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From: David G. Johnston <[email protected]>
To: Ron Johnson <[email protected]>
Cc: pgsql-general <[email protected]>
Subject: Re: Loading the latest N rows into the cache seems way too fast.
Date: Mon, 17 Feb 2025 14:58:53 -0700
Message-ID: <CAKFQuwbLa1d4DMRQsYMzJ7OwcTCHYvuKcG7RD5meC0ryP_Za4g@mail.gmail.com> (raw)
In-Reply-To: <CANzqJaDPR_sshr1hXuMsiZQDf7nH4BthNioCHJ4cVcGKxiSg_Q@mail.gmail.com>
References: <CANzqJaBTPgTJ_M3dGiOa5H-FvAo71oCCa1QHejbzK+joKdrSyw@mail.gmail.com>
	<[email protected]>
	<CANzqJaDPR_sshr1hXuMsiZQDf7nH4BthNioCHJ4cVcGKxiSg_Q@mail.gmail.com>

On Mon, Feb 17, 2025 at 2:41 PM Ron Johnson <[email protected]> wrote:

> On Mon, Feb 17, 2025 at 4:36 PM Tom Lane <[email protected]> wrote:
>
>> Ron Johnson <[email protected]> writes:
>> > The bigint "id" column in "mytbl" is populated from a sequence, and so
>> is
>> > monotonically increasing: the newest records will have the biggest id
>> > values.
>> > The table also has a bytea column that averages about 100KB.
>>
>> > Loading 200K rows is more than 200MB.  I expected this "prewarm"
>> statement
>> > to take much longer than 1/2 second.  Am I still in the dark ages of
>> > computer speed, or is this statement not doing what I hope it's doing?
>>
>> It's not pulling in the TOAST storage where the bytea column lives.
>> (pg_prewarm wouldn't have either, without special pushups.)
>>
>
> Puzzling, since I ran "PERFORM *".  What if I explicitly mentioned the
> bytea column's name?
>
>
It's more about the system optimizing away data retrieval because you've
indicated you don't care about the contents due to using PERFORM.  All it
needs is a pointer to represent the future data, not the data itself.  And
PERFORM will never resolve that pointer by itself - so as Tom said your
query would need to force pointer resolution by computing on the data.

David J.

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