStatDBHashXact, topXid))
-		{
-			Assert(!pgStatRunningInCollector);
-			pgstat_read_statsfile(&pgStatDBHash, MyDatabaseId);
-			pgStatDBHashXact = topXid;
-		}
-	}
+
+/* ----------
+ * pgstat_setup_memcxt() -
+ *
+ *	Create pgStatLocalContext, if not already done.
+ * ----------
+ */
+static void
+pgstat_setup_memcxt(void)
+{
+	if (!pgStatLocalContext)
+		pgStatLocalContext = AllocSetContextCreate(TopMemoryContext,
+												   "Statistics snapshot",
+												   ALLOCSET_SMALL_MINSIZE,
+												   ALLOCSET_SMALL_INITSIZE,
+												   ALLOCSET_SMALL_MAXSIZE);
 }
 
+
+/* ----------
+ * pgstat_clear_snapshot() -
+ *
+ *	Discard any data collected in the current transaction.  Any subsequent
+ *	request will cause new snapshots to be read.
+ *
+ *	This is also invoked during transaction commit or abort to discard
+ *	the no-longer-wanted snapshot.
+ * ----------
+ */
+void
+pgstat_clear_snapshot(void)
+{
+	/* In an autovacuum process we keep the stats forever */
+	if (IsAutoVacuumProcess())
+		return;
+
+	/* Release memory, if any was allocated */
+	if (pgStatLocalContext)
+		MemoryContextDelete(pgStatLocalContext);
+
+	/* Reset variables */
+	pgStatLocalContext = NULL;
+	pgStatDBHash = NULL;
+	localBackendStatusTable = NULL;
+	localNumBackends = 0;
+}
+
+
 /* ----------
  * pgstat_recv_tabstat() -
  *
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 94631969d0e..d011fff76f8 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/pgstatfuncs.c,v 1.37 2007/01/05 22:19:41 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/pgstatfuncs.c,v 1.38 2007/02/07 23:11:29 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -39,7 +39,6 @@ extern Datum pg_stat_get_last_autoanalyze_time(PG_FUNCTION_ARGS);
 
 extern Datum pg_stat_get_backend_idset(PG_FUNCTION_ARGS);
 extern Datum pg_backend_pid(PG_FUNCTION_ARGS);
-extern Datum pg_stat_reset(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_backend_pid(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_backend_dbid(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_backend_userid(PG_FUNCTION_ARGS);
@@ -57,6 +56,9 @@ extern Datum pg_stat_get_db_xact_rollback(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_db_blocks_fetched(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_db_blocks_hit(PG_FUNCTION_ARGS);
 
+extern Datum pg_stat_clear_snapshot(PG_FUNCTION_ARGS);
+extern Datum pg_stat_reset(PG_FUNCTION_ARGS);
+
 
 Datum
 pg_stat_get_numscans(PG_FUNCTION_ARGS)
@@ -336,16 +338,6 @@ pg_backend_pid(PG_FUNCTION_ARGS)
 	PG_RETURN_INT32(MyProcPid);
 }
 
-/*
- * Built-in function for resetting the counters
- */
-Datum
-pg_stat_reset(PG_FUNCTION_ARGS)
-{
-	pgstat_reset_counters();
-
-	PG_RETURN_BOOL(true);
-}
 
 Datum
 pg_stat_get_backend_pid(PG_FUNCTION_ARGS)
@@ -678,3 +670,23 @@ pg_stat_get_db_blocks_hit(PG_FUNCTION_ARGS)
 
 	PG_RETURN_INT64(result);
 }
+
+
+/* Discard the active statistics snapshot */
+Datum
+pg_stat_clear_snapshot(PG_FUNCTION_ARGS)
+{
+	pgstat_clear_snapshot();
+
+	PG_RETURN_VOID();
+}
+
+
+/* Reset all counters for the current database */
+Datum
+pg_stat_reset(PG_FUNCTION_ARGS)
+{
+	pgstat_reset_counters();
+
+	PG_RETURN_VOID();
+}
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
index 154b56b32dd..9ebc4e9b9fd 100644
--- a/src/include/catalog/catversion.h
+++ b/src/include/catalog/catversion.h
@@ -37,7 +37,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.381 2007/02/06 02:59:12 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.382 2007/02/07 23:11:29 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	200702051
+#define CATALOG_VERSION_NO	200702071
 
