Re: JSONB index not in use, but is TOAST the real cause of slow query?

Adrian, Tom, thanks for the input. Based on that, it occurred to me to
create some "dummy" rows and - almost magically - the index kicks in!
Before is 2500ms:

============
Seq Scan on paiyroll_payrun  (cost=0.00..52.43 rows=17 width=32)
(actual time=53.127..2567.024 rows=104 loops=1)
  Filter: ((snapshot -> 'employee'::text) ? '2209'::text)
  Rows Removed by Filter: 1835
Planning Time: 0.060 ms
Execution Time: 2567.044 ms
(5 rows)
============

After is 300ms:

============
Bitmap Heap Scan on paiyroll_payrun  (cost=36.11..64.67 rows=14
width=32) (actual time=4.189..311.932 rows=104 loops=1)
  Recheck Cond: ((snapshot -> 'employee'::text) ? '2209'::text)
  Rows Removed by Index Recheck: 1
  Heap Blocks: exact=8
  ->  Bitmap Index Scan on idx1  (cost=0.00..36.10 rows=14 width=0)
(actual time=0.087..0.087 rows=105 loops=1)
        Index Cond: ((snapshot -> 'employee'::text) ? '2209'::text)
Planning Time: 0.167 ms
Execution Time: 311.962 ms
(8 rows)
============

Woot!

Unfortunately, the real query which I think should behave very
similarly is still at the several-seconds level despite using the
index. Before 3600ms:

============
SELECT "paiyroll_payrun"."actual_t"
  FROM "paiyroll_payrun"
 WHERE ("paiyroll_payrun"."company_id" = 173 AND
("paiyroll_payrun"."snapshot" -> 'employee') ? '16376'
   AND NOT (("paiyroll_payrun"."snapshot" #>
ARRAY['employee','16376','last_run_of_employment']) = 'true'
   AND ("paiyroll_payrun"."snapshot" #>
ARRAY['employee','16376','pay_graph']) = '0'
   AND ("paiyroll_payrun"."snapshot" #>
ARRAY['employee','16376','state','employment','-1','2']) > '0'))
 ORDER BY "paiyroll_payrun"."actual_t" DESC
 LIMIT 1
============

============
Limit (cost=31.33..31.33 rows=1 width=4) (actual
time=3595.174..3595.176 rows=1 loops=1)
  -> Sort (cost=31.33..31.33 rows=3 width=4) (actual
time=3595.174..3595.174 rows=1 loops=1)
    Sort Key: actual_t DESC
    Sort Method: top-N heapsort Memory: 25kB
    -> Bitmap Heap Scan on paiyroll_payrun (cost=6.43..31.31 rows=3
width=4) (actual time=44.575..3595.082 rows=62 loops=1)
       Recheck Cond: (company_id = 173)
       Filter: (((snapshot -> 'employee'::text) ? '16376'::text) AND
(((snapshot #> '{employee,16376,last_run_of_employment}'::text[]) <>
'true'::jsonb) OR ((snapshot #> '{employee,16376,pay_graph}'::text[])
<> '0'::jsonb) OR ((snapshot #>
'{employee,16376,state,employment,-1,2}'::text[]) <= '0'::jsonb)))
Rows Removed by Filter: 242
       Heap Blocks: exact=9
         -> Bitmap Index Scan on paiyroll_payrun_company_id_ce341888
(cost=0.00..6.43 rows=304 width=0) (actual time=0.013..0.013 rows=304
loops=1)
         Index Cond: (company_id = 173)
Planning Time: 0.258 ms
Execution Time: 3595.195 ms
============

After, with the index in use, 2200ms:

================
Limit (cost=30.92..30.93 rows=1 width=4) (actual
time=2258.989..2258.990 rows=1 loops=1)
  -> Sort (cost=30.92..30.93 rows=1 width=4) (actual
time=2258.988..2258.989 rows=1 loops=1)
    Sort Key: actual_t DESC
    Sort Method: top-N heapsort Memory: 25kB
    -> Bitmap Heap Scan on paiyroll_payrun (cost=26.88..30.91 rows=1
width=4) (actual time=32.488..2258.891 rows=62 loops=1)
      Recheck Cond: ((company_id = 173) AND ((snapshot ->
'employee'::text) ? '16376'::text))
      Filter: (((snapshot #>
'{employee,16376,last_run_of_employment}'::text[]) <> 'true'::jsonb)
OR ((snapshot #> '{employee,16376,pay_graph}'::text[]) <> '0'::jsonb)
OR ((snapshot #> '{employee,16376,state,employment,-1,2}'::text[]) <=
'0'::jsonb))
      Heap Blocks: exact=5
        -> BitmapAnd (cost=26.88..26.88 rows=1 width=0) (actual
time=0.038..0.039 rows=0 loops=1)
          -> Bitmap Index Scan on paiyroll_payrun_company_id_ce341888
(cost=0.00..6.56 rows=304 width=0) (actual time=0.016..0.016 rows=304
loops=1)
                Index Cond: (company_id = 173)
          -> Bitmap Index Scan on idx1 (cost=0.00..20.07 rows=9
width=0) (actual time=0.021..0.021 rows=62 loops=1)
                Index Cond: ((snapshot -> 'employee'::text) ? '16376'::text)
Planning Time: 0.245 ms
Execution Time: 2259.019 ms
===================

IIUC, at the bottom, the indices are doing their thing, but a couple
of layers up, the "Bitmap Heap Scan" jumps from ~30ms to 2200ms. But I
cannot quite see why. Have I missed a needed index or what? I'm pretty
much a novice at SQL, so it is quite possible I've missed something
very basic.

Thanks, Shaheed

On Sat, 28 May 2022 at 20:59, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Shaheed Haque <shaheedhaque@gmail.com> writes:
> > One last thought about TOAST. If the cost of the -> retrieving the
> > data cannot be obviated, is there any way to tweak how that works?
>
> The only thing that's readily tweakable is to disable data compression
> for the out-of-line values (see ALTER TABLE ... SET STORAGE, and note
> that you have to rebuild the table for already-stored values to be
> changed).  It seems unlikely that that will help you much though,
> since doing that would save CPU at the cost of more disk I/O, and
> it seems that the I/O side is your bottleneck already.  It *would*
> help if jsonb had logic for partial fetches, because that'd require
> uncompressed storage to work.  But AFAICS that's not there yet.
> I distinctly recall that that was planned for when we were designing
> jsonb's on-disk layout, but I see no code actually doing anything
> of that sort ... the -> operator certainly doesn't.
>
>                         regards, tom lane