Message-ID: <44C13909.9050900@borderware.com>
Date: Fri, 21 Jul 2006 16:28:57 -0400
From: Kevin Keith <kkeith@borderware.com>
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To: pgsql-performance@postgresql.org
Subject: Re: Partitioned tables in queries
References: <44C12852.1040909@borderware.com>
In-Reply-To: <44C12852.1040909@borderware.com>
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My post might have been a little premature - and I apologize for that.

I have figured out what was causing the problem:
1. Constraint exclusion was disabled. I re-enabled.
2. I found that using the now() function - and arbitrary interval will 
produce a different execution plan that using a specific date. For example:
    assuming the current time is 16:00:
    a) where start_date > now() - interval '4 hours' scans all child tables.
    b) where start_date > '2006-07-21 12:00:00' only scans the child 
table with today's data.

So am I to assume that the value in the query must be a constant, and 
cannot be a result of a built-in function in order for 
constraint_exclusion to work correctly?

Thanks,

Kevin


Kevin Keith wrote:
> I have a case where I am partitioning tables based on a date range in 
> version 8.1.4. For example:
>
> table_with_millions_of_records
> interaction_id  char(16) primary key
> start_date   timestamp (without timezone) - indexed
> .. other columns
>
> child_1   start_date >= 2006-07-21 00:00:00
> child_2   start_date >= 2006-07-20 00:00:00 and start_date < 
> 2006-07-21 00:00:00
> ...
> child_5   start_date >= 2006-07-17 00:00:00 and start_date < 
> 2006-07-18 00:00:00
>
> with rules on the parent and child tables that redirect the data to 
> the appropriate child table based on the start_date.
>
> Because this table is going to grow very large (very quickly), and 
> will need to be purged daily, I created partitions, or child tables to 
> hold data for each day. I have done the same thing in Oracle in the 
> past, and the PostgreSQL solution works great. The archival process is 
> very simple - drop the expired child table. I am having one problem.
>
> If I run a query on the full table (there are 5 child tables with data 
> for the last 5 days), and my where clause contains data for the 
> current day only:
> where start_date > date_trunc('day', now())
> all 5 child tables are scanned when I look at the output from explain 
> analyze.
>
> My question is - can I force the planner to only scan the relevant 
> child table - when the key related to the partitioned data it part of 
> the where clause?
>
> Thanks,
>
> Kevin
>
>
> ...
>