Re: [PATCH] Incremental sort (was: PoC: Partial sort) - Mailing list pgsql-hackers

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@gmail.com> wrote:
>>
>> On Tue, Mar 10, 2020 at 10:44 PM Tomas Vondra
>> <tomas.vondra@2ndquadrant.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?

BTW, I might have copied the wrong query - can you try with a higher
value in the LIMIT clause? For example:

     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 1000000;

I think this might be the differenc ewhy you don't see the memory leak.
Or maybe it was because of asserts? I'm not sure if I had enabled them
in the build ...


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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