Re: Strange performance problem

Alessandro Manzoni <manzoni(dot)alessandro4(at)gmail(dot)com> writes:
> Then I have a complex query that has two complete different
> performances, in these two cases:

> case a)
> with getfieldone as (select * from tableone where fieldone = 'one')
> select * from getfieldone
> inner join tabletwo on ... [and other joins]

> case b)
> select * from viewone
> inner join tabletwo on ... [and other joins, the same as case a)]

> I expected the same performance, but case a) lasts a few hundreds ms,
> while case b) lasts more than 12 seconds.

WITH is an optimization fence (at present --- there is discussion
of relaxing that) so it's not exactly surprising that these queries
perform differently. I am a little surprised though that case a)
is the faster one; usually, preventing the planner from optimizing
the whole query as one problem makes things worse.

Saying more than that would require a lot more detail.

https://wiki.postgresql.org/wiki/Slow_Query_Questions

regards, tom lane