| From: | Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> |
|---|---|
| To: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
| Cc: | Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>, "houzj(dot)fnst(at)fujitsu(dot)com" <houzj(dot)fnst(at)fujitsu(dot)com>, vignesh C <vignesh21(at)gmail(dot)com>, Greg Nancarrow <gregn4422(at)gmail(dot)com>, "tanghy(dot)fnst(at)fujitsu(dot)com" <tanghy(dot)fnst(at)fujitsu(dot)com>, "osumi(dot)takamichi(at)fujitsu(dot)com" <osumi(dot)takamichi(at)fujitsu(dot)com>, Alexey Lesovsky <lesovsky(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Skipping logical replication transactions on subscriber side |
| Date: | 2021-12-08 11:22:32 |
| Message-ID: | CAD21AoATauhXzBV4RP+W67bR3bSM7t0g_LwoKKiX6nHcfyGe=w@mail.gmail.com |
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On Wed, Dec 8, 2021 at 4:05 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > Okay, I understand those cases but note always checking if the
> > prepared xact exists during commit prepared has a cost and that is why
> > we avoided it at the first place.
BTW what costs were we concerned about? Looking at LookupGXact(), we
look for the 2PC state data on shmem while acquiring TwoPhaseStateLock
in shared mode. And we check origin_lsn and origin_timestamp of 2PC by
reading WAL or 2PC state file only if gid matched. On the other hand,
committing the prepared transaction does WAL logging, waits for
synchronous replication, and calls post-commit callbacks, and removes
2PC state file etc. And it requires acquiring TwoPhaseStateLock in
exclusive mode to remove 2PC state entry. So it looks like always
checking if the prepared transaction exists and skipping it if not is
cheaper than always committing prepared transactions.
Regards,
--
Masahiko Sawada
EDB: https://www.enterprisedb.com/
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