Re: Global temporary tables

On 16.08.2019 20:17, Pavel Stehule wrote:
>
>
> pá 16. 8. 2019 v 16:12 odesílatel Konstantin Knizhnik
> <k(dot)knizhnik(at)postgrespro(dot)ru <mailto: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
>
 One more test which is used to simulate access to temp tables under
high load.
I am using "upsert" into temp table in multiple connections.

create global temp table gtemp (x integer primary key, y bigint);

upsert.sql:
insert into gtemp values (random() * 1000000, 0) on conflict(x) do
update set y=gtemp.y+1;

pgbench -c 10 -M prepared -T 100 -P 1 -n -f upsert.sql postgres

I failed to find some standard way in pgbech to perform per-session
initialization to create local temp table,
so I just insert this code in pgbench code:

diff --git a/src/bin/pgbench/pgbench.c b/src/bin/pgbench/pgbench.c
index 570cf33..af6a431 100644
--- a/src/bin/pgbench/pgbench.c
+++ b/src/bin/pgbench/pgbench.c
@@ -5994,6 +5994,7 @@ threadRun(void *arg)
                {
                        if ((state[i].con = doConnect()) == NULL)
                                goto done;
+                       executeStatement(state[i].con, "create temp
table ltemp(x integer primary key, y bigint)");
                }
        }

Results are the following:
Global temp table: 117526 TPS
Local temp table:   107802 TPS

So even for this workload global temp table with shared buffers are a
little bit faster.
I will be pleased if you can propose some other testing scenario.