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
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