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Subject: Re: optimize lookups in snapshot [sub]xip arrays
From: Zhang Mingli <zmlpostgres@gmail.com>
In-Reply-To: <20220725040856.GB4085281@nathanxps13>
Date: Mon, 25 Jul 2022 12:28:23 +0800
Cc: pgsql-hackers@postgresql.org
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Message-Id: <E5777552-8152-49B7-8F82-5C6FB6287D90@gmail.com>
References: <20220713170950.GA3116318@nathanxps13>
 <76ACEAF0-8050-41D5-B2F5-01164533BE99@gmail.com>
 <20220725040856.GB4085281@nathanxps13>
To: Nathan Bossart <nathandbossart@gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/E5777552-8152-49B7-8F82-5C6FB6287D90%40gmail.com>
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Got it, thanks.


Regards,
Zhang Mingli


> On Jul 25, 2022, at 12:08, Nathan Bossart <nathandbossart@gmail.com> =
wrote:
>=20
> On Sun, Jul 24, 2022 at 12:48:25PM +0800, Zhang Mingli wrote:
>> If snaphost->suboverflowed is  false then the subxcnt must be less =
than PGPROC_MAX_CACHED_SUBXIDS which is 64 now.
>>=20
>> And we won=E2=80=99t use hash if the xcnt is less than =
XIP_HASH_MIN_ELEMENTS which is 128 currently during discussion.
>>=20
>> So that, subxid=E2=80=99s hash table will never be used, right?
>=20
> This array will store up to TOTAL_MAX_CACHED_SUBXIDS transactions, =
which
> will typically be much greater than 64.  When there isn't any =
overflow,
> subxip stores all of the subxids for all of the entries in the =
procarray.
>=20
> --=20
> Nathan Bossart
> Amazon Web Services: https://aws.amazon.com