Re: [HACKERS] Runtime Partition Pruning

On 19 April 2018 at 03:13, Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Apr 16, 2018 at 10:46 PM, David Rowley
> <david.rowley@2ndquadrant.com> wrote:
>> I did go and start working on a patch to test how possible this would
>> be and came up with the attached. I've left a stray
>> MemoryContextStatsDetail call in there which does indicate that
>> something is not being freed. I'm just not sure what it is yet.
>>
>> The patch does happen to improve performance slightly, but that is
>> most likely due to the caching of the ExprStates rather than the
>> change of memory management. It's not really possible to do that with
>> the reset unless we stored the executor's memory context in
>> PartitionPruneContext and did a context switch back inside
>> partkey_datum_from_expr before calling ExecInitExpr.
>
> 10% is more than a "slight" improvement, I'd say!  It's certainly got
> to be worth avoiding the repeated calls to ExecInitExpr, whatever we
> do about the memory contexts.

I've attached a patch which does just this. On benchmarking again with
this single change performance has improved 15% over master.

Also, out of curiosity, I also checked what this performed like before
the run-time pruning patch was committed (5c0675215). Taking the
average of the times below, it seems without this patch the
performance of this case has improved about 356% and about 410% with
this patch. So, I agree, it might be worth considering.

create table p (a int, value int) partition by hash (a);
select 'create table p'||x|| ' partition of p for values with (modulus
10, remainder '||x||');' from generate_series(0,9) x;
\gexec
create table t1 (a int);

insert into p select x,x from generate_Series(1,1000) x;
insert into t1 select x from generate_series(1,1000) x;

create index on p(a);

set enable_hashjoin = 0;
set enable_mergejoin = 0;
explain analyze select count(*) from t1 inner join p on t1.a=p.a;

-- Unpatched
Execution Time: 20413.975 ms
Execution Time: 20232.050 ms
Execution Time: 20229.116 ms

-- Patched
Execution Time: 17758.111 ms
Execution Time: 17645.151 ms
Execution Time: 17492.260 ms

-- 5c0675215e153ba1297fd494b34af2fdebd645d1
Execution Time: 72875.161 ms
Execution Time: 71817.757 ms
Execution Time: 72411.730 ms

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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