On Thu, Dec 30, 2010 at 10:45 AM, Tom Lane <
tgl@sss.pgh.pa.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.)