Re: How to craft a query that uses memory?

Note that after cranking up work mem you then need a query that can
use it. Quickest way to use LOTS of memory is to do a lot of
unconstrained joins.

select * from table a join table b on (true) join table c on (true)
join table d on (true); Result set is size of a*b*c*d

On Fri, Jun 19, 2015 at 7:21 AM,
<Holger(dot)Friedrich-Fa-Trivadis(at)it(dot)nrw(dot)de> wrote:
> Albe Laurenz wrote on Friday, June 19, 2015 12:56 PM:
>> Holger Friedrich wrote:
>> > So how do I craft a query that actually does use lots of memory?
>
>> You increase the parameter "work_mem".
>
>> You can do that globally in postgresql.conf or with SET for one session or with SET LOCAL for one transaction.
>
> Thank you to both Albe Laurenz and Andreas Kretschmer, who both gave the same advice. It was spot-on: my query actually seems to be good for a performance study, setting work_mem to various values causes the query planner to either sort in memory or on disk, and of course this has some impact on performance.
>
> (Sorry for erroneously asking for suggesting a "better" query, rather than for tuning my configuration, which turned out to be the sensible thing to do...)
>
> Best regards,
> Holger Friedrich
>
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