Re: [SQL] Sub-select speed.

> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> >> There has been some talk of automatically rewriting queries to eliminate
> >> unnecessary sub-selects, but I don't foresee it getting done for a
> >> while yet.
>
> > Tom, you mentioned that subselects use nested join, but they could be
> > hardcoded to use hash join. My opinion is that this should be done if
> > it is easy.
>
> That would probably help for uncorrelated subselects (where no variable
> from the outer query is referenced in the inner one). For correlated
> subselects, such as this one is, hashing the subselect result isn't
> going to help much --- the real problem is that the subselect is
> repeated afresh for each outer tuple.

Oh, I was thinking in general of our EXISTS() workaround and if we could
help that by forcing hash joins.

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