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help / color / mirror / Atom feedFrom: David Mullineux <[email protected]>
To: James Pang <[email protected]>
Cc: [email protected]
Subject: Re: huge shared_blocks_hit one select but manually run very fast
Date: Sat, 21 Dec 2024 16:40:54 +0000
Message-ID: <CAGsyd8U0cgD=2wTG5HAsbs=e=9ynz3r55tH56xzQeUQYju94Yw@mail.gmail.com> (raw)
In-Reply-To: <CAHgTRff_1mTsU-QSaDBY5k9+D-i9sV0aQeRY9ShffV=6b7jOWQ@mail.gmail.com>
References: <CAHgTRff_1mTsU-QSaDBY5k9+D-i9sV0aQeRY9ShffV=6b7jOWQ@mail.gmail.com>
Depends on a lot of thongs...Visibility map sounds like it's impacted here.
Are your inserts towards the index (like a monotonically increasing serial
id) or scattered around the index values ? How big is the table index
and shared buffers ? An example would really help
On Sat, 21 Dec 2024, 11:51 James Pang, <[email protected]> wrote:
> Hi,
> we have a simple select .... from table where ... (that mache the
> index) , table has 80million rows. when many application sessions run the
> query and at the same time some other sessions doing insert into ... this
> table. from pg_stat_statements, shared_blks_hit show 31652 / per call. we
> see very high cpu almost 100% cpu during application workload test, and
> high LWLock BufferMapping waiting for these querys. But manually run the
> sql show only 2148 shared_blks_hit/ per call. this is a simple sql, from
> pg_profile we did see it use same index scan as manually running. What
> could be possible reason leading so big difference with shared_blks_hit ?
> PGv14.8
>
> Thanks,
>
> James
>
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Subject: Re: huge shared_blocks_hit one select but manually run very fast
In-Reply-To: <CAGsyd8U0cgD=2wTG5HAsbs=e=9ynz3r55tH56xzQeUQYju94Yw@mail.gmail.com>
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