Re: LIMIT on partitioned-table!?

On Tue, Feb 15, 2011 at 21:33, Kim A. Brandt <kimabrandt(at)gmx(dot)de> wrote:
> removing the ORDER BY worked. But I am afraid to ask this. How can I order
> by partition? It seams that the planner has picked a random(!?) order of
> partition to select from. The returned records, from the selected partition,
> are correctly sorted bythe index though.

If a single query accesses more than one partition, PostgreSQL
currently cannot read the values in index-sorted order. Hence with
ORDER BY and LIMIT, PostgreSQL cannot return *any* results before it
has read all matching rows and then sorted them. Adding a LIMIT
doesn't help much. Your only bet is to reduce the number of matched
rows, or make sure that you only access a single partition.

Increasing work_mem may speed up the sort step if you're hitting the
disk (EXPLAIN ANALYZE VERBOSE will tell you whether that's the case).

This will change in PostgreSQL 9.1 which has a new Merge Append plan node.

Regards,
Marti