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From: Josh Berkus <josh@agliodbs.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
To: Claudio Freire <klaussfreire@gmail.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: On partitioning
Date: Fri, 12 Dec 2014 14:40:35 -0800
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On 12/12/2014 02:10 PM, Tom Lane wrote:
> Actually, I'm not sure that's what we want.  I thought what we really
> wanted here was to postpone partition-routing decisions to runtime,
> so that the behavior would be efficient whether or not the decision
> could be predetermined at plan time.
> 
> This still leads to the same point Robert is making: the routing
> decisions have to be cheap and fast.  But it's wrong to think of it
> in terms of planner proofs.

The other reason I'd really like to have the new partitioning taken out
of the planner: expressions.

Currently, if you have partitions with constraints on, day,
"event_date", the following WHERE clause will NOT use CE and will scan
all partitions:

WHERE event_date BETWEEN ( '2014-12-11' - interval '1 month' ) and
'2014-12-11'.

This is despite the fact that the expression above gets rewritten to a
constant by the time the query is executed; by then it's too late.  To
say nothing of functions like to_timestamp(), now(), etc.

As long as partitions need to be chosen at plan time, I don't see a good
way to fix the expression problem.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com


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