|From:||Jeff Davis <pgsql(at)j-davis(dot)com>|
|To:||Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>|
|Subject:||Re: PATCH: decreasing memory needlessly consumed by array_agg|
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On Wed, 2015-01-28 at 23:25 +0100, Tomas Vondra wrote:
> 3) moves the assert into the 'if (release)' branch
You missed one, but I got it.
> 4) includes the comments proposed by Ali Akbar in his reviews
> Warnings at makeArrayResult/makeMdArrayResult about freeing memory
> with private subcontexts.
I also edited the comments substantially.
> Regarding the performance impact of decreasing the size of the
> preallocated array from 64 to just 8 elements, I tried this.
> CREATE TABLE test AS
> SELECT mod(i,100000) a, i FROM generate_series(1,64*100000) s(i);
> SELECT a, array_agg(i) AS x FRM test GROUP BY 1;
> or actually (to minimize transfer overhead):
> SELECT COUNT(x) FROM (
> SELECT a, array_agg(i) AS x FRM test GROUP BY 1
> ) foo;
That's actually a bogus test -- array_agg is never executed. See the
test script I used (attached). Many small groups has a significant
improvement with your patch (median 167ms unpatched, 125ms patched), and
the one large group is not measurably different (median 58ms for both).
The test script uses a small dataset of 100K tuples, because 1M tuples
will run out of memory for small groups without the patch.
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