On Tue, Oct 14, 2014 at 10:08 AM, Dave Johansen <davejohan...@gmail.com> 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
