Re: Optimizing Multiply Joins ???

Meszaros Attila <tilla@draconis.elte.hu> writes:
> Until we join no more than 10 tables the response time is below 0.2 s.
> joining the 11th table comes with a dramatic change: response time
> usually grows up to 5-7 s, 

That's interesting; apparently the regular optimizer is faster than the
GEQO optimizer for your style of query.  Try increasing the GEQO
threshold (pg_option "geqo_rels") to more than 11.

>     I have examined the output of explain in all 3 cases, and I have
>     the feeling that the planner simply forgets the best solutions
>     in 2nd and 3rd case.

The GEQO planner does not guarantee to find an optimal solution, it
just does a random search through a limited number of possible solutions
and uses the best one it happened across.

> But I think (hope) we could have a solution, because all of our
> complex joins are following foreign keys. 

Actually, as the 7.1 code currently stands, a query that uses explicit
JOIN operators like yours does will always be implemented in exactly
the given join order, with no searching.  I haven't quite decided if
that's a bug or a feature ...
        regards, tom lane