Re: Partitioned tables in queries

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On Jul 21, 2006, at 12:17 PM, 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?

Yes. You'll need non-overlapping check constraints in each child table and to set constraint_exclusion to "on" in postgresql.conf.

See http://www.postgresql.org/docs/8.1/static/ddl-partitioning.html for the gory details.

Cheers,
  Steve


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