Allow bitmap scans to operate as index-only scans when possible.

If we don't have to return any columns from heap tuples, and there's
no need to recheck qual conditions, and the heap page is all-visible,
then we can skip fetching the heap page altogether.

Skip prefetching pages too, when possible, on the assumption that the
recheck flag will remain the same from one page to the next.  While that
assumption is hardly bulletproof, it seems like a good bet most of the
time, and better than prefetching pages we don't need.

This commit installs the executor infrastructure, but doesn't change
any planner cost estimates, thus possibly causing bitmap scans to
not be chosen in cases where this change renders them the best choice.
I (tgl) am not entirely convinced that we need to account for this
behavior in the planner, because I think typically the bitmap scan would
get chosen anyway if it's the best bet.  In any case the submitted patch
took way too many shortcuts, resulting in too many clearly-bad choices,
to be committable.

Alexander Kuzmenkov, reviewed by Alexey Chernyshov, and whacked around
rather heavily by me.

Discussion: https://postgr.es/m/239a8955-c0fc-f506-026d-c837e86c827b@postgrespro.ru

1 files changed, 9 insertions, 1 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 8698c8a50c0..d209ec012c5 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -507,7 +507,7 @@ typedef struct EState
 	bool	   *es_epqTupleSet; /* true if EPQ tuple is provided */
 	bool	   *es_epqScanDone; /* true if EPQ tuple has been fetched */
 
-	bool		es_use_parallel_mode; /* can we use parallel workers? */
+	bool		es_use_parallel_mode;	/* can we use parallel workers? */
 
 	/* The per-query shared memory area to use for parallel execution. */
 	struct dsa_area *es_query_dsa;
@@ -1331,6 +1331,10 @@ typedef struct ParallelBitmapHeapState
  *		tbm				   bitmap obtained from child index scan(s)
  *		tbmiterator		   iterator for scanning current pages
  *		tbmres			   current-page data
+ *		can_skip_fetch	   can we potentially skip tuple fetches in this scan?
+ *		skip_fetch		   are we skipping tuple fetches on this page?
+ *		vmbuffer		   buffer for visibility-map lookups
+ *		pvmbuffer		   ditto, for prefetched pages
  *		exact_pages		   total number of exact pages retrieved
  *		lossy_pages		   total number of lossy pages retrieved
  *		prefetch_iterator  iterator for prefetching ahead of current page
@@ -1351,6 +1355,10 @@ typedef struct BitmapHeapScanState
 	TIDBitmap  *tbm;
 	TBMIterator *tbmiterator;
 	TBMIterateResult *tbmres;
+	bool		can_skip_fetch;
+	bool		skip_fetch;
+	Buffer		vmbuffer;
+	Buffer		pvmbuffer;
 	long		exact_pages;
 	long		lossy_pages;
 	TBMIterator *prefetch_iterator;