| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | PostgreSQL - Hans-Jürgen Schönig <postgres(at)cybertec(dot)at> |
| Cc: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, PostgreSQL-development Hackers <pgsql-hackers(at)postgresql(dot)org>, Boszormenyi Zoltan <zb(at)cybertec(dot)at> |
| Subject: | Re: cross column correlation revisted |
| Date: | 2010-07-14 14:22:01 |
| Message-ID: | 4820.1279117321@sss.pgh.pa.us |
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=?iso-8859-1?Q?PostgreSQL_-_Hans-J=FCrgen_Sch=F6nig?= <postgres(at)cybertec(dot)at> writes:
> i think that having stats on an index is a problem by itself for 2 reasons - for cross column correlation at least:
> a.) joins cannot be covered by an index on two tables - we would fix "inside a table correlation problems" but not joins.
Your proposed command didn't cover the two-table case either, and anyway
what the heck do you mean by cross-correlation across tables?
Cross-correlation is about the correlation between values in the same
row.
> b.) who says that there is actually an index in place?
If the combination of columns is actually interesting, there might well
be an index in place, or the DBA might be willing to create it. For
that matter, have you considered the idea of examining the index
contents to derive the statistics? Might work better than trying to get
numbers via ANALYZE.
regards, tom lane
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