| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | josh(at)agliodbs(dot)com, Greg Stark <gsstark(at)mit(dot)edu>, Marko Ristola <marko(dot)ristola(at)kolumbus(dot)fi>, pgsql-perform <pgsql-performance(at)postgresql(dot)org>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: [HACKERS] Bad n_distinct estimation; hacks suggested? |
| Date: | 2005-04-25 15:23:00 |
| Message-ID: | 19276.1114442580@sss.pgh.pa.us |
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| Lists: | pgsql-hackers pgsql-performance |
Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> My suggested hack for PostgreSQL is to have an option to *not* sample,
> just to scan the whole table and find n_distinct accurately.
> ...
> What price a single scan of a table, however large, when incorrect
> statistics could force scans and sorts to occur when they aren't
> actually needed ?
It's not just the scan --- you also have to sort, or something like
that, if you want to count distinct values. I doubt anyone is really
going to consider this a feasible answer for large tables.
regards, tom lane
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