Re: [PATCH] Incremental sort (was: PoC: Partial sort)

On Fri, Mar 13, 2020 at 8:23 PM James Coleman <jtc331(at)gmail(dot)com> wrote:
>
> On Friday, March 13, 2020, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>>
>> On Fri, Mar 13, 2020 at 04:31:16PM -0400, James Coleman wrote:
>>>
>>> On Fri, Mar 13, 2020 at 2:23 PM James Coleman <jtc331(at)gmail(dot)com> wrote:
>>>>
>>>>
>>>> On Tue, Mar 10, 2020 at 10:44 PM Tomas Vondra
>>>> <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>>>> > 3) Most of the execution plans look reasonable, except that some of the
>>>> > plans look like this:
>>>> >
>>>> >
>>>> > QUERY PLAN
>>>> > ---------------------------------------------------------
>>>> > Limit
>>>> > -> GroupAggregate
>>>> > Group Key: t.a, t.b, t.c, t.d
>>>> > -> Incremental Sort
>>>> > Sort Key: t.a, t.b, t.c, t.d
>>>> > Presorted Key: t.a, t.b, t.c
>>>> > -> Incremental Sort
>>>> > Sort Key: t.a, t.b, t.c
>>>> > Presorted Key: t.a, t.b
>>>> > -> Index Scan using t_a_b_idx on t
>>>> > (10 rows)
>>>> >
>>>> > i.e. there are two incremental sorts on top of each other, with
>>>> > different prefixes. But this this is not a new issue - it happens with
>>>> > queries like this:
>>>> >
>>>> > SELECT a, b, c, d, count(*) FROM (
>>>> > SELECT * FROM t ORDER BY a, b, c
>>>> > ) foo GROUP BY a, b, c, d limit 1000;
>>>> >
>>>> > i.e. there's a subquery with a subset of pathkeys. Without incremental
>>>> > sort the plan looks like this:
>>>> >
>>>> > QUERY PLAN
>>>> > ---------------------------------------------
>>>> > Limit
>>>> > -> GroupAggregate
>>>> > Group Key: t.a, t.b, t.c, t.d
>>>> > -> Sort
>>>> > Sort Key: t.a, t.b, t.c, t.d
>>>> > -> Sort
>>>> > Sort Key: t.a, t.b, t.c
>>>> > -> Seq Scan on t
>>>> > (8 rows)
>>>> >
>>>> > so essentially the same plan shape. What bugs me though is that there
>>>> > seems to be some sort of memory leak, so that this query consumes
>>>> > gigabytes os RAM before it gets killed by OOM. But the memory seems not
>>>> > to be allocated in any memory context (at least MemoryContextStats don't
>>>> > show anything like that), so I'm not sure what's going on.
>>>> >
>>>> > Reproducing it is fairly simple:
>>>> >
>>>> > CREATE TABLE t (a bigint, b bigint, c bigint, d bigint);
>>>> > INSERT INTO t SELECT
>>>> > 1000*random(), 1000*random(), 1000*random(), 1000*random()
>>>> > FROM generate_series(1,10000000) s(i);
>>>> > CREATE INDEX idx ON t(a,b);
>>>> > ANALYZE t;
>>>> >
>>>> > EXPLAIN ANALYZE SELECT a, b, c, d, count(*)
>>>> > FROM (SELECT * FROM t ORDER BY a, b, c) foo GROUP BY a, b, c, d
>>>> > LIMIT 100;
>>>>
>>>> While trying to reproduce this, instead of lots of memory usage, I got
>>>> the attached assertion failure instead.
>>>
>>>
>>> And, without the EXPLAIN ANALYZE was able to get this one, which will
>>> probably be a lot more helpful.
>>>
>>
>> Hmmm, I'll try reproducing it, but can you investigate the values in the
>> Assert? I mean, it fails on this:
>>
>> Assert(total_allocated == context->mem_allocated);
>>
>> so can you get a core or attach to the process using gdb, and see what's
>> the expected / total value?

I've reproduced this on multiple machines (though all are Ubuntu or
Debian derivatives...I don't think that's likely to matter). A core
dump is ~150MB, so I've uploaded to Dropbox [1].

I didn't find an obvious first-level member of Tuplesortstate that was
covered by either of the two blocks in the AllocSet (both are 8KB in
size).

James

[1]: https://www.dropbox.com/s/jwndwp4634hzywk/aset_assertion_failure.core?dl=0