Re: Different execution plans in PG17 and pgBouncer...

On 5/5/25 11:00, Mladen Marinović wrote:
>
>
> On Mon, May 5, 2025 at 11:24 AM Achilleas Mantzios
> <a(dot)mantzios(at)cloud(dot)gatewaynet(dot)com> wrote:
>
>
> On 5/5/25 09:52, Mladen Marinović wrote:
>> Hi,
>>
>> We recently migrated our production instances from PG11 to PG17.
>> While doing so we upgraded our pgBouncer instances from 1.12 to
>> 1.24. As everything worked on the test servers we pushed this to
>> production a few weeks ago. We did not notice any problems until
>> a few days ago (but the problems were here from the start). The
>> main manifestation of the problems is a service that runs a fixed
>> query to get a backlog of unprocessed data (limited to a 1000
>> rows). When testing the query using pgAdmin connected directly to
>> the database we get a result in cca. 20 seconds. The same query
>> runs for 2 hours when using pgBouncer to connect to the same
>> database.
>
>
> That's a huge jump, I hope you guys did extensive testing of your
> app. In which language is your app written? If java, then define
> prepareThreshold=0 in your jdbc and set max_prepared_statements =
> 0 in pgbouncer.
>
> Mainly python, but the problem was noticed in a java service.
> Prepare treshold was already set to 0. We changed the
> max_prepared_statements to 0 from the default (200) but no change was
> noticed.
>
> How about search paths ? any difference on those between the two
> runs ? Do you set search_path in pgbouncer ? what is "cca." btw ?
>
>>
>> The more interesting part is that when we issue an explain of the
>> same query we get different plans. We did this a few seconds
>> apart so there should be no difference in collected statistics.
>> We ruled out prepared statements, as we suspected the generic
>> plan might be the problem, but it is not. Is there any pgBouncer
>> or PG17 parameter that might be the cause of this?
>
>
> Does this spawn any connections (such as dblink) ? are there
> limits per user/db pool_size in pgbouncer ?
>
> No additional connection nor dbling. Just plain SQL (CTE, SELECT,
> INSERT, UPDATE, DELETE,...) There are limits, but they are not hit.
> The query just uses a different plan and runs slower because of that.
>
> Pgbouncer, in contrast to its old friend PgPool-II is completely
> passive, just passes through SQL to the server as fast as possible
> as it can. But I am sure you know that. Good luck, keep us posted!
>
> Yes, that is what puzzles me.

What is the pgbouncer's timeout in the server connections ?

How about "idle in transaction" ? do you get any of those? What's the
isolation level ?

How about the user ? is this the same user doing pgadmin queries VS via
the app ?

Can you identify the user under which the problem is manifested and :

ALTER user "unlucky_user" SET log_statement = 'all';

ALTER user "unlucky_user" SET log_min_duration_statement = 0; -- to help
you debug the prepared statements .. just in case , and other stuff not
printed by log_statement = all.

> Regards,
> Mladen Marinović