Re: Global temporary tables

On 16.08.2019 20:17, Pavel Stehule wrote:

pá 16. 8. 2019 v 16:12 odesílatel Konstantin Knizhnik <k.knizhnik@postgrespro.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.