Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Cc: Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions
Date: 2020-03-29 15:31:05
Message-ID: 20200329153105.mdryxzz562tg65pk@development
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On Sun, Mar 29, 2020 at 11:19:21AM +0530, Amit Kapila wrote:
>On Sun, Mar 29, 2020 at 6:29 AM Tomas Vondra
><tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>> On Sat, Mar 28, 2020 at 03:29:34PM +0530, Amit Kapila wrote:
>> >On Sat, Mar 28, 2020 at 2:19 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>> >
>> >How about if instead of writing an XLOG_XACT_ASSIGNMENT WAL, we set a
>> >flag in TransactionStateData and then log that as special information
>> >whenever we write next WAL record for a new subtransaction? Then
>> >during recovery, we can only call ProcArrayApplyXidAssignment when we
>> >find that special flag is set in a WAL record. One idea could be to
>> >use a flag bit in XLogRecord.xl_info. If that is feasible then the
>> >solution can work as it is now, without any overhead or change in the
>> >way we maintain KnownAssignedXids.
>> >
>> Ummm, how is that different from what the patch is doing now? I mean, we
>> only write the top-level XID for the first WAL record in each subxact,
>> right? Or what would be the difference with your approach?
>We have to do what the patch is currently doing and additionally, we
>will set this flag after PGPROC_MAX_CACHED_SUBXIDS which would allow
>us to call ProcArrayApplyXidAssignment during WAL replay only after
>PGPROC_MAX_CACHED_SUBXIDS number of subxacts. It will help us in
>clearing the KnownAssignedXids at the same time as we do now, so no
>additional performance overhead.

Hmmm. So we'd still log assignment twice? Or would we keep just the
immediate assignments (embedded into xlog records), and cache the
subxids on the replica somehow?


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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