Re: unstable query plan on pg 16,17,18

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.

--
regards, Andrei Lepikhov,
pgEdge