Re: improve transparency of bitmap-only heap scans - Mailing list pgsql-hackers

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..508.440rows=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..147.036rows=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..130.366rows=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