From: Dean Gibson (DB Administrator) <postgresql@mailpen.com>
To: pgsql-performance@lists.postgresql.org
Subject: Re: AWS forcing PG upgrade from v9.6 a disaster
Date: Tue, 1 Jun 2021 10:44:54 -0700
Message-ID: <a38c45cf-3bbd-b43a-0f8a-11539c1c6dc1@mailpen.com> (raw)
In-Reply-To: <20210601041635.GQ2082@telsasoft.com>
References: <88dec70b-eb63-0b72-0b5d-253851ef127b@mailpen.com>
<92cc8563-195f-8727-d887-5f2bb63f231f@mailpen.com>
<20210601041635.GQ2082@telsasoft.com>
On 2021-05-31 21:16, Justin Pryzby wrote:
>> 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.
I originally upgraded from 9.6.20 to v12.6. When that (otherwise
successful) upgrade had performance problems, I upgraded the v9.6.20
copy to v9.6.21, & tried again, with the same result.
Interestingly, on v13.2 I have now run into another (similar)
performance issue. I've solved it by setting the following to values I
used with v9.x:
join_collapse_limit & from_collapse_limit = 16
geqo_threshold = 32
I pretty sure I tried those settings (on v10 & above) with the earlier
performance problem, to no avail. However, I now wonder what would have
been the result if I have doubled those values before re-architecting
some of my tables (moving from certain JOINs to specific sub-selects).
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Subject: Re: AWS forcing PG upgrade from v9.6 a disaster
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