From: Andy Fan <zhihuifan1213@163.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Subject: Re: Why does exec_simple_query requires 2 snapshots
In-Reply-To: <1925004.1739932245@sss.pgh.pa.us> (Tom Lane's message of "Tue,
	18 Feb 2025 21:30:45 -0500")
References: <87o6yzbhip.fsf@163.com> <1241667.1739893420@sss.pgh.pa.us>
	<87jz9mbuyq.fsf@163.com> <1925004.1739932245@sss.pgh.pa.us>
Date: Wed, 19 Feb 2025 08:50:05 +0000
Message-ID: <87frkab9oi.fsf@163.com>
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Tom Lane <tgl@sss.pgh.pa.us> writes:

> Andy Fan <zhihuifan1213@163.com> writes:
>> Tom Lane <tgl@sss.pgh.pa.us> writes:
>>> Many years ago, we committed a patch to do exactly that.  We had
>>> to back it out again because it broke too many real-world scenarios.
>>> I'm too lazy to search the archives for you, but you might be able
>>> to find the commit and revert by searching the git history.
>
>> Commit id is 532994299e2, thank you Tom!
>
> links in commit messages till some years after that.

Links in commit messages is a really great change.

I have found more details at [1], the issue can be summaried as IIUC:

The current strategy is below:
t1: snapshot1
parser&planner
t2: snapshot2
executor.

which makes the committed tuple between (t1, t2] are visible to user, but if
we share the same snapshot, the committed tuple would not be visible to
user, Chaning it would be a behavior change. 

[1] https://www.postgresql.org/message-id/5075D8DF.6050500%40fuzzy.cz

-- 
Best Regards
Andy Fan