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
>
>
> ...
>
