Message-ID: <44C12852.1040909@borderware.com>
Date: Fri, 21 Jul 2006 15:17:38 -0400
From: Kevin Keith <kkeith@borderware.com>
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To: pgsql-performance@postgresql.org
Subject: Partitioned tables in queries
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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


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