Re: GEQO vs join order restrictions

Andres Freund <andres@anarazel.de> writes:
> This also explains why I saw nearly no improvement during the genetic search 
> itself. The paths out of random_init_pool were already hugely selected, so 
> there were not that many improvements to find and a change was relatively like 
> to yield a impossible ordering.

Yeah, I suspect most of the variants tried during that phase simply
failed.

> I do even less know how feasible this is, but given that joins in the right 
> hand side of a LEFT JOIN are not really useful to study from the outside in 
> the general case, would it be possible to "hide" them below the join during 
> join order planning?

We could refrain from collapsing the sub-problem during joinlist
formation.  But the trouble with that is it creates a "hard" join order
restriction.  Most of the restrictions are "soft" to some extent, ie,
you can do some rearrangements but not others.  It might be worth
looking at though; in the cases where there is no chance of a
rearrangement, it would save cycles for either regular or GEQO planning.
        regards, tom lane