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From: Nathan Bossart <[email protected]>
To: Tom Lane <[email protected]>
Cc: Aleksander Alekseev <[email protected]>
Cc: Amit Langote <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: The documentation for READ COMMITTED may be incomplete or wrong
Date: Thu, 18 May 2023 14:34:42 -0700
Message-ID: <20230518213442.GA3364784@nathanxps13> (raw)
In-Reply-To: <[email protected]>
References: <CAJ7c6TMBTN3rcz4=AjYhLPD_w3FFT0Wq_C15jxCDn8U4tZnH1g@mail.gmail.com>
	<[email protected]>
	<[email protected]>
	<20230518195123.GA3362715@nathanxps13>
	<[email protected]>

On Thu, May 18, 2023 at 04:03:36PM -0400, Tom Lane wrote:
> Yeah.  I see the problem: when starting up an EPQ recheck, we stuff
> the tuple-to-test into the epqstate->relsubs_slot[] entry for the
> relation it came from, but we do nothing to the EPQ state for the
> other target relations, which allows the EPQ plan to fetch rows
> from those relations as usual.  If it finds a (non-updated) row
> passing the qual, kaboom!  We decide the EPQ check passed.

Ah, so the EPQ check only fails for the last tuple because we won't fetch
rows from the other relations.  I think that explains the behavior I'm
seeing.

> What we need to do, I think, is set epqstate->relsubs_done[] for
> all target relations except the one we are stuffing a tuple into.

This seems generally reasonable to me.

> While nodeModifyTable can certainly be made to do that, things are
> complicated by the fact that currently ExecScanReScan thinks it ought
> to clear all the relsubs_done flags, which would break things again.
> I wonder if we can simply delete that code.  Dropping the
> FDW/Custom-specific code there is a bit scary, but on the whole that
> looks like code that got cargo-culted in rather than anything we
> actually need.

I see that part was added in 385f337 [0].  I haven't had a chance to
evaluate whether it seems necessary.

[0] https://postgr.es/m/9A28C8860F777E439AA12E8AEA7694F80117370C%40BPXM15GP.gisp.nec.co.jp

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

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