Change seqscan logic so that we check visibility of all tuples on a page

when we first read the page, rather than checking them one at a time.
This allows us to take and release the buffer content lock just once
per page, instead of once per tuple.  Since it's a shared lock the
contention penalty for holding the lock longer shouldn't be too bad.
We can safely do this only when using an MVCC snapshot; else the
assumption that visibility won't change over time is uncool.  Therefore
there are now two code paths depending on the snapshot type.  I also
made the same change in nodeBitmapHeapscan.c, where it can be done always
because we only support MVCC snapshots for bitmap scans anyway.
Also make some incidental cleanups in the APIs of these functions.
Per a suggestion from Qingqing Zhou.

1 files changed, 161 insertions, 86 deletions

diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
index 3d4f7d38392..16b0453023f 100644
--- a/src/backend/executor/nodeBitmapHeapscan.c
+++ b/src/backend/executor/nodeBitmapHeapscan.c
@@ -21,7 +21,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapHeapscan.c,v 1.5 2005/11/25 04:24:48 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapHeapscan.c,v 1.6 2005/11/26 03:03:07 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -44,6 +44,7 @@
 
 
 static TupleTableSlot *BitmapHeapNext(BitmapHeapScanState *node);
+static void bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres);
 
 
 /* ----------------------------------------------------------------
@@ -57,7 +58,7 @@ BitmapHeapNext(BitmapHeapScanState *node)
 {
 	EState	   *estate;
 	ExprContext *econtext;
-	HeapScanDesc scandesc;
+	HeapScanDesc scan;
 	Index		scanrelid;
 	TIDBitmap  *tbm;
 	TBMIterateResult *tbmres;
@@ -70,7 +71,7 @@ BitmapHeapNext(BitmapHeapScanState *node)
 	estate = node->ss.ps.state;
 	econtext = node->ss.ps.ps_ExprContext;
 	slot = node->ss.ss_ScanTupleSlot;
-	scandesc = node->ss.ss_currentScanDesc;
+	scan = node->ss.ss_currentScanDesc;
 	scanrelid = ((BitmapHeapScan *) node->ss.ps.plan)->scan.scanrelid;
 	tbm = node->tbm;
 	tbmres = node->tbmres;
@@ -123,6 +124,9 @@ BitmapHeapNext(BitmapHeapScanState *node)
 
 	for (;;)
 	{
+		Page		dp;
+		ItemId		lp;
+
 		/*
 		 * Get next page of results if needed
 		 */
@@ -141,134 +145,199 @@ BitmapHeapNext(BitmapHeapScanState *node)
 			 * AccessShareLock before performing any of the indexscans, but
 			 * let's be safe.)
 			 */
-			if (tbmres->blockno >= scandesc->rs_nblocks)
+			if (tbmres->blockno >= scan->rs_nblocks)
 			{
 				node->tbmres = tbmres = NULL;
 				continue;
 			}
 
 			/*
-			 * Acquire pin on the current heap page.  We'll hold the pin until
-			 * done looking at the page.  We trade in any pin we held before.
+			 * Fetch the current heap page and identify candidate tuples.
 			 */
-			scandesc->rs_cbuf = ReleaseAndReadBuffer(scandesc->rs_cbuf,
-													 scandesc->rs_rd,
-													 tbmres->blockno);
-
-			/*
-			 * Determine how many entries we need to look at on this page. If
-			 * the bitmap is lossy then we need to look at each physical item
-			 * pointer; otherwise we just look through the offsets listed in
-			 * tbmres.
-			 */
-			if (tbmres->ntuples >= 0)
-			{
-				/* non-lossy case */
-				node->minslot = 0;
-				node->maxslot = tbmres->ntuples - 1;
-			}
-			else
-			{
-				/* lossy case */
-				Page		dp;
-
-				LockBuffer(scandesc->rs_cbuf, BUFFER_LOCK_SHARE);
-				dp = (Page) BufferGetPage(scandesc->rs_cbuf);
-
-				node->minslot = FirstOffsetNumber;
-				node->maxslot = PageGetMaxOffsetNumber(dp);
-
-				LockBuffer(scandesc->rs_cbuf, BUFFER_LOCK_UNLOCK);
-			}
+			bitgetpage(scan, tbmres);
 
