Re: Global temporary tables

pá 16. 8. 2019 v 16:12 odesílatel Konstantin Knizhnik <
k(dot)knizhnik(at)postgrespro(dot)ru> napsal:

> I did more investigations of performance of global temp tables with shared
> buffers vs. vanilla (local) temp tables.
>
> 1. Combination of persistent and temporary tables in the same query.
>
> Preparation:
> create table big(pk bigint primary key, val bigint);
> insert into big values
> (generate_series(1,100000000),generate_series(1,100000000));
> create temp table lt(key bigint, count bigint);
> create global temp table gt(key bigint, count bigint);
>
> Size of table is about 6Gb, I run this test on desktop with 16GB of RAM
> and postgres with 1Gb shared buffers.
> I run two queries:
>
> insert into T (select count(*),pk/P as key from big group by key);
> select sum(count) from T;
>
> where P is (100,10,1) and T is name of temp table (lt or gt).
> The table below contains times of both queries in msec:
>
> Percent of selected data
> 1%
> 10%
> 100%
> Local temp table
> 44610
> 90
> 47920
> 891
> 63414
> 21612
> Global temp table
> 44669
> 35
> 47939
> 298
> 59159
> 26015
>
> As you can see, time of insertion in temporary table is almost the same
> and time of traversal of temporary table is about twice smaller for global
> temp table
> when it fits in RAM together with persistent table and slightly worser
> when it doesn't fit.
>
>
>
> 2. Temporary table only access.
> The same system, but Postgres is configured with shared_buffers=10GB,
> max_parallel_workers = 4, max_parallel_workers_per_gather = 4
>
> Local temp tables:
> create temp table local_temp(x1 bigint, x2 bigint, x3 bigint, x4 bigint,
> x5 bigint, x6 bigint, x7 bigint, x8 bigint, x9 bigint);
> insert into local_temp values
> (generate_series(1,100000000),0,0,0,0,0,0,0,0);
> select sum(x1) from local_temp;
>
> Global temp tables:
> create global temporary table global_temp(x1 bigint, x2 bigint, x3 bigint,
> x4 bigint, x5 bigint, x6 bigint, x7 bigint, x8 bigint, x9 bigint);
> insert into global_temp values
> (generate_series(1,100000000),0,0,0,0,0,0,0,0);
> select sum(x1) from global_temp;
>
> Results (msec):
>
> Insert
> Select
> Local temp table 37489
> 48322
> Global temp table 44358
> 3003
>
> So insertion in local temp table is performed slightly faster but select
> is 16 times slower!
>
> Conclusion:
> In the assumption then temp table fits in memory, global temp tables with
> shared buffers provides better performance than local temp table.
> I didn't consider here global temp tables with local buffers because for
> them results should be similar with local temp tables.
>

Probably there is not a reason why shared buffers should be slower than
local buffers when system is under low load.

access to shared memory is protected by spin locks (are cheap for few
processes), so tests in one or few process are not too important (or it is
just one side of space)

another topic can be performance on MS Sys - there are stories about not
perfect performance of shared memory there.

Regards

Pavel

>
> --
> Konstantin Knizhnik
> Postgres Professional: http://www.postgrespro.com
> The Russian Postgres Company
>
>