From: | Justin Pryzby <pryzby(at)telsasoft(dot)com> |
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To: | Lars Bergeson <larsavatar(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-performance(at)lists(dot)postgresql(dot)org |
Subject: | Re: Query is slower with a large proportion of NULLs in several columns |
Date: | 2021-12-21 04:51:38 |
Message-ID: | 20211221045138.GZ17618@telsasoft.com |
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Lists: | pgsql-performance |
On Mon, Dec 20, 2021 at 08:11:42PM -0800, Lars Bergeson wrote:
> ok, here are results after I did:
> set max_parallel_workers_per_gather = 0;
>
> HashAggregate (cost=1676432.13..1676432.16 rows=3 width=15) (actual time=19908.343..19908.345 rows=5 loops=1)
> I/O Timings: read=532369.898
> Execution Time: 19908.383 ms
> HashAggregate (cost=1390580.70..1390580.72 rows=2 width=15) (actual time=30369.758..30369.761 rows=5 loops=1)
> I/O Timings: read=6440851.540
> Execution Time: 30369.796 ms
> Still taking 10X more I/O to read the smaller table. Very odd.
If I'm not wrong, it's even worse than that ?
It takes 20 or 30sec to run the query - but it says the associated I/O times
are ~500sec or ~6000sec ?
What architecture and OS/version are you running ?
How did you install postgres? From a package or compiled from source ?
It might be interesting to know the output from something like this command,
depending on whether and where the headers like pg_config_x86_64.h are installed.
grep -r HAVE_CLOCK_GETTIME /usr/pgsql-13/include
--
Justin
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