Re: Partitioned tables in queries

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