Re: Major performance problem after upgrade from 8.3 to 8.4

Gerhard Wiesinger <lists(at)wiesinger(dot)com> writes:
> I know the drawbacks of an EAV design but I don't want to discuss that. I
> want to discuss the major performance decrease of PostgreSQL 8.3
> (performance was ok) to PostgreSQL 8.4 (performance is NOT ok).

> Any further ideas how I can track this down?
> Can someone explain the difference in query plan from an optimizer point
> of view?

Since you haven't shown us the 8.3 plan, it's kind of hard to speculate ;-)

One thing that jumped out at me was that 8.4 appears to be expecting
multiple matches in each of the left-joined tables, which is why the
total rowcount estimate balloons so fast. I rather imagine that you are
expecting at most one match in reality, else the query isn't going to
behave nicely. Is this correct? Are you *sure* you analyzed all these
tables? And if that is how the data looks, where is the actual
performance problem? A bad rowcount estimate isn't in itself going
to kill you.

FWIW, in a similar albeit toy example, I don't see any difference
between the 8.3 and 8.4 plans or cost estimates.

regards, tom lane