| From: | Lincoln Yeoh <lylyeoh(at)mecomb(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, karl(at)debisschop(dot)net |
| Cc: | ddd(at)genesis(dot)homeip(dot)net, pgsql-general(at)hub(dot)org |
| Subject: | Re: Am I really stupid??? |
| Date: | 2000-05-19 01:05:54 |
| Message-ID: | 3.0.5.32.20000519090554.008e9ad0@pop.mecomb.po.my |
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| Thread: | |
| Lists: | pgsql-general |
At 11:18 AM 17-05-2000 -0400, Tom Lane wrote:
>In fact, IN (subselect), INTERSECT, and EXCEPT are all pretty much the
>same thing, and they're all pretty slow in the current code :-(, because
>they all work by rescanning the inner query for each outer tuple --- in
>other words, they're all implemented like plain nestloop joins. EXISTS
>is marginally better because the planner can figure out how to use an
>index on the inner table, if there is one.
Does that mean for two tables, one 1000 rows another 2000 rows, it's a
total of 1000 X 2000 rows scanned?
I suppose one way to slightly reduce the number of rows would be to select
stuff into temp tables first.
Cheerio,
Link.
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