On Thu, Dec 30, 2010 at 11:50 PM, Robert Haas <robertmh...@gmail.com> wrote:
> On Thu, Dec 30, 2010 at 10:45 AM, Tom Lane <t...@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.) > > 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, http://dbcrusade.blogspot.com/2008/01/oracle-hash-join-right-outer.html And Oracle 11g has the full hash join. http://www.dba-oracle.com/oracle11g/oracle_11g_full_hash_join.htm Haven't checked whether other DBMS have this feature. Thanks, Li Jie
