| 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 21:10:55 |
| Message-ID: | 24355.1114463455@sss.pgh.pa.us |
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| Lists: | pgsql-hackers pgsql-performance |
Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> On Mon, 2005-04-25 at 11:23 -0400, Tom Lane wrote:
>> 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.
> Assuming you don't use the HashAgg plan, which seems very appropriate
> for the task? (...but I understand the plan otherwise).
The context here is a case with a very large number of distinct
values... keep in mind also that we have to do this for *all* the
columns of the table. A full-table scan for each column seems
right out to me.
regards, tom lane
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