Re: query not using index

On 19 December 2013 16:48, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Johann Spies <johann(dot)spies(at)gmail(dot)com> writes:
> > I would appreciate some help optimising the following query:
>
> It's a mistake to imagine that indexes are going to help much with
> a join of this size. Hash or merge join is going to be a lot better
> than nestloop. What you need to do is make sure those will perform
> as well as possible, and to that end, it'd likely help to raise
> work_mem. I'm not sure if you can sanely put it high enough to
> make the query operate totally in memory --- it looks like you'd
> need work_mem of 500MB or more to prevent any of the sorts or
> hashes from spilling to disk, and keep in mind that this query
> is going to use several times work_mem because there are multiple
> sorts/hashes going on. But if you can transiently dedicate a lot
> of RAM to this query, that should help some. I'd suggest increasing
> work_mem via a SET command in the particular session running this
> query --- you don't want such a high value to be the global default.
>

Thanks Tom. Raising work_mem from 384MB to 512MB made a significant
difference.

You said "hash or merge join id going to be a lot better than nestloop".
Is that purely in the hands of the query planner or what can I do to get
the planner to use that options apart from raising the work_mem?

Regards
Johann

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