Re: Postgres: Queries are too slow after upgrading to PG17 from PG15

On Thu, 31 Jul 2025 at 07:49, Todd Cook <cookt(at)blackduck(dot)com> wrote:
> I work at the same company as Sajith, but on a different product. The reproducer he
> provided is just a sample; it's not the only problem. Load testing in my team shows
> that PG 17 is about 4x slower than PG 15 across the board. It's bordering on unusable
> for production deployments.
>
> Unfortunately, the load testing setup doesn't really help isolate individual, regressing
> queries. However, I'm more than willing to help support any further investigation if
> needed or helpful.

Unfortunately, we can't really work with that much information. It's
not like we have a list of things we know are slower in newer versions
vs older versions. Changes generally go through a large amount of
testing to help ensure these regressions don't happen, so if you
report one, unless someone else beat you to it, there's a decent
chance we didn't know about it. There's nothing we can really do to
help you based on this much information. There's just no chance we'd
have shipped PG17 if it was known to be x4 slower than some previous
version.

You may be able to narrow down what's slower using pg_stat_statements.
If you can, then use EXPLAIN and compare the plans. Did PG17 choose a
different plan? Does EXPLAIN ANALYZE reveal any inaccurate statistics?
Are both instances configured the same way?

Once you find a specific query that's causing the issue, then a report
similar to what Sajith has done is a good way to get help.

David