From: | Gregory Stark <stark(at)enterprisedb(dot)com> |
---|---|
To: | "Joshua Tolley" <eggyknap(at)gmail(dot)com> |
Cc: | PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Cross-column statistics revisited |
Date: | 2008-10-15 13:51:37 |
Message-ID: | 871vyiarva.fsf@oxford.xeocode.com |
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Lists: | pgsql-hackers |
"Joshua Tolley" <eggyknap(at)gmail(dot)com> writes:
> I've been interested in what it would take to start tracking
> cross-column statistics. A review of the mailing lists as linked from
> the TODO item on the subject [1] suggests the following concerns:
>
> 1) What information exactly would be tracked?
> 2) How would it be kept from exploding in size?
> 3) For which combinations of columns would statistics be kept?
I think then you have
4) How would we form estimates from these stats
> The major concern in #1 seemed to be that the most suitable form for
> keeping most common value lists, histograms, etc. is in an array, and
> at the time of the posts I read, arrays of composite types weren't
> possible. This seems much less of a concern now -- perhaps in greatest
> part because a test I just did against a recent 8.4devel sure makes it
> look like stats on composite type columns aren't even kept. The most
> straightforward is that we'd keep a simple multi-dimensional
> histogram, but that leads to a discussion of #2.
"multi-dimensional histogram" isn't such a simple concept, at least not to me.
Histograms aren't a bar chart of equal widths and various heights like I was
taught in school. They're actually bars of various widths arranged such that
they all of the same heights.
It's not clear how to extend that concept into two dimensions. I imagine
there's research on this though. What do the GIST statistics functions store?
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's 24x7 Postgres support!
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