| From: | Magnus Hagander <magnus(at)hagander(dot)net> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | pgsql-committers(at)postgresql(dot)org |
| Subject: | Re: pgsql: Teach tuplesort.c about "top N" sorting, in which only the first |
| Date: | 2007-05-04 16:26:38 |
| Message-ID: | 20070504162638.GD30617@svr2.hagander.net |
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| Lists: | pgsql-committers pgsql-hackers |
On Fri, May 04, 2007 at 10:04:08AM -0400, Tom Lane wrote:
> Magnus Hagander <magnus(at)hagander(dot)net> writes:
> > Is there some way to see in the generated query plan if this optimisation
> > is used?
>
> If there's a SORT just below a LIMIT (that has a limit, ie it's not just
> an OFFSET), then it's potentially used. Whether it's actually used
> depends on actual row counts and widths at runtime --- you'd have to
> turn on trace_sort and look at the log output to determine that.
Could we show it in EXPLAIN ANALYZE somehow? I'm thinking it would be good
to see at runtime (for example as a hint that if you put in a bit more
work_mem it might get used)
//Magnus
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