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From: Andres Freund <[email protected]>
To: Hannu Krosing <[email protected]>
Cc: Tomas Vondra <[email protected]>
Cc: Peter Eisentraut <[email protected]>
Cc: vignesh C <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Petr Jelinek <[email protected]>
Subject: Re: logical decoding and replication of sequences
Date: Mon, 22 Nov 2021 17:01:14 -0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAMT0RQQeDR51xs8zTa25YpfKB1B34nS-Q4hhsRPznVsjMB_P1w@mail.gmail.com>
References: <[email protected]>
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	<CAMT0RQQeDR51xs8zTa25YpfKB1B34nS-Q4hhsRPznVsjMB_P1w@mail.gmail.com>

Hi,

On 2021-09-25 22:05:43 +0200, Hannu Krosing wrote:
> If our aim is just to make sure that all user-visible data in
> *transactional* tables is consistent with sequence state then  one
> very much simplified approach to this could be to track the results of
> nextval() calls in a transaction at COMMIT put the latest sequence
> value in WAL (or just track the sequences affected and put the latest
> sequence state in WAL at commit which needs extra read of sequence but
> protects against race conditions with parallel transactions which get
> rolled back later)

I think this is a bad idea. It's architecturally more complicated and prevents
use cases because sequence values aren't guaranteed to be as new as on the
original system. You'd need to track all sequence use somehow *even if there
is no relevant WAL generated* in a transaction. There's simply no evidence of
sequence use in a transaction if that transaction uses a previously logged
sequence value.

Greetings,

Andres Freund





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