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Re: WIP: cross column correlation ...

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Nathan Boley <npboley(at)gmail(dot)com>
Cc: PostgreSQL - Hans-Jürgen Schönig <postgres(at)cybertec(dot)at>, pgsql-hackers Hackers <pgsql-hackers(at)postgresql(dot)org>, Boszormenyi Zoltan <zb(at)cybertec(dot)at>
Subject: Re: WIP: cross column correlation ...
Date: 2011-02-23 14:46:34
Message-ID: AANLkTind81SDsTqrsSnvoeMJeKbNafAtJXdW4=U1NkRa@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, Feb 23, 2011 at 12:50 AM, Nathan Boley <npboley(at)gmail(dot)com> wrote:
>> Personally, I think the first thing we ought to do is add a real, bona
>> fide planner hint to override the selectivity calculation manually,
>> maybe something like this:
>>
>> WHERE (x < 5 AND y = 1) SELECTIVITY (0.1);
>
> If you're going to go that far, why not just collect statistics on
> that specific predicate?
>
> ie,  ANALYZE SELECTIVITY ON tablename (x, y) WHERE (x < 5 AND y = 1);
>
> Then it won't fall subject to all of the pitfalls that Tom outlines below.
>
> Selectivities are easy to estimate if we know the predicate. They only
> become hard when they have to work for every possible predicate.

Fair point.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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