On Thu, Dec 30, 2010 at 11:50 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Dec 30, 2010 at 10:45 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > I had an epiphany about this topic, or actually two of them.
> > 1. Whether or not you think there's a significant performance reason
> > to support hash right joins, there's a functionality reason. The
> > infrastructure for right join could just as easily do full joins.
> > And AFAICS, a hash full join would only require one hashable join
> > clause --- the other FULL JOIN ON conditions could be anything at all.
> > This is unlike the situation for merge join, where all the JOIN ON
> > conditions have to be mergeable or it doesn't work right. So we could
> > greatly reduce the scope of the dreaded "FULL JOIN is only supported
> > with merge-joinable join conditions" error. (Well, okay, it's not
> > *that* dreaded, but people complain about it occasionally.)
> Yeah, that would be neat. It might be a lot faster in some cases, too.
Yeah, PostgreSQL should have this great feature.
Actually Oracle 10g already has the right hash join,
And Oracle 11g has the full hash join.
Haven't checked whether other DBMS have this feature.
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