 #endif
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 500d2395422..e8014c2bda8 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/pg_proc.h,v 1.442 2007/02/03 14:06:55 petere Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/pg_proc.h,v 1.443 2007/02/07 23:11:30 tgl Exp $
  *
  * NOTES
  *	  The script catalog/genbki.sh reads this file and generates .bki
@@ -2938,8 +2938,6 @@ DATA(insert OID = 1936 (  pg_stat_get_backend_idset		PGNSP PGUID 12 1 100 f f t
 DESCR("Statistics: Currently active backend IDs");
 DATA(insert OID = 2026 (  pg_backend_pid				PGNSP PGUID 12 1 0 f f t f s 0 23 "" _null_ _null_ _null_ pg_backend_pid - _null_ ));
 DESCR("Statistics: Current backend PID");
-DATA(insert OID = 2274 (  pg_stat_reset				PGNSP PGUID 12 1 0 f f f f v 0 16  "" _null_ _null_ _null_	pg_stat_reset - _null_ ));
-DESCR("Statistics: Reset collected statistics");
 DATA(insert OID = 1937 (  pg_stat_get_backend_pid		PGNSP PGUID 12 1 0 f f t f s 1 23 "23" _null_ _null_ _null_ pg_stat_get_backend_pid - _null_ ));
 DESCR("Statistics: PID of backend");
 DATA(insert OID = 1938 (  pg_stat_get_backend_dbid		PGNSP PGUID 12 1 0 f f t f s 1 26 "23" _null_ _null_ _null_ pg_stat_get_backend_dbid - _null_ ));
@@ -2970,6 +2968,10 @@ DATA(insert OID = 1944 (  pg_stat_get_db_blocks_fetched PGNSP PGUID 12 1 0 f f t
 DESCR("Statistics: Blocks fetched for database");
 DATA(insert OID = 1945 (  pg_stat_get_db_blocks_hit		PGNSP PGUID 12 1 0 f f t f s 1 20 "26" _null_ _null_ _null_ pg_stat_get_db_blocks_hit - _null_ ));
 DESCR("Statistics: Blocks found in cache for database");
+DATA(insert OID = 2230 (  pg_stat_clear_snapshot		PGNSP PGUID 12 1 0 f f f f v 0 2278  "" _null_ _null_ _null_	pg_stat_clear_snapshot - _null_ ));
+DESCR("Statistics: Discard current transaction's statistics snapshot");
+DATA(insert OID = 2274 (  pg_stat_reset					PGNSP PGUID 12 1 0 f f f f v 0 2278  "" _null_ _null_ _null_	pg_stat_reset - _null_ ));
+DESCR("Statistics: Reset collected statistics for current database");
 
 DATA(insert OID = 1946 (  encode						PGNSP PGUID 12 1 0 f f t f i 2 25 "17 25" _null_ _null_ _null_	binary_encode - _null_ ));
 DESCR("Convert bytea value into some ascii-only text string");
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index e64fb8bf550..896cc5ad68c 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -5,7 +5,7 @@
  *
  *	Copyright (c) 2001-2007, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/include/pgstat.h,v 1.52 2007/01/05 22:19:50 momjian Exp $
+ *	$PostgreSQL: pgsql/src/include/pgstat.h,v 1.53 2007/02/07 23:11:30 tgl Exp $
  * ----------
  */
 #ifndef PGSTAT_H
@@ -380,6 +380,7 @@ extern void pgstat_report_tabstat(void);
 extern void pgstat_vacuum_tabstat(void);
 extern void pgstat_drop_relation(Oid relid);
 
+extern void pgstat_clear_snapshot(void);
 extern void pgstat_reset_counters(void);
 
 extern void pgstat_report_autovac(Oid dboid);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index d6b17157b06..fc76d142259 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -27,61 +27,28 @@ SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
   FROM pg_catalog.pg_stat_user_tables AS t,
        pg_catalog.pg_statio_user_tables AS b
  WHERE t.relname='tenk2' AND b.relname='tenk2';
--- enable statistics
-SET stats_block_level = on;
-SET stats_row_level = on;
--- do a seqscan
-SELECT count(*) FROM tenk2;
- count 
--------
- 10000
-(1 row)
-
--- do an indexscan
-SELECT count(*) FROM tenk2 WHERE unique1 = 1;
- count 
--------
-     1
-(1 row)
-
--- All of the thrashing here is to wait for the stats collector to update,
--- without waiting too long (in fact, we'd like to try to measure how long
--- we wait).  Watching for change in the stats themselves wouldn't work
--- because the backend only reads them once per transaction.  The stats file
--- mod timestamp isn't too helpful because it may have resolution of only one
--- second, or even worse.  So, we touch a new table and then watch for change
--- in the size of the stats file.  Ugh.
--- save current stats-file size
-CREATE TEMP TABLE prevfilesize AS
-  SELECT size FROM pg_stat_file('global/pgstat.stat');
--- make and touch a previously nonexistent table
-CREATE TABLE stats_hack (f1 int);
-SELECT * FROM stats_hack;
- f1 
-----
-(0 rows)
-
--- wait for stats collector to update
+-- function to wait for counters to advance
 create function wait_for_stats() returns void as $$
 declare
   start_time timestamptz := clock_timestamp();
-  oldsize bigint;
-  newsize bigint;
+  updated bool;
 begin
-  -- fetch previous stats-file size
-  select size into oldsize from prevfilesize;
-
   -- we don't want to wait forever; loop will exit after 30 seconds
   for i in 1 .. 300 loop
 
