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

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?

Yours,
Laurenz Albe