| From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
|---|---|
| To: | Tom Raney <raneyt(at)cecs(dot)pdx(dot)edu> |
| Cc: | Postgres Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Planner question |
| Date: | 2008-09-05 18:41:23 |
| Message-ID: | 1220640083.11403.32.camel@dell.linuxdev.us.dell.com |
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| Lists: | pgsql-hackers |
On Fri, 2008-09-05 at 11:21 -0700, Tom Raney wrote:
> Why does the planner consider both input variations of each symmetric merge join? The README says "there is not a lot of difference" between the two options. When are there any differences?
>
> -Tom Raney
>
http://archives.postgresql.org/pgsql-general/2008-08/msg00967.php
My understanding from that thread is that if one table has high
ndistinct and the other has low ndistinct, one plan may require more
re-scanning than the other.
Regards,
Jeff Davis
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