From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Bartosz Polnik <bartoszpolnik(at)gmail(dot)com>, PostgreSQL mailing lists <pgsql-bugs(at)lists(dot)postgresql(dot)org> |
Subject: | Re: BUG #15577: Query returns different results when executed multiple times |
Date: | 2019-01-10 17:38:13 |
Message-ID: | 2887.1547141893@sss.pgh.pa.us |
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Lists: | pgsql-bugs |
I wrote:
> Well, if we posit that the problems are limited to false Param-sharing,
> my inclination is to fix it by simplifying rather than adding complexity.
> To wit, let's just assign a new PARAM_EXEC slot for every NestLoopParam
> and never try to share them. The argument that sharing them would work
> for NLPs representing the same Var hinged on the assumption that nested
> NestLoops would necessarily execute "in sync", which we now see is false
> at least when Gathers are in between.
Here's a POC patch for that. It's seriously ugly, but it passes
check-world and I can no longer reproduce the problem using
mockup_bug15577_v2.sql.
At this point assign_nestloop_param_var and
assign_nestloop_param_placeholdervar are dead code, and there's a bunch
of incorrect comments in subselect.c, and I really need to refactor
the division of labor between createplan.c and subselect.c (for one
thing, this is an abuse of the documented purpose of
SS_make_initplan_output_param). But functionally I think it does the
right thing. Please test and verify that you no longer see the race
condition.
regards, tom lane
Attachment | Content-Type | Size |
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remove-NLP-slot-sharing-poc.patch | text/x-diff | 3.8 KB |
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