Re: Bug? Query plans / EXPLAIN using gigabytes of memory

On 26/04/12 13:11, Tom Lane wrote:
> Toby Corkindale<toby(dot)corkindale(at)strategicdata(dot)com(dot)au> writes:
>> Just wondering if anyone else has thoughts on this?
>
>> I'm still suspicious that this is a bug.
>
> Well, if you were to provide a reproducible test case, somebody might be
> motivated to look into it. There could be a memory leak in the planner
> somewhere, but without a test case it's not very practical to go look
> for it.

Hi Tom,
Thanks for responding.. I'm trying to work on a test case, but it's
quite tricky.
It'll need to be something like a script that generates a tonne of
partitions at the very least. I don't know if the actual amount of data
in the partitions is part of the problem or not.
Would a Perl-based script that built up a database like that be a useful
test case for you?

For what it's worth, I discovered something quite interesting. The
memory usage only blows out when I do an update based on the results of
the query. But not if I just select the results on their own, nor if I
do the update using those values on its own.

ie.

Method #1, uses all the memory and doesn't return it:
explain update line set status = 'foo'
where file_id=725 and line.lineno in (
select line from complex_view
where file_id=725
);

Method #2, also uses all the memory:
explain update line set status = 'foo'
from complex_view v
where line.lineno = v.line
and line.file_id=725
and v.file_id=725;

Method #3, which uses next to no memory:
explain select line from complex_view
where file_id=725;

Method #4, which also uses next to no memory:
explain create temp table foo as
select line from complex_view;

where file_id=725;
update line set status = 'foo'
from foo
where line.lineno=foo.line
and file_id=725;

-Toby