Re: No hash join across partitioned tables?

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, Kris Jurka <books(at)ejurka(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: No hash join across partitioned tables?
Date: 2010-06-09 19:47:55
Message-ID: AANLkTinx8lLTEKWcyEQ1rxVz6WMJVKNezfXW5TKnNAU6@mail.gmail.com
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On Tue, Mar 2, 2010 at 12:23 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Thu, Feb 25, 2010 at 7:03 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Partially.  There are stats now but autovacuum is not bright about
>>> when to update them.
>
>> Is that something you're planning to fix for 9.0?  If not, we at least
>> need to document what we intend for people to do about it.
>
> I want to look at it, but I'm not sure whether the fix will be small
> enough that we want to put it in during beta.

In going back through emails I had marked as possibly needing another
look before 9.0 is released, I came across this issue again. As I
understand it, analyze (or analyse) now collects statistics for both
the parent individually, and for the parent and its children together.
However, as I further understand it, autovacuum won't actually fire
off an analyze unless there's enough activity on the parent table
considered individually to warrant it. So if you have an empty parent
and a bunch of children with data in it, your stats will still stink,
unless you analyze by hand.

Assuming my understanding of the problem is correct, we could:

(a) fix it,
(b) document that you should consider periodic manual analyze commands
in this situation, or
(c) do nothing.

Thoughts?

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

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