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temp table "on commit delete rows": transaction overhead

From: Artiom Makarov <artiom(dot)makarov(at)gmail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: temp table "on commit delete rows": transaction overhead
Date: 2010-03-30 11:46:26
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
We have a postgres database which accessed by clients app via PL/PGSQL
stored procedures ( 8.4.1 on x86_64 ubuntu 8.04 server).

For some reasons we use about 25 temp tables "on commit delete rows".
It widely used by our SP.

When temp tables with "on commit delete rows" exists, I can see a
strange delay at any “begin” and “commit”.

2010-03-09 15:14:01 MSK logrus 32102 amber LOG:  duration: 20.809 ms
statement: BEGIN
2010-03-09 15:14:01 MSK logrus 32102 amber LOG:  duration: 0.809 ms
statement: SELECT  empl.BL_CustomerFreeCLGet('384154676925391',
'8189', NULL)
010-03-09 15:14:01 MSK logrus 32102 amber LOG:  duration: 0.283 ms
statement: FETCH ALL IN "<unnamed portal 165>"; --
2010-03-09 15:14:01 MSK logrus 32102 amber LOG:  duration: 19.895 ms
statement: COMMIT

The more system load and more temp table quantity in session, then
more “begin” and “commit” delays.

Test example below:

create database test;
create language plpgsql;
CREATE OR REPLACE FUNCTION test_connectionprepare(in_create
bool,in_IsTemp bool,in_DelOnCommit bool,in_TableCount int)
 RETURNS boolean AS $$

 m_count int := 50;
 m_isTemp  bool;


m_count := coalesce(in_TableCount,m_count);

FOR i IN 0..m_count LOOP

if in_create then
   execute 'create ' || case when in_IsTemp then ' temp ' else ' ' end
||' table tmp_table_'
             || i::text || '(id int,pid int,name text) '
             || case when in_DelOnCommit then ' on commit delete rows
' else ' ' end || ';';
   execute 'drop table if exists tmp_table_' || i::text ||';';
end if;


 return in_create;

Now run pgScript:
SET @I = 1;
WHILE @I <= 100

select now();

   SET @I = @I + 1;

It spent about 2200-2300 ms on my server.

Let's create 50 temp tables: select test_connectionprepare(true,true,true,100);

and run script again.

We can see 2-3 times slowing!

Here temp tables quantity vs test run time:

0 - 2157-2187
10 - 2500-2704
50 - 5900-6000
100 - 7900-8000
500 - 43000+

I can to suppose, that all tables are truncated before and after every
transactions. Very strange method for read only transactions!

Sorry for my English.

My server info:
"PostgreSQL 8.4.1 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.2.real
(GCC) 4.2.4 (Ubuntu 4.2.4-1ubuntu4), 64-bit"
Linux u16 2.6.24-24-server #1 SMP Tue Jul 7 19:39:36 UTC 2009 x86_64 GNU/Linux
4xOpteron 16 processor cores.


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