 			/*
-			 * Set curslot to first slot to examine
+			 * Set rs_cindex to first slot to examine
 			 */
-			node->curslot = node->minslot;
+			scan->rs_cindex = 0;
 		}
 		else
 		{
 			/*
-			 * Continuing in previously obtained page; advance curslot
+			 * Continuing in previously obtained page; advance rs_cindex
 			 */
-			node->curslot++;
+			scan->rs_cindex++;
 		}
 
 		/*
 		 * Out of range?  If so, nothing more to look at on this page
 		 */
-		if (node->curslot < node->minslot || node->curslot > node->maxslot)
+		if (scan->rs_cindex < 0 || scan->rs_cindex >= scan->rs_ntuples)
 		{
 			node->tbmres = tbmres = NULL;
 			continue;
 		}
 
 		/*
-		 * Okay to try to fetch the tuple
+		 * Okay to fetch the tuple
+		 */
+		targoffset = scan->rs_vistuples[scan->rs_cindex];
+		dp = (Page) BufferGetPage(scan->rs_cbuf);
+		lp = PageGetItemId(dp, targoffset);
+		Assert(ItemIdIsUsed(lp));
+
+		scan->rs_ctup.t_data = (HeapTupleHeader) PageGetItem((Page) dp, lp);
+		scan->rs_ctup.t_len = ItemIdGetLength(lp);
+		ItemPointerSet(&scan->rs_ctup.t_self, tbmres->blockno, targoffset);
+
+		pgstat_count_heap_fetch(&scan->rs_pgstat_info);
+
+		/*
+		 * Set up the result slot to point to this tuple. Note that the
+		 * slot acquires a pin on the buffer.
 		 */
+		ExecStoreTuple(&scan->rs_ctup,
+					   slot,
+					   scan->rs_cbuf,
+					   false);
+
+		/*
+		 * If we are using lossy info, we have to recheck the qual
+		 * conditions at every tuple.
+		 */
+		if (tbmres->ntuples < 0)
+		{
+			econtext->ecxt_scantuple = slot;
+			ResetExprContext(econtext);
+
+			if (!ExecQual(node->bitmapqualorig, econtext, false))
+			{
+				/* Fails recheck, so drop it and loop back for another */
+				ExecClearTuple(slot);
+				continue;
+			}
+		}
+
+		/* OK to return this tuple */
+		return slot;
+	}
+
+	/*
+	 * if we get here it means we are at the end of the scan..
+	 */
+	return ExecClearTuple(slot);
+}
+
+/*
+ * bitgetpage - subroutine for BitmapHeapNext()
+ *
+ * This routine reads and pins the specified page of the relation, then
+ * builds an array indicating which tuples on the page are both potentially
+ * interesting according to the bitmap, and visible according to the snapshot.
+ */
+static void
+bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres)
+{
+	BlockNumber	page = tbmres->blockno;
+	Buffer		buffer;
+	Snapshot	snapshot;
+	Page		dp;
+	int			ntup;
+	int			curslot;
+	int			minslot;
+	int			maxslot;
+	int			maxoff;
+
+	/*
+	 * Acquire pin on the target heap page, trading in any pin we held before.
+	 */
+	Assert(page < scan->rs_nblocks);
+
+	scan->rs_cbuf = ReleaseAndReadBuffer(scan->rs_cbuf,
+										 scan->rs_rd,
+										 page);
+	buffer = scan->rs_cbuf;
+	snapshot = scan->rs_snapshot;
+
+	/*
+	 * We must hold share lock on the buffer content while examining
+	 * tuple visibility.  Afterwards, however, the tuples we have found
+	 * to be visible are guaranteed good as long as we hold the buffer pin.
+	 */
+	LockBuffer(buffer, BUFFER_LOCK_SHARE);
+
+	dp = (Page) BufferGetPage(buffer);
+	maxoff = PageGetMaxOffsetNumber(dp);
+
+	/*
+	 * Determine how many entries we need to look at on this page. If
+	 * the bitmap is lossy then we need to look at each physical item
+	 * pointer; otherwise we just look through the offsets listed in
+	 * tbmres.
+	 */
+	if (tbmres->ntuples >= 0)
+	{
+		/* non-lossy case */
+		minslot = 0;
+		maxslot = tbmres->ntuples - 1;
+	}
+	else
+	{
+		/* lossy case */
+		minslot = FirstOffsetNumber;
+		maxslot = maxoff;
+	}
+
+	ntup = 0;
+	for (curslot = minslot; curslot <= maxslot; curslot++)
+	{
+		OffsetNumber targoffset;
+		ItemId		lp;
+		HeapTupleData loctup;
+		bool		valid;
+
 		if (tbmres->ntuples >= 0)
 		{
 			/* non-lossy case */
-			targoffset = tbmres->offsets[node->curslot];
+			targoffset = tbmres->offsets[curslot];
 		}
 		else
 		{
 			/* lossy case */
-			targoffset = (OffsetNumber) node->curslot;
+			targoffset = (OffsetNumber) curslot;
 		}
 
