Fix EXPLAIN Bitmap heap scan to count pages with no visible tuples

Previously, bitmap heap scans only counted lossy and exact pages for
explain when there was at least one visible tuple on the page.

heapam_scan_bitmap_next_block() returned true only if there was a
"valid" page with tuples to be processed. However, the lossy and exact
page counters in EXPLAIN should count the number of pages represented
in a lossy or non-lossy way in the constructed bitmap, regardless of
whether or not the pages ultimately contained visible tuples.

Backpatch to all supported versions.

Author: Melanie Plageman
Discussion: https://www.postgresql.org/message-id/CAAKRu_ZwCwWFeL_H3ia26bP2e7HiKLWt0ZmGXPVwPO6uXq0vaA@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/CAAKRu_bxrXeZ2rCnY8LyeC2Ls88KpjWrQ%2BopUrXDRXdcfwFZGA@mail.gmail.com

src/backend/executor/nodeBitmapHeapscan.c

@@ -204,6 +204,11 @@ BitmapHeapNext(BitmapHeapScanState *node)
 
 			BitmapAdjustPrefetchIterator(node, tbmres);
 
+			if (tbmres->ntuples >= 0)
+				node->exact_pages++;
+			else
+				node->lossy_pages++;
+
 			/*
 			 * We can skip fetching the heap page if we don't need any fields
 			 * from the heap, and the bitmap entries don't need rechecking,
@@ -235,11 +240,6 @@ BitmapHeapNext(BitmapHeapScanState *node)
 				continue;
 			}
 
-			if (tbmres->ntuples >= 0)
-				node->exact_pages++;
-			else
-				node->lossy_pages++;
-
 			/* Adjust the prefetch target */
 			BitmapAdjustPrefetchTarget(node);
 		}

src/test/regress/expected/partition_prune.out

@@ -2848,6 +2848,7 @@ update ab_a1 set b = 3 from ab where ab.a = 1 and ab.a = ab_a1.a;
                            Index Cond: (a = 1)
                ->  Bitmap Heap Scan on ab_a1_b3 ab_a1_3 (actual rows=0 loops=1)
                      Recheck Cond: (a = 1)
+                     Heap Blocks: exact=1
                      ->  Bitmap Index Scan on ab_a1_b3_a_idx (actual rows=1 loops=1)
                            Index Cond: (a = 1)
          ->  Materialize (actual rows=1 loops=1)
@@ -2863,9 +2864,10 @@ update ab_a1 set b = 3 from ab where ab.a = 1 and ab.a = ab_a1.a;
                                  Index Cond: (a = 1)
                      ->  Bitmap Heap Scan on ab_a1_b3 ab_3 (actual rows=0 loops=1)
                            Recheck Cond: (a = 1)
+                           Heap Blocks: exact=1
                            ->  Bitmap Index Scan on ab_a1_b3_a_idx (actual rows=1 loops=1)
                                  Index Cond: (a = 1)
-(34 rows)
+(36 rows)
 
 table ab;
  a | b