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From: Attila Soki <[email protected]>
To: Andrei Lepikhov <[email protected]>
To: [email protected]
Subject: Re: unstable query plan on pg 16,17,18
Date: Mon, 23 Feb 2026 18:03:33 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>

> On 23 Feb 2026, at 16:54, Andrei Lepikhov <[email protected]> wrote:
> 
> On 23/2/26 10:41, Laurenz Albe wrote:
>> On Mon, 2026-02-23 at 10:37 +0100, Attila Soki wrote:
>>> When upgrading from PostgreSQL 14.4, I noticed that one of my somewhat complex
>>> analytical queries sometimes gets an inefficient plan under PostgreSQL 16, 17, and 18.
>>> Under 14.4, the query runs with a stable plan and completes in 19 to 22 seconds.
>>> In newer versions, the plan seems to be unstable, sometimes the query completes
>>> in 17 to 20 seconds, sometimes it runs for 5 to 18 minutes with the inefficient plan.
>>> This also happens even if the data is not significantly changed.
>> This is very likely owing to a bad estimate.
>> Could you turn on "track_io_timing" and send us the EXPLAIN (ANALYZE, BUFFERS) output
>> for both the good and the bad plan?
> 
> Since PG16, the optimiser changed a lot. So, there are plenty of possibilities that might happen - table statistics update, for example. So, we need at least EXPLAIN ANALYSE for 'good' and 'bad' cases to begin a discussion.

Hi Andrei,

see my previous answer:
https://www.postgresql.org/message-id/1695A676-062B-47C5-B302-91E2357DC874%40gmx.net

but here are the plans again:
In order to be able to publish the plans here, I have obfuscated the table and field names, but this is reversible, so I can provide more info if needed.

plan-ok:
https://explain.depesz.com/s/hQvM

plan-wrong:
https://explain.depesz.com/s/uLvl


regards,
Attila

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