Horton

Re: BUG #18280: logical decoding build wrong snapshot for subtransactions

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: feichanghong <feichanghong@qq.com>
Cc: pgsql-bugs <pgsql-bugs@lists.postgresql.org>
Date: 2024-01-23T11:42:34Z
Lists: pgsql-bugs

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  1. Fix catalog lookup due to wrong snapshot for subtransactions during decoding.

  2. Fix catalog lookup with the wrong snapshot during logical decoding.

On Wed, Jan 10, 2024 at 7:56 PM feichanghong <feichanghong@qq.com> wrote:
>
> > SnapBuildXidSetCatalogChanges (introduced in 272248a) skipping the check for
> > the sub-transactions when the parent transaction has been marked as
> > containing catalog changes should be the root cause of the problem.
>
> My fix is to remove the ReorderBufferXidHasCatalogChanges condition from the SnapBuildXidSetCatalogChanges function. This approach has a false positive; the subtransactions that didn't change catalog to the snapshot are also added to the commit list. But that won't be a problem since we use snapshot built during decoding only to read system catalogs, as 272248a's commit message says.
>

Thanks, your analysis sounds correct to me. BTW, I think this should
occur only in 15 and 14 because in prior branches we didn't check if
the xact has catalog changes before marking its subxacts. From 16
onwards, we already accurately serialize the xacts and subxacts that
have made any catalog changes, so this problem shouldn't be there. Can
you please once verify the same?

-- 
With Regards,
Amit Kapila.