Bitmap Table Scans use unified TBMIterator

With the repurposing of TBMIterator as an interface for both parallel
and serial iteration through TIDBitmaps in commit 7f9d4187e7,
bitmap table scans may now use it.

Modify bitmap table scan code to use the TBMIterator. This requires
moving around a bit of code, so a few variables are initialized
elsewhere.

Author: Melanie Plageman
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/c736f6aa-8b35-4e20-9621-62c7c82e2168%40vondra.me

src/backend/access/heap/heapam_handler.c

@@ -2135,10 +2135,7 @@ heapam_scan_bitmap_next_block(TableScanDesc scan,
 	{
 		CHECK_FOR_INTERRUPTS();
 
-		if (scan->st.bitmap.rs_shared_iterator)
-			tbmres = tbm_shared_iterate(scan->st.bitmap.rs_shared_iterator);
-		else
-			tbmres = tbm_private_iterate(scan->st.bitmap.rs_iterator);
+		tbmres = tbm_iterate(&scan->st.rs_tbmiterator);
 
 		if (tbmres == NULL)
 			return false;

src/backend/executor/nodeBitmapHeapscan.c

@@ -95,8 +95,7 @@ BitmapHeapNext(BitmapHeapScanState *node)
 	 */
 	if (!node->initialized)
 	{
-		TBMPrivateIterator *tbmiterator = NULL;
-		TBMSharedIterator *shared_tbmiterator = NULL;
+		TBMIterator tbmiterator;
 
 		if (!pstate)
 		{
@@ -106,70 +105,62 @@ BitmapHeapNext(BitmapHeapScanState *node)
 				elog(ERROR, "unrecognized result from subplan");
 
 			node->tbm = tbm;
-			tbmiterator = tbm_begin_private_iterate(tbm);
 
 #ifdef USE_PREFETCH
 			if (node->prefetch_maximum > 0)
-			{
-				node->prefetch_iterator = tbm_begin_private_iterate(tbm);
-				node->prefetch_pages = 0;
-				node->prefetch_target = -1;
-			}
+				node->prefetch_iterator =
+					tbm_begin_iterate(node->tbm, dsa,
+									  pstate ?
+									  pstate->prefetch_iterator :
+									  InvalidDsaPointer);
 #endif							/* USE_PREFETCH */
 		}
-		else
+		else if (BitmapShouldInitializeSharedState(pstate))
 		{
 			/*
 			 * The leader will immediately come out of the function, but
 			 * others will be blocked until leader populates the TBM and wakes
 			 * them up.
 			 */
-			if (BitmapShouldInitializeSharedState(pstate))
-			{
-				tbm = (TIDBitmap *) MultiExecProcNode(outerPlanState(node));
-				if (!tbm || !IsA(tbm, TIDBitmap))
-					elog(ERROR, "unrecognized result from subplan");
+			tbm = (TIDBitmap *) MultiExecProcNode(outerPlanState(node));
+			if (!tbm || !IsA(tbm, TIDBitmap))
+				elog(ERROR, "unrecognized result from subplan");
 
-				node->tbm = tbm;
+			node->tbm = tbm;
 
-				/*
-				 * Prepare to iterate over the TBM. This will return the
-				 * dsa_pointer of the iterator state which will be used by
-				 * multiple processes to iterate jointly.
-				 */
-				pstate->tbmiterator = tbm_prepare_shared_iterate(tbm);
-#ifdef USE_PREFETCH
-				if (node->prefetch_maximum > 0)
-				{
-					pstate->prefetch_iterator =
-						tbm_prepare_shared_iterate(tbm);
-
-					/*
-					 * We don't need the mutex here as we haven't yet woke up
-					 * others.
-					 */
-					pstate->prefetch_pages = 0;
-					pstate->prefetch_target = -1;
-				}
-#endif
-
-				/* We have initialized the shared state so wake up others. */
-				BitmapDoneInitializingSharedState(pstate);
-			}
-
-			/* Allocate a private iterator and attach the shared state to it */
-			shared_tbmiterator =
-				tbm_attach_shared_iterate(dsa, pstate->tbmiterator);
+			/*
+			 * Prepare to iterate over the TBM. This will return the
+			 * dsa_pointer of the iterator state which will be used by
+			 * multiple processes to iterate jointly.
+			 */
+			pstate->tbmiterator = tbm_prepare_shared_iterate(tbm);
 
 #ifdef USE_PREFETCH
 			if (node->prefetch_maximum > 0)
 			{
-				node->shared_prefetch_iterator =
-					tbm_attach_shared_iterate(dsa, pstate->prefetch_iterator);
+				pstate->prefetch_iterator =
+					tbm_prepare_shared_iterate(tbm);
 			}
 #endif							/* USE_PREFETCH */
+
+			/* We have initialized the shared state so wake up others. */
+			BitmapDoneInitializingSharedState(pstate);
 		}
 
+		tbmiterator = tbm_begin_iterate(tbm, dsa,
+										pstate ?
+										pstate->tbmiterator :
+										InvalidDsaPointer);
+
+#ifdef USE_PREFETCH
+		if (node->prefetch_maximum > 0)
+			node->prefetch_iterator =
+				tbm_begin_iterate(tbm, dsa,
+								  pstate ?
+								  pstate->prefetch_iterator :
+								  InvalidDsaPointer);
+#endif							/* USE_PREFETCH */
+
 		/*
 		 * If this is the first scan of the underlying table, create the table
 		 * scan descriptor and begin the scan.
@@ -198,8 +189,7 @@ BitmapHeapNext(BitmapHeapScanState *node)
 			node->ss.ss_currentScanDesc = scan;
 		}
 
-		scan->st.bitmap.rs_iterator = tbmiterator;
-		scan->st.bitmap.rs_shared_iterator = shared_tbmiterator;
+		scan->st.rs_tbmiterator = tbmiterator;
 		node->initialized = true;
 
 		goto new_page;
@@ -285,7 +275,7 @@ new_page:
 		 * ahead of the current block.
 		 */
 		if (node->pstate == NULL &&
-			node->prefetch_iterator &&
+			!tbm_exhausted(&node->prefetch_iterator) &&
 			node->prefetch_blockno < node->blockno)
 			elog(ERROR,
 				 "prefetch and main iterators are out of sync. pfblockno: %d. blockno: %d",
@@ -332,16 +322,16 @@ BitmapAdjustPrefetchIterator(BitmapHeapScanState *node)
 
 	if (pstate == NULL)
 	{
-		TBMPrivateIterator *prefetch_iterator = node->prefetch_iterator;
+		TBMIterator *prefetch_iterator = &node->prefetch_iterator;
 
 		if (node->prefetch_pages > 0)
 		{
 			/* The main iterator has closed the distance by one page */
 			node->prefetch_pages--;
 		}
-		else if (prefetch_iterator)
+		else if (!tbm_exhausted(prefetch_iterator))
 		{
-			tbmpre = tbm_private_iterate(prefetch_iterator);
+			tbmpre = tbm_iterate(prefetch_iterator);
 			node->prefetch_blockno = tbmpre ? tbmpre->blockno :
 				InvalidBlockNumber;
 		}
@@ -359,7 +349,7 @@ BitmapAdjustPrefetchIterator(BitmapHeapScanState *node)
 	 */
 	if (node->prefetch_maximum > 0)
 	{
-		TBMSharedIterator *prefetch_iterator = node->shared_prefetch_iterator;
+		TBMIterator *prefetch_iterator = &node->prefetch_iterator;
 
 		SpinLockAcquire(&pstate->mutex);
 		if (pstate->prefetch_pages > 0)
@@ -380,9 +370,9 @@ BitmapAdjustPrefetchIterator(BitmapHeapScanState *node)
 			 * we don't validate the blockno here as we do in non-parallel
 			 * case.
 			 */
-			if (prefetch_iterator)
+			if (!tbm_exhausted(prefetch_iterator))
 			{
-				tbmpre = tbm_shared_iterate(prefetch_iterator);
+				tbmpre = tbm_iterate(prefetch_iterator);
 				node->prefetch_blockno = tbmpre ? tbmpre->blockno :
 					InvalidBlockNumber;
 			}
@@ -446,21 +436,19 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan)
 
 	if (pstate == NULL)
 	{
-		TBMPrivateIterator *prefetch_iterator = node->prefetch_iterator;
+		TBMIterator *prefetch_iterator = &node->prefetch_iterator;
 
-		if (prefetch_iterator)
+		if (!tbm_exhausted(prefetch_iterator))
 		{
 			while (node->prefetch_pages < node->prefetch_target)
 			{
-				TBMIterateResult *tbmpre;
+				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
 				bool		skip_fetch;
 
-				tbmpre = tbm_private_iterate(prefetch_iterator);
 				if (tbmpre == NULL)
 				{
 					/* No more pages to prefetch */
-					tbm_end_private_iterate(prefetch_iterator);
-					node->prefetch_iterator = NULL;
+					tbm_end_iterate(prefetch_iterator);
 					break;
 				}
 				node->prefetch_pages++;
@@ -488,9 +476,9 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan)
 
 	if (pstate->prefetch_pages < pstate->prefetch_target)
 	{
-		TBMSharedIterator *prefetch_iterator = node->shared_prefetch_iterator;
+		TBMIterator *prefetch_iterator = &node->prefetch_iterator;
 
-		if (prefetch_iterator)
+		if (!tbm_exhausted(prefetch_iterator))
 		{
 			while (1)
 			{
@@ -513,12 +501,11 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan)
 				if (!do_prefetch)
 					return;
 
-				tbmpre = tbm_shared_iterate(prefetch_iterator);
+				tbmpre = tbm_iterate(prefetch_iterator);
 				if (tbmpre == NULL)
 				{
 					/* No more pages to prefetch */
-					tbm_end_shared_iterate(prefetch_iterator);
-					node->shared_prefetch_iterator = NULL;
+					tbm_end_iterate(prefetch_iterator);
 					break;
 				}
 
@@ -587,39 +574,30 @@ ExecReScanBitmapHeapScan(BitmapHeapScanState *node)
 		/*
 		 * End iteration on iterators saved in scan descriptor.
 		 */
-		if (scan->st.bitmap.rs_shared_iterator)
-		{
-			tbm_end_shared_iterate(scan->st.bitmap.rs_shared_iterator);
-			scan->st.bitmap.rs_shared_iterator = NULL;
-		}
-
-		if (scan->st.bitmap.rs_iterator)
-		{
-			tbm_end_private_iterate(scan->st.bitmap.rs_iterator);
-			scan->st.bitmap.rs_iterator = NULL;
-		}
+		tbm_end_iterate(&scan->st.rs_tbmiterator);
 
 		/* rescan to release any page pin */
 		table_rescan(node->ss.ss_currentScanDesc, NULL);
 	}
 
+	/* If we did not already clean up the prefetch iterator, do so now. */
+	if (!tbm_exhausted(&node->prefetch_iterator))
+		tbm_end_iterate(&node->prefetch_iterator);
+
 	/* release bitmaps and buffers if any */
-	if (node->prefetch_iterator)
-		tbm_end_private_iterate(node->prefetch_iterator);
-	if (node->shared_prefetch_iterator)
-		tbm_end_shared_iterate(node->shared_prefetch_iterator);
 	if (node->tbm)
 		tbm_free(node->tbm);
 	if (node->pvmbuffer != InvalidBuffer)
 		ReleaseBuffer(node->pvmbuffer);
 	node->tbm = NULL;
-	node->prefetch_iterator = NULL;
 	node->initialized = false;
-	node->shared_prefetch_iterator = NULL;
 	node->pvmbuffer = InvalidBuffer;
 	node->recheck = true;
+	/* Only used for serial BHS */
 	node->blockno = InvalidBlockNumber;
 	node->prefetch_blockno = InvalidBlockNumber;
+	node->prefetch_pages = 0;
+	node->prefetch_target = -1;
 
 	ExecScanReScan(&node->ss);
 
@@ -678,17 +656,7 @@ ExecEndBitmapHeapScan(BitmapHeapScanState *node)
 		/*
 		 * End iteration on iterators saved in scan descriptor.
 		 */
-		if (scanDesc->st.bitmap.rs_shared_iterator)
-		{
-			tbm_end_shared_iterate(scanDesc->st.bitmap.rs_shared_iterator);
-			scanDesc->st.bitmap.rs_shared_iterator = NULL;
-		}
-
-		if (scanDesc->st.bitmap.rs_iterator)
-		{
-			tbm_end_private_iterate(scanDesc->st.bitmap.rs_iterator);
-			scanDesc->st.bitmap.rs_iterator = NULL;
-		}
+		tbm_end_iterate(&scanDesc->st.rs_tbmiterator);
 
 		/*
 		 * close table scan
@@ -696,15 +664,15 @@ ExecEndBitmapHeapScan(BitmapHeapScanState *node)
 		table_endscan(scanDesc);
 	}
 
+	/* If we did not already clean up the prefetch iterator, do so now. */
+	if (!tbm_exhausted(&node->prefetch_iterator))
+		tbm_end_iterate(&node->prefetch_iterator);
+
 	/*
 	 * release bitmaps and buffers if any
 	 */
-	if (node->prefetch_iterator)
-		tbm_end_private_iterate(node->prefetch_iterator);
 	if (node->tbm)
 		tbm_free(node->tbm);
-	if (node->shared_prefetch_iterator)
-		tbm_end_shared_iterate(node->shared_prefetch_iterator);
 	if (node->pvmbuffer != InvalidBuffer)
 		ReleaseBuffer(node->pvmbuffer);
 }
@@ -744,11 +712,9 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 	/* Zero the statistics counters */
 	memset(&scanstate->stats, 0, sizeof(BitmapHeapScanInstrumentation));
 
-	scanstate->prefetch_iterator = NULL;
 	scanstate->prefetch_pages = 0;
-	scanstate->prefetch_target = 0;
+	scanstate->prefetch_target = -1;
 	scanstate->initialized = false;
-	scanstate->shared_prefetch_iterator = NULL;
 	scanstate->pstate = NULL;
 	scanstate->recheck = true;
 	scanstate->blockno = InvalidBlockNumber;
@@ -907,7 +873,7 @@ ExecBitmapHeapInitializeDSM(BitmapHeapScanState *node,
 	/* Initialize the mutex */
 	SpinLockInit(&pstate->mutex);
 	pstate->prefetch_pages = 0;
-	pstate->prefetch_target = 0;
+	pstate->prefetch_target = -1;
 	pstate->state = BM_INITIAL;
 
 	ConditionVariableInit(&pstate->cv);
@@ -944,6 +910,8 @@ ExecBitmapHeapReInitializeDSM(BitmapHeapScanState *node,
 		return;
 
 	pstate->state = BM_INITIAL;
+	pstate->prefetch_pages = 0;
+	pstate->prefetch_target = -1;
 
 	if (DsaPointerIsValid(pstate->tbmiterator))
 		tbm_free_shared_area(dsa, pstate->tbmiterator);

src/include/access/relscan.h

@@ -16,6 +16,7 @@
 
 #include "access/htup_details.h"
 #include "access/itup.h"
+#include "nodes/tidbitmap.h"
 #include "port/atomics.h"
 #include "storage/buf.h"
 #include "storage/relfilelocator.h"
@@ -25,9 +26,6 @@
 
 struct ParallelTableScanDescData;
 
-struct TBMPrivateIterator;
-struct TBMSharedIterator;
-
 /*
  * Generic descriptor for table scans. This is the base-class for table scans,
  * which needs to be embedded in the scans of individual AMs.
@@ -45,12 +43,8 @@ typedef struct TableScanDescData
 	 */
 	union
 	{
-		/* Iterators for Bitmap Table Scans */
-		struct
-		{
-			struct TBMPrivateIterator *rs_iterator;
-			struct TBMSharedIterator *rs_shared_iterator;
-		}			bitmap;
+		/* Iterator for Bitmap Table Scans */
+		TBMIterator rs_tbmiterator;
 
 		/*
 		 * Range of ItemPointers for table_scan_getnextslot_tidrange() to

src/include/access/tableam.h

@@ -957,17 +957,13 @@ static inline TableScanDesc
 table_beginscan_bm(Relation rel, Snapshot snapshot,
 				   int nkeys, struct ScanKeyData *key, bool need_tuple)
 {
-	TableScanDesc result;
 	uint32		flags = SO_TYPE_BITMAPSCAN | SO_ALLOW_PAGEMODE;
 
 	if (need_tuple)
 		flags |= SO_NEED_TUPLES;
 
-	result = rel->rd_tableam->scan_begin(rel, snapshot, nkeys, key,
-										 NULL, flags);
-	result->st.bitmap.rs_shared_iterator = NULL;
-	result->st.bitmap.rs_iterator = NULL;
-	return result;
+	return rel->rd_tableam->scan_begin(rel, snapshot, nkeys, key,
+									   NULL, flags);
 }
 
 /*

src/include/nodes/execnodes.h

@@ -1838,7 +1838,6 @@ typedef struct SharedBitmapHeapInstrumentation
  *		prefetch_target    current target prefetch distance
  *		prefetch_maximum   maximum value for prefetch_target
  *		initialized		   is node is ready to iterate
- *		shared_prefetch_iterator shared iterator for prefetching
  *		pstate			   shared state for parallel bitmap scan
  *		sinstrument		   statistics for parallel workers
  *		recheck			   do current page's tuples need recheck
@@ -1853,12 +1852,11 @@ typedef struct BitmapHeapScanState
 	TIDBitmap  *tbm;
 	Buffer		pvmbuffer;
 	BitmapHeapScanInstrumentation stats;
-	TBMPrivateIterator *prefetch_iterator;
+	TBMIterator prefetch_iterator;
 	int			prefetch_pages;
 	int			prefetch_target;
 	int			prefetch_maximum;
 	bool		initialized;
-	TBMSharedIterator *shared_prefetch_iterator;
 	ParallelBitmapHeapState *pstate;
 	SharedBitmapHeapInstrumentation *sinstrument;
 	bool		recheck;

src/include/nodes/tidbitmap.h

@@ -92,4 +92,14 @@ extern void tbm_end_iterate(TBMIterator *iterator);
 
 extern TBMIterateResult *tbm_iterate(TBMIterator *iterator);
 
+static inline bool
+tbm_exhausted(TBMIterator *iterator)
+{
+	/*
+	 * It doesn't matter if we check the private or shared iterator here. If
+	 * tbm_end_iterate() was called, they will be NULL
+	 */
+	return !iterator->i.private_iterator;
+}
+
 #endif							/* TIDBITMAP_H */