| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "Jim 'Decibel!' Nasby" <jnasby(at)cashnetusa(dot)com> |
| Cc: | Richard Huxton <dev(at)archonet(dot)com>, postgresql performance list <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Oddity with view (now with test case) |
| Date: | 2008-11-12 02:34:50 |
| Message-ID: | 21846.1226457290@sss.pgh.pa.us |
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| Lists: | pgsql-performance |
"Jim 'Decibel!' Nasby" <jnasby(at)cashnetusa(dot)com> writes:
> So here's something odd... in both 8.3 and HEAD from a while ago it
> gives a better plan for SELECT * than for SELECT count(*):
The short answer is that the Subquery Scan nodes can be dropped out
when they are no-ops, which is to say producing the same set of columns
their input produces (and not testing any filter conditions, but that's
not relevant here). SELECT count(*) doesn't want to know about any
columns so the output of the UNION arm doesn't match ...
regards, tom lane
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