Re: left-deep plans?

Neil Conway <neilc(at)samurai(dot)com> writes:
> Tom Lane wrote:
>> Once we get into GEQO territory, we are using the left-deep-only
>> heuristic because that's the only kind of plan GEQO can construct.

> I think most applications would prefer an exhaustive, deterministic
> search of a subset of the search space over a non-exhaustive,
> non-deterministic search of the same subset, given approximately the
> same performance.

I am not by any means standing up to defend GEQO as being the best
way to do partial searches ;-). Just saying that in the regime where
we can hope to do complete searches, we shouldn't exclude bushy plans.

> Speaking of which, why does GEQO restrict its search to left-deep plans
> only?

Well, because it's really a traveling-salesman algorithm, and it models
the "find a good join tree" problem as "find a good tour". I've
commented before that I don't believe this is a particularly good model
--- intuitively it doesn't seem that the cost functions have the same
structure. But I've not had time to look for a better heuristic
algorithm. Just one of the many things on the TODO list ...

regards, tom lane