-    -- look for update of stats file
-    select size into newsize from pg_stat_file('global/pgstat.stat');
+    -- check to see if indexscan has been sensed
+    SELECT (st.idx_scan >= pr.idx_scan + 1) INTO updated
+      FROM pg_stat_user_tables AS st, pg_class AS cl, prevstats AS pr
+     WHERE st.relname='tenk2' AND cl.relname='tenk2';
 
-    exit when newsize != oldsize;
+    exit when updated;
 
     -- wait a little
     perform pg_sleep(0.1);
 
+    -- reset stats snapshot so we can test again
+    perform pg_stat_clear_snapshot();
+
   end loop;
 
   -- report time waited in postmaster log (where it won't change test output)
@@ -89,13 +56,30 @@ begin
     extract(epoch from clock_timestamp() - start_time);
 end
 $$ language plpgsql;
+-- enable statistics
+SET stats_block_level = on;
+SET stats_row_level = on;
+-- do a seqscan
+SELECT count(*) FROM tenk2;
+ count 
+-------
+ 10000
+(1 row)
+
+-- do an indexscan
+SELECT count(*) FROM tenk2 WHERE unique1 = 1;
+ count 
+-------
+     1
+(1 row)
+
+-- wait for stats collector to update
 SELECT wait_for_stats();
  wait_for_stats 
 ----------------
  
 (1 row)
 
-DROP TABLE stats_hack;
 -- check effects
 SELECT st.seq_scan >= pr.seq_scan + 1,
        st.seq_tup_read >= pr.seq_tup_read + cl.reltuples,
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index dca0031470b..cde38b3a379 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -21,52 +21,28 @@ SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
        pg_catalog.pg_statio_user_tables AS b
  WHERE t.relname='tenk2' AND b.relname='tenk2';
 
--- enable statistics
-SET stats_block_level = on;
-SET stats_row_level = on;
-
--- do a seqscan
-SELECT count(*) FROM tenk2;
--- do an indexscan
-SELECT count(*) FROM tenk2 WHERE unique1 = 1;
-
--- All of the thrashing here is to wait for the stats collector to update,
--- without waiting too long (in fact, we'd like to try to measure how long
--- we wait).  Watching for change in the stats themselves wouldn't work
--- because the backend only reads them once per transaction.  The stats file
--- mod timestamp isn't too helpful because it may have resolution of only one
--- second, or even worse.  So, we touch a new table and then watch for change
--- in the size of the stats file.  Ugh.
-
--- save current stats-file size
-CREATE TEMP TABLE prevfilesize AS
-  SELECT size FROM pg_stat_file('global/pgstat.stat');
-
--- make and touch a previously nonexistent table
-CREATE TABLE stats_hack (f1 int);
-SELECT * FROM stats_hack;
-
--- wait for stats collector to update
+-- function to wait for counters to advance
 create function wait_for_stats() returns void as $$
 declare
   start_time timestamptz := clock_timestamp();
-  oldsize bigint;
-  newsize bigint;
+  updated bool;
 begin
-  -- fetch previous stats-file size
-  select size into oldsize from prevfilesize;
-
   -- we don't want to wait forever; loop will exit after 30 seconds
   for i in 1 .. 300 loop
 
-    -- look for update of stats file
-    select size into newsize from pg_stat_file('global/pgstat.stat');
+    -- check to see if indexscan has been sensed
+    SELECT (st.idx_scan >= pr.idx_scan + 1) INTO updated
+      FROM pg_stat_user_tables AS st, pg_class AS cl, prevstats AS pr
+     WHERE st.relname='tenk2' AND cl.relname='tenk2';
 
-    exit when newsize != oldsize;
+    exit when updated;
 
     -- wait a little
     perform pg_sleep(0.1);
 
+    -- reset stats snapshot so we can test again
+    perform pg_stat_clear_snapshot();
+
   end loop;
 
   -- report time waited in postmaster log (where it won't change test output)
@@ -75,9 +51,17 @@ begin
 end
 $$ language plpgsql;
 
-SELECT wait_for_stats();
+-- enable statistics
+SET stats_block_level = on;
+SET stats_row_level = on;
 
-DROP TABLE stats_hack;
+-- do a seqscan
+SELECT count(*) FROM tenk2;
+-- do an indexscan
+SELECT count(*) FROM tenk2 WHERE unique1 = 1;
+
+-- wait for stats collector to update
+SELECT wait_for_stats();
 
 -- check effects
 SELECT st.seq_scan >= pr.seq_scan + 1,