Teach heapam code to know the difference between a real seqscan and the

pseudo HeapScanDesc created for a bitmap heap scan.  This avoids some useless
overhead during a bitmap scan startup, in particular invoking the syncscan
code.  (We might someday want to do that, but right now it's merely useless
contention for shared memory, to say nothing of possibly pushing useful
entries out of syncscan's small LRU list.)  This also allows elimination of
ugly pgstat_discount_heap_scan() kluge.

src/backend/access/heap/heapam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.235 2007/06/08 18:23:52 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.236 2007/06/09 18:49:54 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -58,6 +58,10 @@
 #include "utils/syscache.h"
 
 
+static HeapScanDesc heap_beginscan_internal(Relation relation,
+											Snapshot snapshot,
+											int nkeys, ScanKey key,
+											bool is_bitmapscan);
 static XLogRecPtr log_heap_update(Relation reln, Buffer oldbuf,
 		   ItemPointerData from, Buffer newbuf, HeapTuple newtup, bool move);
 
@@ -95,8 +99,9 @@ initscan(HeapScanDesc scan, ScanKey key)
 	 *
 	 * During a rescan, don't make a new strategy object if we don't have to.
 	 */
-	if (scan->rs_nblocks > NBuffers / 4 &&
-		!scan->rs_rd->rd_istemp)
+	if (!scan->rs_bitmapscan &&
+		!scan->rs_rd->rd_istemp &&
+		scan->rs_nblocks > NBuffers / 4)
 	{
 		if (scan->rs_strategy == NULL)
 			scan->rs_strategy = GetAccessStrategy(BAS_BULKREAD);
@@ -114,8 +119,6 @@ initscan(HeapScanDesc scan, ScanKey key)
 		scan->rs_startblock = 0;
 	}
 
-	/* rs_pageatatime was set when the snapshot was filled in */
-
 	scan->rs_inited = false;
 	scan->rs_ctup.t_data = NULL;
 	ItemPointerSetInvalid(&scan->rs_ctup.t_self);
@@ -133,7 +136,12 @@ initscan(HeapScanDesc scan, ScanKey key)
 	if (key != NULL)
 		memcpy(scan->rs_key, key, scan->rs_nkeys * sizeof(ScanKeyData));
 
-	pgstat_count_heap_scan(scan->rs_rd);
+	/*
+	 * Currently, we don't have a stats counter for bitmap heap scans
+	 * (but the underlying bitmap index scans will be counted).
+	 */
+	if (!scan->rs_bitmapscan)
+		pgstat_count_heap_scan(scan->rs_rd);
 }
 
 /*
@@ -1037,11 +1045,30 @@ heap_openrv(const RangeVar *relation, LOCKMODE lockmode)
 
 /* ----------------
  *		heap_beginscan	- begin relation scan
+ *
+ * heap_beginscan_bm is an alternate entry point for setting up a HeapScanDesc
+ * for a bitmap heap scan.  Although that scan technology is really quite
+ * unlike a standard seqscan, there is just enough commonality to make it
+ * worth using the same data structure.
  * ----------------
  */
 HeapScanDesc
 heap_beginscan(Relation relation, Snapshot snapshot,
 			   int nkeys, ScanKey key)
+{
+	return heap_beginscan_internal(relation, snapshot, nkeys, key, false);
+}
+
+HeapScanDesc
+heap_beginscan_bm(Relation relation, Snapshot snapshot,
+				  int nkeys, ScanKey key)
+{
+	return heap_beginscan_internal(relation, snapshot, nkeys, key, true);
+}
+
+static HeapScanDesc
+heap_beginscan_internal(Relation relation, Snapshot snapshot,
+						int nkeys, ScanKey key, bool is_bitmapscan)
 {
 	HeapScanDesc scan;
 
@@ -1062,6 +1089,7 @@ heap_beginscan(Relation relation, Snapshot snapshot,
 	scan->rs_rd = relation;
 	scan->rs_snapshot = snapshot;
 	scan->rs_nkeys = nkeys;
+	scan->rs_bitmapscan = is_bitmapscan;
 	scan->rs_strategy = NULL;	/* set in initscan */
 
 	/*

src/backend/executor/nodeBitmapHeapscan.c

@@ -21,7 +21,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapHeapscan.c,v 1.17 2007/05/27 03:50:39 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapHeapscan.c,v 1.18 2007/06/09 18:49:55 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -388,9 +388,6 @@ ExecBitmapHeapReScan(BitmapHeapScanState *node, ExprContext *exprCtxt)
 	/* rescan to release any page pin */
 	heap_rescan(node->ss.ss_currentScanDesc, NULL);
 
