| From: | "Jim C(dot) Nasby" <decibel(at)decibel(dot)org> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Neil Conway <neilc(at)samurai(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Hash vs. HashJoin nodes |
| Date: | 2005-04-01 00:32:40 |
| Message-ID: | 20050401003240.GU53309@decibel.org |
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| Lists: | pgsql-hackers |
On Thu, Mar 31, 2005 at 12:03:37AM -0500, Tom Lane wrote:
> > Right; I was planning to bail and only do this for inner joins.
>
> Well, for outer joins the optimal strategy is simple: pull from the
> outer query first. If it's empty then you needn't touch the inner
> query at all. Otherwise you have to build the hash table.
What about the case of an empty inner query? Granted, you still have to
read in the outer query, but would there be any reason to generate
hashes for it's results?
--
Jim C. Nasby, Database Consultant decibel(at)decibel(dot)org
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