From: Justin Pryzby <pryzby@telsasoft.com>
To: Dean Gibson (DB Administrator) <postgresql@mailpen.com>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: AWS forcing PG upgrade from v9.6 a disaster
Date: Mon, 31 May 2021 23:16:35 -0500
Message-ID: <20210601041635.GQ2082@telsasoft.com> (raw)
In-Reply-To: <92cc8563-195f-8727-d887-5f2bb63f231f@mailpen.com>
References: <88dec70b-eb63-0b72-0b5d-253851ef127b@mailpen.com>
<92cc8563-195f-8727-d887-5f2bb63f231f@mailpen.com>
> Here's the FROM clause that bit me:
>
> FROM lic_en
> JOIN govt_region USING (territory_id, country_id)
> LEFT JOIN zip_code USING (territory_id, country_id, zip5)
> LEFT JOIN "County" USING (territory_id, country_id, fips_county);
I'm guessing that there's a dependency/correlation between
territory/country/county, and that's probably related to a misestimate causing
a bad plan.
> The first two JOINs are not the problem, & are in fact retained in my
> solution. The problem is the third JOIN, where "fips_county" from "County" is
> actually matched with the corresponding field from the "zip_code" VIEW. Works
> fine, if you don't mind the performance impact in v10 & above. It has now
> been rewritten, to be a sub-query for an output field. Voila ! Back to
> sub-second query times.
What version of 9.6.X were you upgrading *from* ?
v9.6 added selectivity estimates based on FKs, so it's not surprising if there
was a plan change migrating *to* v9.6.
...but there were a number of fixes to that, and it seems possible the plans
changed between 9.6.0 and 9.6.22, and anything backpatched to 9.X would also be
in v10+. So you might've gotten the bad plan on 9.6.22, also.
I found these commits that might be relevant.
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=1f184426bhttps://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=7fa93eec4https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=770671062
ad1c36b07 wasn't backpatched and probably not relevant to your issue.
--
Justin
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To: pgsql-performance@postgresql.org
Cc: pryzby@telsasoft.com, postgresql@mailpen.com, pgsql-performance@lists.postgresql.org
Subject: Re: AWS forcing PG upgrade from v9.6 a disaster
In-Reply-To: <20210601041635.GQ2082@telsasoft.com>
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