Re: Query running longer - Mailing list pgsql-general

On Fri, 2024-02-02 at 10:14 +0530, veem v wrote:
> On Fri, 2 Feb 2024 at 02:43, Laurenz Albe <laurenz.albe@cybertec.at> wrote:
> > On Fri, 2024-02-02 at 02:27 +0530, veem v wrote:
> > > We have the below query which is running for ~45 seconds on postgres aurora reader instance.
> > > I have captured the explain analyze. Want to understand, where exactly the resources are
> > > getting spent and if we can be able to optimize it further.
> >
> > Aurora <> PostgreSQL, but here is what I can see:
> >
> > - The index scan on SCHEMA1."TAB2" has to check 2 million extra
> >   rows because "work_mem" is too small.  Almost the complete time
> >   is spent there.
> >
> > - You may be getting a bad plan, because the statistics on
> >   SCHEMA1.TAB4 are either out of date or not detailed enough,
> >   which makes PostgreSQL underestimate the result size.
>
> As you mentioned below, So wondering how you got to know, if this is the step where majority of the DB resources get
spent.And as total time the query ran was ~45 seconds, and out of that how much time it spent in this step, how can i
getthat? And to fix this one line with regards to TAB2, should we increase "work_mem" here? 
>
> - The index scan on SCHEMA1."TAB2" has to check 2 million extra
>   rows because "work_mem" is too small.  Almost the complete time
>   is spent there.
>
>  ->  Parallel Bitmap Heap Scan on SCHEMA1.""TAB2"" TAB2  (cost=84860.50..13040301.00 rows=1175611 width=80) (actual
time=713.054..26942.082rows=956249 loops=5)" 
> "                     Output: TAB2.TAB2_time, TAB2.PRI, TAB2.AID"
>   Recheck Cond: (TAB2.MID = 'XXXXX'::numeric)
>   Rows Removed by Index Recheck: 2137395
>   Filter: ((TAB2.TAB2_time >= '2024-01-01 00:00:00+00'::timestamp with time zone) AND (TAB2.TAB2_time <= '2024-01-31
00:00:00+00'::timestampwith time zone)) 
>   Heap Blocks: exact=5300 lossy=782577
>   Buffers: shared hit=1651569 read=2245157
>   I/O Timings: shared/local read=29063.286  

The bitmap index scan builds a bitmap, the size of which is limited by "work_mem".
If that is big enough, the bitmap will contain a bit for each table row, if not,
it only contains a bit per table row for some table blocks (the "exact" ones),
then it degrades to a bit per block (whe "lossy" ones).

For lossy blocks, *all* rows in the block have to be rechecked, which is
overhead.  Given that only a small part of the time (26942.082 - 713.054) * 5
is spent doing I/O (29063.286), I guess that the rest is spent processing
table rows.

> Another point you mentioned as below , for this , should we run vacuum analyze on the table TAB4?
>
> - You may be getting a bad plan, because the statistics on
>   SCHEMA1.TAB4 are either out of date or not detailed enough,
>   which makes PostgreSQL underestimate the result size.
>   
>   ->  Parallel Bitmap Heap Scan on SCHEMA1.TAB4 TAB4  (cost=26.39..7042.63 rows=1049 width=37) (actual
time=23.650..201.606rows=27613 loops=5) 
>        ->  Bitmap Index Scan on TAB4_idx1  (cost=0.00..25.95 rows=1784 width=0) (actual time=23.938..23.938
rows=138067loops=1) 
>            Index Cond: ((TAB4.TAB4_code)::text = 'XX'::text)
>            Buffers: shared hit=72

An ANALYZE might be enough.

If not, you can try to collect more detailed statistics for the column:

  ALTER TABLE SCHEMA1.TAB4 ALTER TAB4_code SET STATISTICS 1000;
  ANALYZE SCHEMA1.TAB4;

Yours,
Laurenz Albe