Avoiding repeated ON COMMIT truncation for temporary tables

Currently, if you create a temporary table with the ON COMMIT action of
DELETE ROWS, the table will truncated for every commit, whether there is
any data in the table or not.

I measured the overhead using this test:

$ (echo 'CREATE TEMPORARY TABLE TEST2 (x int);'; jot -b 'SELECT 1;'
10000) | time psql test > /dev/null
6.93 real 0.93 user 0.78 sys
$ (echo 'CREATE TEMPORARY TABLE TEST2 (x int) ON COMMIT DELETE ROWS;';
jot -b 'SELECT 1;' 10000) | time psql test > /dev/null
7.93 real 1.02 user 0.72 sys

The overhead measures 14%. Is there a simple way to avoid the repeated
truncation overhead of such cases? Is this a TODO?

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +

On Fri, Mar 11, 2011 at 8:07 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Currently, if you create a temporary table with the ON COMMIT action of
> DELETE ROWS, the table will truncated for every commit, whether there is
> any data in the table or not.
>
> I measured the overhead using this test:
>
>        $ (echo 'CREATE TEMPORARY TABLE TEST2 (x int);'; jot -b 'SELECT 1;'
>        10000) | time psql  test > /dev/null
>                6.93 real         0.93 user         0.78 sys
>        $ (echo 'CREATE TEMPORARY TABLE TEST2 (x int) ON COMMIT DELETE ROWS;';
>        jot -b 'SELECT 1;' 10000) | time psql  test > /dev/null
>                7.93 real         1.02 user         0.72 sys
>
> The overhead measures 14%.  Is there a simple way to avoid the repeated
> truncation overhead of such cases?  Is this a TODO?

We might be able to make PreCommit_on_commit_actions() exit quickly
without doing anything if MyXactAccessedTempRel is false. I haven't
tested that solution and am not 100% confident that it's safe, but if
it is I believe it would address your concern.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Mon, Mar 14, 2011 at 8:52 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Mar 11, 2011 at 8:07 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>> Currently, if you create a temporary table with the ON COMMIT action of
>> DELETE ROWS, the table will truncated for every commit, whether there is
>> any data in the table or not.
>>
>> I measured the overhead using this test:
>>
>>        $ (echo 'CREATE TEMPORARY TABLE TEST2 (x int);'; jot -b 'SELECT 1;'
>>        10000) | time psql  test > /dev/null
>>                6.93 real         0.93 user         0.78 sys
>>        $ (echo 'CREATE TEMPORARY TABLE TEST2 (x int) ON COMMIT DELETE ROWS;';
>>        jot -b 'SELECT 1;' 10000) | time psql  test > /dev/null
>>                7.93 real         1.02 user         0.72 sys
>>
>> The overhead measures 14%.  Is there a simple way to avoid the repeated
>> truncation overhead of such cases?  Is this a TODO?
>
> We might be able to make PreCommit_on_commit_actions() exit quickly
> without doing anything if MyXactAccessedTempRel is false.  I haven't
> tested that solution and am not 100% confident that it's safe, but if
> it is I believe it would address your concern.

aside: I find 'on commit drop' tables to be quite useless, especially
in plpgsql due to performance issues. it's much better to organize
data around a permanent table organized around txid_current(), with
something swooping along periodically and cleaning it up.

merlin

Robert Haas wrote:
> On Fri, Mar 11, 2011 at 8:07 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > Currently, if you create a temporary table with the ON COMMIT action of
> > DELETE ROWS, the table will truncated for every commit, whether there is
> > any data in the table or not.
> >
> > I measured the overhead using this test:
> >
> > ? ? ? ?$ (echo 'CREATE TEMPORARY TABLE TEST2 (x int);'; jot -b 'SELECT 1;'
> > ? ? ? ?10000) | time psql ?test > /dev/null
> > ? ? ? ? ? ? ? ?6.93 real ? ? ? ? 0.93 user ? ? ? ? 0.78 sys
> > ? ? ? ?$ (echo 'CREATE TEMPORARY TABLE TEST2 (x int) ON COMMIT DELETE ROWS;';
> > ? ? ? ?jot -b 'SELECT 1;' 10000) | time psql ?test > /dev/null
> > ? ? ? ? ? ? ? ?7.93 real ? ? ? ? 1.02 user ? ? ? ? 0.72 sys
> >
> > The overhead measures 14%. ?Is there a simple way to avoid the repeated
> > truncation overhead of such cases? ?Is this a TODO?
>
> We might be able to make PreCommit_on_commit_actions() exit quickly
> without doing anything if MyXactAccessedTempRel is false. I haven't
> tested that solution and am not 100% confident that it's safe, but if
> it is I believe it would address your concern.

Added to TODO:

Prevent temporary tables created with ON COMMIT DELETE ROWS from
repeatedly truncating the table on every commit if the table is already
empty

* http://archives.postgresql.org/pgsql-hackers/2011-03/msg00842.php

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +