Add and use BitmapHeapScanDescData struct

Move the several members of HeapScanDescData which are specific to
Bitmap Heap Scans into a new struct, BitmapHeapScanDescData, which
inherits from HeapScanDescData.

This reduces the size of the HeapScanDescData for other types of scans
and will allow us to add additional bitmap heap scan-specific members in
the future without fear of bloating the HeapScanDescData.

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

src/backend/access/heap/heapam.c

@@ -1048,7 +1048,16 @@ heap_beginscan(Relation relation, Snapshot snapshot,
 	/*
 	 * allocate and initialize scan descriptor
 	 */
-	scan = (HeapScanDesc) palloc(sizeof(HeapScanDescData));
+	if (flags & SO_TYPE_BITMAPSCAN)
+	{
+		BitmapHeapScanDesc bscan = palloc(sizeof(BitmapHeapScanDescData));
+
+		bscan->rs_vmbuffer = InvalidBuffer;
+		bscan->rs_empty_tuples_pending = 0;
+		scan = (HeapScanDesc) bscan;
+	}
+	else
+		scan = (HeapScanDesc) palloc(sizeof(HeapScanDescData));
 
 	scan->rs_base.rs_rd = relation;
 	scan->rs_base.rs_snapshot = snapshot;
@@ -1056,8 +1065,6 @@ heap_beginscan(Relation relation, Snapshot snapshot,
 	scan->rs_base.rs_flags = flags;
 	scan->rs_base.rs_parallel = parallel_scan;
 	scan->rs_strategy = NULL;	/* set in initscan */
-	scan->rs_vmbuffer = InvalidBuffer;
-	scan->rs_empty_tuples_pending = 0;
 
 	/*
 	 * Disable page-at-a-time mode if it's not a MVCC-safe snapshot.
@@ -1173,18 +1180,23 @@ heap_rescan(TableScanDesc sscan, ScanKey key, bool set_params,
 	if (BufferIsValid(scan->rs_cbuf))
 		ReleaseBuffer(scan->rs_cbuf);
 
-	if (BufferIsValid(scan->rs_vmbuffer))
+	if (scan->rs_base.rs_flags & SO_TYPE_BITMAPSCAN)
 	{
-		ReleaseBuffer(scan->rs_vmbuffer);
-		scan->rs_vmbuffer = InvalidBuffer;
-	}
+		BitmapHeapScanDesc bscan = (BitmapHeapScanDesc) scan;
 
-	/*
-	 * Reset rs_empty_tuples_pending, a field only used by bitmap heap scan,
-	 * to avoid incorrectly emitting NULL-filled tuples from a previous scan
-	 * on rescan.
-	 */
-	scan->rs_empty_tuples_pending = 0;
+		/*
+		 * Reset empty_tuples_pending, a field only used by bitmap heap scan,
+		 * to avoid incorrectly emitting NULL-filled tuples from a previous
+		 * scan on rescan.
+		 */
+		bscan->rs_empty_tuples_pending = 0;
+
+		if (BufferIsValid(bscan->rs_vmbuffer))
+		{
+			ReleaseBuffer(bscan->rs_vmbuffer);
+			bscan->rs_vmbuffer = InvalidBuffer;
+		}
+	}
 
 	/*
 	 * The read stream is reset on rescan. This must be done before
@@ -1213,8 +1225,14 @@ heap_endscan(TableScanDesc sscan)
 	if (BufferIsValid(scan->rs_cbuf))
 		ReleaseBuffer(scan->rs_cbuf);
 
-	if (BufferIsValid(scan->rs_vmbuffer))
-		ReleaseBuffer(scan->rs_vmbuffer);
+	if (scan->rs_base.rs_flags & SO_TYPE_BITMAPSCAN)
+	{
+		BitmapHeapScanDesc bscan = (BitmapHeapScanDesc) sscan;
+
+		bscan->rs_empty_tuples_pending = 0;
+		if (BufferIsValid(bscan->rs_vmbuffer))
+			ReleaseBuffer(bscan->rs_vmbuffer);
+	}
 
 	/*
 	 * Must free the read stream before freeing the BufferAccessStrategy.

src/backend/access/heap/heapam_handler.c

@@ -2118,13 +2118,16 @@ heapam_scan_bitmap_next_block(TableScanDesc scan,
 							  BlockNumber *blockno, bool *recheck,
 							  uint64 *lossy_pages, uint64 *exact_pages)
 {
-	HeapScanDesc hscan = (HeapScanDesc) scan;
+	BitmapHeapScanDesc bscan = (BitmapHeapScanDesc) scan;
+	HeapScanDesc hscan = (HeapScanDesc) bscan;
 	BlockNumber block;
 	Buffer		buffer;
 	Snapshot	snapshot;
 	int			ntup;
 	TBMIterateResult *tbmres;
 
+	Assert(scan->rs_flags & SO_TYPE_BITMAPSCAN);
+
 	hscan->rs_cindex = 0;
 	hscan->rs_ntuples = 0;
 
@@ -2162,13 +2165,13 @@ heapam_scan_bitmap_next_block(TableScanDesc scan,
 	 */
 	if (!(scan->rs_flags & SO_NEED_TUPLES) &&
 		!tbmres->recheck &&
-		VM_ALL_VISIBLE(scan->rs_rd, tbmres->blockno, &hscan->rs_vmbuffer))
+		VM_ALL_VISIBLE(scan->rs_rd, tbmres->blockno, &bscan->rs_vmbuffer))
 	{
 		/* can't be lossy in the skip_fetch case */
 		Assert(tbmres->ntuples >= 0);
-		Assert(hscan->rs_empty_tuples_pending >= 0);
+		Assert(bscan->rs_empty_tuples_pending >= 0);
 
-		hscan->rs_empty_tuples_pending += tbmres->ntuples;
+		bscan->rs_empty_tuples_pending += tbmres->ntuples;
 
 		return true;
 	}
@@ -2282,18 +2285,19 @@ static bool
 heapam_scan_bitmap_next_tuple(TableScanDesc scan,
 							  TupleTableSlot *slot)
 {
-	HeapScanDesc hscan = (HeapScanDesc) scan;
+	BitmapHeapScanDesc bscan = (BitmapHeapScanDesc) scan;
+	HeapScanDesc hscan = (HeapScanDesc) bscan;
 	OffsetNumber targoffset;
 	Page		page;
 	ItemId		lp;
 
-	if (hscan->rs_empty_tuples_pending > 0)
+	if (bscan->rs_empty_tuples_pending > 0)
 	{
 		/*
 		 * If we don't have to fetch the tuple, just return nulls.
 		 */
 		ExecStoreAllNullTuple(slot);
-		hscan->rs_empty_tuples_pending--;
+		bscan->rs_empty_tuples_pending--;
 		return true;
 	}
 

src/include/access/heapam.h

@@ -92,16 +92,6 @@ typedef struct HeapScanDescData
 	 */
 	ParallelBlockTableScanWorkerData *rs_parallelworkerdata;
 
-	/*
-	 * These fields are only used for bitmap scans for the "skip fetch"
-	 * optimization. Bitmap scans needing no fields from the heap may skip
-	 * fetching an all visible block, instead using the number of tuples per
-	 * block reported by the bitmap to determine how many NULL-filled tuples
-	 * to return.
-	 */
-	Buffer		rs_vmbuffer;
-	int			rs_empty_tuples_pending;
-
 	/* these fields only used in page-at-a-time mode and for bitmap scans */
 	uint32		rs_cindex;		/* current tuple's index in vistuples */
 	uint32		rs_ntuples;		/* number of visible tuples on page */
@@ -109,6 +99,24 @@ typedef struct HeapScanDescData
 }			HeapScanDescData;
 typedef struct HeapScanDescData *HeapScanDesc;
 
+typedef struct BitmapHeapScanDescData
+{
+	HeapScanDescData rs_heap_base;
+
+	/*
+	 * These fields are only used for bitmap scans for the "skip fetch"
+	 * optimization. Bitmap scans needing no fields from the heap may skip
+	 * fetching an all visible block, instead using the number of tuples per
+	 * block reported by the bitmap to determine how many NULL-filled tuples
+	 * to return. They are common to parallel and serial BitmapHeapScans
+	 */
+
+	/* page of VM containing info for current block */
+	Buffer		rs_vmbuffer;
+	int			rs_empty_tuples_pending;
+}			BitmapHeapScanDescData;
+typedef struct BitmapHeapScanDescData *BitmapHeapScanDesc;
+
 /*
  * Descriptor for fetches from heap via an index.
  */

src/tools/pgindent/typedefs.list

@@ -264,6 +264,7 @@ BitmapAndState
 BitmapHeapPath
 BitmapHeapScan
 BitmapHeapScanInstrumentation
+BitmapHeapScanDesc
 BitmapHeapScanState
 BitmapIndexScan
 BitmapIndexScanState