Re: GEQO and join_collapse_limit correlation

Hi,

> After some testing in order to lower the planning time we ended bringing
> > down the GEQO values, and we have the best results with:
>
> > from_collapse_limit = 150
> > join_collapse_limit = 150
> > geqo_threshold = 2
> > geqo_effort= 2
>
> Hmm. The trouble with this approach is that you're relying on GEQO
> to find a good plan, and that's only probabilistic --- especially so
> when you're reducing geqo_effort, meaning it doesn't try as many
> possibilities as it otherwise might. Basically, therefore, the
> fear is that every so often you'll get a bad plan.
>

What we felt odd was having to find a balance between geqo_threshold and
join_collapse_limit, lowering one was only effective after raising the
other. The geqo_effort was only mofidied after we found this path, and some
more testing.

In an environment with geqo_threshold=1 and join_collapse_limit=1, would
the planner be GEQO exclusive (and syntactic)?

If the queries are fairly stylized, you might be able to get good
> results by exploiting rather than bypassing join_collapse_limit:
> determine what a good join order is, and then write the FROM clause
> as an explicit JOIN nest in that order, and then *reduce* not raise
> join_collapse_limit to force the planner to follow the syntactic
> join order. In this way you'd get rid of most of the run-time
> join order search effort. Don't know how cooperative your ORM
> would be with such an approach though.
>

The ORM seems to build the join path just the other way round of what would
be good for the planner. The thing we should take a good look at if it is
really needed looking at +120 tables for a query that gets a pretty trivial
result, but that is completely off topic.

> regards, tom lane
>

Thanks for your repply.

Regards,

Juan José Santamaría