| From: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Jie Li <jay23jack(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: small table left outer join big table |
| Date: | 2010-12-29 12:34:05 |
| Message-ID: | 1293626045.1892.4409.camel@ebony |
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| Thread: | |
| Lists: | pgsql-hackers |
On Wed, 2010-12-29 at 07:17 -0500, Robert Haas wrote:
> >
> > Here I have a puzzle, why not choose the small table to build hash table? It
> > can avoid multiple batches thus save significant I/O cost, isn't it?
>
> Yeah, you'd think. Can you post a full reproducible test case?
It's not a bug, that's the way it currently works. We don't need a test
case for that.
I agree that the optimisation would be a useful one.
It allows you to ask the query "Show me sales for each of my stores"
efficiently, rather than being forced to request the inner join query
"Show me the sales for each of my stores for which there have been
sales", which is a much less useful query.
--
Simon Riggs http://www.2ndQuadrant.com/books/
PostgreSQL Development, 24x7 Support, Training and Services
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