Re: Partitions and work_mem?

[Jeff Janes <jeff.janes@xxxxxxxxx>]

On Tue, Oct 14, 2014 at 10:08 AM, Dave Johansen <davejohansen@xxxxxxxxx> wrote:

I'm running Postgres 8.4 on RHEL 6 64-bit and I had a question about how work_mem and partitions interact.

https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server#work_mem

The above wiki states that "if a query involves doing merge sorts of 8 tables, that requires 8 times work_mem." If I have a table that is partitioned does each partition count as a "table" and get its on work_mem?

For example, say I have the following table partitioned by the time column:

CREATE TABLE values (time TIMESTAMP, value INTEGER);

If I do the following query will it require 1 work_mem or N work_mem's (where N is the number of partitions)?

SELECT * FROM values ORDER BY time;

The specific query you show should do the append first and then the sort on the result, and so would only use 1 work_mem.

However, other queries could cause it to use one (or more) sorts per partition, for example a self-join which it decides to run as a sort-merge join.

 

Cheers,

Jeff