-	/* undo bogus "seq scan" count (see notes in ExecInitBitmapHeapScan) */
-	pgstat_discount_heap_scan(node->ss.ss_currentScanDesc->rs_rd);
-
 	if (node->tbm)
 		tbm_free(node->tbm);
 	node->tbm = NULL;
@@ -522,20 +519,12 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 
 	/*
 	 * Even though we aren't going to do a conventional seqscan, it is useful
-	 * to create a HeapScanDesc --- this checks the relation size and sets up
-	 * statistical infrastructure for us.
-	 */
-	scanstate->ss.ss_currentScanDesc = heap_beginscan(currentRelation,
-													  estate->es_snapshot,
-													  0,
-													  NULL);
-
-	/*
-	 * One problem is that heap_beginscan counts a "sequential scan" start,
-	 * when we actually aren't doing any such thing.  Reverse out the added
-	 * scan count.	(Eventually we may want to count bitmap scans separately.)
+	 * to create a HeapScanDesc --- most of the fields in it are usable.
 	 */
-	pgstat_discount_heap_scan(scanstate->ss.ss_currentScanDesc->rs_rd);
+	scanstate->ss.ss_currentScanDesc = heap_beginscan_bm(currentRelation,
+														 estate->es_snapshot,
+														 0,
+														 NULL);
 
 	/*
 	 * get the scan type from the relation descriptor.

src/include/access/heapam.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/access/heapam.h,v 1.125 2007/06/08 18:23:53 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/heapam.h,v 1.126 2007/06/09 18:49:55 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -141,6 +141,8 @@ extern Relation heap_openrv(const RangeVar *relation, LOCKMODE lockmode);
 
 extern HeapScanDesc heap_beginscan(Relation relation, Snapshot snapshot,
 			   int nkeys, ScanKey key);
+extern HeapScanDesc heap_beginscan_bm(Relation relation, Snapshot snapshot,
+			   int nkeys, ScanKey key);
 extern void heap_rescan(HeapScanDesc scan, ScanKey key);
 extern void heap_endscan(HeapScanDesc scan);
 extern HeapTuple heap_getnext(HeapScanDesc scan, ScanDirection direction);

src/include/access/relscan.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/access/relscan.h,v 1.55 2007/06/08 18:23:53 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/relscan.h,v 1.56 2007/06/09 18:49:55 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -26,12 +26,13 @@ typedef struct HeapScanDescData
 	Snapshot	rs_snapshot;	/* snapshot to see */
 	int			rs_nkeys;		/* number of scan keys */
 	ScanKey		rs_key;			/* array of scan key descriptors */
+	bool		rs_bitmapscan;	/* true if this is really a bitmap scan */
+	bool		rs_pageatatime;	/* verify visibility page-at-a-time? */
 
 	/* state set up at initscan time */
 	BlockNumber rs_nblocks;		/* number of blocks to scan */
 	BlockNumber	rs_startblock;	/* block # to start at */
 	BufferAccessStrategy rs_strategy;	/* access strategy for reads */
-	bool		rs_pageatatime; /* verify visibility page-at-a-time? */
 	bool		rs_syncscan;	/* report location to syncscan logic? */
 
 	/* scan current state */
@@ -42,7 +43,7 @@ typedef struct HeapScanDescData
 	/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	ItemPointerData rs_mctid;	/* marked scan position, if any */
 
-	/* these fields only used in page-at-a-time mode */
+	/* these fields only used in page-at-a-time mode and for bitmap scans */
 	int			rs_cindex;		/* current tuple's index in vistuples */
 	int			rs_mindex;		/* marked tuple's saved index */
 	int			rs_ntuples;		/* number of visible tuples on page */

src/include/pgstat.h

@@ -5,7 +5,7 @@
  *
  *	Copyright (c) 2001-2007, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/include/pgstat.h,v 1.61 2007/05/27 17:28:36 tgl Exp $
+ *	$PostgreSQL: pgsql/src/include/pgstat.h,v 1.62 2007/06/09 18:49:55 tgl Exp $
  * ----------
  */
 #ifndef PGSTAT_H
@@ -518,12 +518,6 @@ extern void pgstat_initstats(Relation rel);
 		if (pgstat_collect_tuplelevel && (rel)->pgstat_info != NULL)	\
 			(rel)->pgstat_info->t_counts.t_numscans++;					\
 	} while (0)
-/* kluge for bitmap scans: */
-#define pgstat_discount_heap_scan(rel)									\
-	do {																\
-		if (pgstat_collect_tuplelevel && (rel)->pgstat_info != NULL)	\
-			(rel)->pgstat_info->t_counts.t_numscans--;					\
-	} while (0)
 #define pgstat_count_heap_getnext(rel)									\
 	do {																\
 		if (pgstat_collect_tuplelevel && (rel)->pgstat_info != NULL)	\