-		ItemPointerSet(&scandesc->rs_ctup.t_self, tbmres->blockno, targoffset);
-
 		/*
-		 * Fetch the heap tuple and see if it matches the snapshot. We use
-		 * heap_release_fetch to avoid useless bufmgr traffic.
+		 * We'd better check for out-of-range offnum in case of VACUUM since
+		 * the TID was obtained.
 		 */
-		if (heap_release_fetch(scandesc->rs_rd,
-							   scandesc->rs_snapshot,
-							   &scandesc->rs_ctup,
-							   &scandesc->rs_cbuf,
-							   true,
-							   &scandesc->rs_pgstat_info))
-		{
-			/*
-			 * Set up the result slot to point to this tuple. Note that the
-			 * slot acquires a pin on the buffer.
-			 */
-			ExecStoreTuple(&scandesc->rs_ctup,
-						   slot,
-						   scandesc->rs_cbuf,
-						   false);
+		if (targoffset < FirstOffsetNumber || targoffset > maxoff)
+			continue;
 
-			/*
-			 * If we are using lossy info, we have to recheck the qual
-			 * conditions at every tuple.
-			 */
-			if (tbmres->ntuples < 0)
-			{
-				econtext->ecxt_scantuple = slot;
-				ResetExprContext(econtext);
-
-				if (!ExecQual(node->bitmapqualorig, econtext, false))
-				{
-					/* Fails recheck, so drop it and loop back for another */
-					ExecClearTuple(slot);
-					continue;
-				}
-			}
+		lp = PageGetItemId(dp, targoffset);
 
-			/* OK to return this tuple */
-			return slot;
-		}
+		/*
+		 * Must check for deleted tuple.
+		 */
+		if (!ItemIdIsUsed(lp))
+			continue;
 
 		/*
-		 * Failed the snap, so loop back and try again.
+		 * check time qualification of tuple, remember it if valid
 		 */
+		loctup.t_data = (HeapTupleHeader) PageGetItem((Page) dp, lp);
+		loctup.t_len = ItemIdGetLength(lp);
+		ItemPointerSet(&(loctup.t_self), page, targoffset);
+
+		valid = HeapTupleSatisfiesVisibility(&loctup, snapshot, buffer);
+		if (valid)
+			scan->rs_vistuples[ntup++] = targoffset;
 	}
 
-	/*
-	 * if we get here it means we are at the end of the scan..
-	 */
-	return ExecClearTuple(slot);
+	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+
+	Assert(ntup <= MaxHeapTuplesPerPage);
+	scan->rs_ntuples = ntup;
 }
 
 /* ----------------------------------------------------------------
@@ -404,6 +473,12 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate)
 	Relation	currentRelation;
 
 	/*
+	 * Assert caller didn't ask for an unsafe snapshot --- see comments
+	 * at head of file.
+	 */
+	Assert(IsMVCCSnapshot(estate->es_snapshot));
+
+	/*
 	 * create state structure
 	 */
 	scanstate = makeNode(BitmapHeapScanState);