Re: improve transparency of bitmap-only heap scans

On Mon, Mar 16, 2020 at 09:08:36AM -0400, James Coleman wrote:
> Does the original optimization cover parallel bitmap heap scans like this?

It works for parallel bitmap only scans.

template1=# explain analyze select count(*) from exp where a between 25 and 35 and d between 5 and 10;
Finalize Aggregate (cost=78391.68..78391.69 rows=1 width=8) (actual time=525.972..525.972 rows=1 loops=1)
-> Gather (cost=78391.47..78391.68 rows=2 width=8) (actual time=525.416..533.133 rows=3 loops=1)
Workers Planned: 2
Workers Launched: 2
-> Partial Aggregate (cost=77391.47..77391.48 rows=1 width=8) (actual time=518.406..518.406 rows=1 loops=3)
-> Parallel Bitmap Heap Scan on exp (cost=31825.37..77245.01 rows=58582 width=0) (actual time=296.309..508.440 rows=43887 loops=3)
Recheck Cond: ((a >= 25) AND (a <= 35) AND (d >= 5) AND (d <= 10))
Heap Blocks: unfetched=4701 exact=9650
-> BitmapAnd (cost=31825.37..31825.37 rows=140597 width=0) (actual time=282.590..282.590 rows=0 loops=1)
-> Bitmap Index Scan on index_exp_a (cost=0.00..15616.99 rows=1166456 width=0) (actual time=147.036..147.036 rows=1099872 loops=1)
Index Cond: ((a >= 25) AND (a <= 35))
-> Bitmap Index Scan on index_exp_d (cost=0.00..16137.82 rows=1205339 width=0) (actual time=130.366..130.366 rows=1200000 loops=1)
Index Cond: ((d >= 5) AND (d <= 10))

> +++ b/src/backend/commands/explain.c
> @@ -2777,6 +2777,8 @@ show_tidbitmap_info(BitmapHeapScanState *planstate, ExplainState *es)
> {
> if (es->format != EXPLAIN_FORMAT_TEXT)
> {
> + ExplainPropertyInteger("Unfetched Heap Blocks", NULL,
> + planstate->unfetched_pages, es);
> ExplainPropertyInteger("Exact Heap Blocks", NULL,
> planstate->exact_pages, es);
> ExplainPropertyInteger("Lossy Heap Blocks", NULL,
> @@ -2784,10 +2786,14 @@ show_tidbitmap_info(BitmapHeapScanState *planstate, ExplainState *es)
> }
> else
> {
> - if (planstate->exact_pages > 0 || planstate->lossy_pages > 0)
> + if (planstate->exact_pages > 0 || planstate->lossy_pages > 0
> + || planstate->unfetched_pages > 0)
> {
> ExplainIndentText(es);
> appendStringInfoString(es->str, "Heap Blocks:");
> + if (planstate->unfetched_pages > 0)
> + appendStringInfo(es->str, " unfetched=%ld",
> + planstate->unfetched_pages);
> if (planstate->exact_pages > 0)
> appendStringInfo(es->str, " exact=%ld", planstate->exact_pages);
> if (planstate->lossy_pages > 0)

I don't think it matters in nontext mode, but at least in text mode, I think
maybe the Unfetched blocks should be output after the exact and lossy blocks,
in case someone is parsing it, and because bitmap-only is a relatively new
feature. Its output is probably less common than exact/lossy.

--
Justin