| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | "Zhijie Hou (Fujitsu)" <houzj(dot)fnst(at)fujitsu(dot)com> |
| Cc: | "bharath(dot)rupireddyforpostgres(at)gmail(dot)com" <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>, "sawada(dot)mshk(at)gmail(dot)com" <sawada(dot)mshk(at)gmail(dot)com>, "thomas(dot)munro(at)gmail(dot)com" <thomas(dot)munro(at)gmail(dot)com>, "bertranddrouvot(dot)pg(at)gmail(dot)com" <bertranddrouvot(dot)pg(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, "pgsql(at)j-davis(dot)com" <pgsql(at)j-davis(dot)com>, "amit(dot)kapila16(at)gmail(dot)com" <amit(dot)kapila16(at)gmail(dot)com>, "robertmhaas(at)gmail(dot)com" <robertmhaas(at)gmail(dot)com>, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> |
| Subject: | Re: walsender performance regression due to logical decoding on standby changes |
| Date: | 2023-05-22 17:52:54 |
| Message-ID: | 20230522175254.dqfnbqntrgznzdgu@awork3.anarazel.de |
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| Lists: | pgsql-hackers |
Hi,
On 2023-05-22 12:15:07 +0000, Zhijie Hou (Fujitsu) wrote:
> About "a backend doing logical decoding", do you mean the case when a user
> start a backend and invoke pg_logical_slot_get_changes() to do the logical
> decoding ? If so, it seems the logical decoding in a backend won't be waked up
> by startup process because the backend won't be registered as a walsender so
> the backend won't be found in WalSndWakeup().
I meant logical decoding happening inside a walsender instance.
> Or do you mean the deadlock between the real logical walsender and startup
> process ? (I might miss something) I think the logical decoding doesn't lock
> the target user relation when decoding because it normally can get the needed
> information from WAL.
It does lock catalog tables briefly. There's no guarantee that such locks are
released immediately. I forgot the details, but IIRC there's some outfuncs
(enum?) that intentionally delay releasing locks till transaction commit.
Greetings,
Andres Freund
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