Re: Removing unneeded self joins

Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> writes:
> IIUC in DB2 (the clear winner at join elimination in the article you
> mentioned), you get these sorts of things by default (optimisation
> level 5 includes it), but not if you SET CURRENT QUERY OPTIMIZATION =
> 3 as many articles recommend for OLTP work. I think it's interesting
> that they provide that knob rather than something automatic, and
> interesting that there is one linear knob to classify your workload
> rather than N knobs for N optimisations.

There's a lot to be said for that type of approach, as opposed to trying
to drive it off some necessarily-very-inexact preliminary estimate of
query cost. For example, the mere fact that you're joining giant tables
doesn't in itself suggest that extra efforts in query optimization will be
repaid. (If anything, it seems more likely that the user would've avoided
silliness like useless self-joins in such a case.)

A different line of thought is that, to me, the most intellectually
defensible rationale for efforts like const-simplification and join
removal is that opportunities for those things can arise after view
expansion, even in queries where the original query text didn't seem
to contain anything extraneous. (Robert and Andres alluded to this
upthread, but not very clearly.) So maybe we could track how much
the query got changed during rewriting, and use that to drive the
planner's decisions about how hard to work later on. But I'm not
very sure that this'd be superior to having a user-visible knob.

regards, tom lane