Re: Function runtime increases after 5 calls in the same session.

Hi

On Tue, Oct 21, 2014 at 12:53 PM, Ilya I. Ashchepkov <koctep@gmail.com> wrote:
> I wrote a function and during testing it I came across the strange
> behaviour.
> Function runtime is about 200ms first 5 times, 6th and futher calls takes
> ~22000 ms.
> I simplified my schema, you can see it in attached file.
> I've tested on 9.4beta3 and 9.3.5.

I didn't look at the test case. But most likely the problem is that
after 5 executions, plancache decides that a generic plan is no more
expensive than a custom plan, and reverts to executing the generic
one. But in reality there is a big difference in execution time.

See this for how the logic works:
https://github.com/postgres/postgres/blob/master/src/backend/utils/cache/plancache.c#L1036

As a workaround you could use PL/pgSQL EXECUTE to run the query, which
isn't subject to generic plans:
http://www.postgresql.org/docs/current/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN

Regards,
Marti