Re: BUG #15060: Row in table not found when using pg function in an expression

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>
Cc: pgsql-bugs(at)lists(dot)postgresql(dot)org, dejan(dot)petrovic(at)islonline(dot)com
Subject: Re: BUG #15060: Row in table not found when using pg function in an expression
Date: 2018-02-12 20:11:27
Message-ID: 12102.1518466287@sss.pgh.pa.us
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Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
> So what's happening here is that the function get_bug_id, being stable,
> is being called speculatively at plan time for the query where it
> appears in the WHERE clause. For whatever reason, the snapshot it's
> being run in at that time is not the same one actually used for the
> later execution of the query, and the plan-time snapshot doesn't see the
> just-inserted row.

> It looks like what's going on here is that SPI does GetCachedPlan -
> which is where planning will happen - _before_ establishing the new
> snapshot in the non-read-only case (read_only is false here because the
> calling function, test_bug(), is volatile).

Yeah, I came to the same conclusion. I think it's basically accidental
that the given test case works before 9.2: the reason seems to be that
in 9.1, the plancache doesn't pass through the parameter list containing
the value of "my_text", so that the planner is unable to speculatively
execute get_bug_id(). The order of operations in _SPI_execute_plan
is just as wrong though.

The attached patch makes it better and doesn't seem to break any
regression tests. I'm not sure how nervous I am about unexpected
side-effects ...

regards, tom lane

Attachment Content-Type Size
set-snapshot-sooner-in-spi.patch text/x-diff 1.4 KB

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