Re: xmlconcat performance

On Fri, Feb 22, 2013 at 3:21 AM, Davide Berra <d.berra@esitelsrl.it> wrote:
> I got a problem with the performance of a PL/PGsql stored procedure
> outputting an xml.
>
> Server version: PostgreSQL 8.3.6 on i686-pc-linux-gnu, compiled by GCC gcc
> (GCC) 4.1.2 20080704 (Red Hat 4.1.2-46)
> CPU: Intel(R) Core(TM) i3 CPU 540  @ 3.07GHz
> RAM installed: 4GB
> Hard Disk: Seagate 500Gb SATA 2
>
> This is a simplified content of the function showing the xmlconcat
> behaviour.
>
> CREATE OR REPLACE FUNCTION test_function (v_limit int)
>     RETURNS xml AS
> $BODY$
> DECLARE
>     v_xml xml;
> BEGIN
>
>     FOR i IN 1..v_limit LOOP
>         v_xml := xmlconcat(v_xml, xmlelement(name content, 'aaaaaaa'));
>     END LOOP;
>
>     RETURN v_xml ;
> END
> $BODY$
>     LANGUAGE 'plpgsql' SECURITY DEFINER ;
>
>
> As long as the v_limit parameter grows (and then the size of the output xml,
> the time needed increase exponentially.
> Look at this examples:
>
> pang=# explain analyze select test_function(1000);
>                                       QUERY PLAN
> --------------------------------------------------------------------------------------
>  Result  (cost=0.00..0.26 rows=1 width=0) (actual time=65.430..65.431 rows=1
> loops=1)
>  Total runtime: 65.457 ms
> (2 rows)
>
> pang=# explain analyze select test_function(5000);
>                                        QUERY PLAN
> ----------------------------------------------------------------------------------------
>  Result  (cost=0.00..0.26 rows=1 width=0) (actual time=473.318..473.318
> rows=1 loops=1)
>  Total runtime: 473.340 ms
> (2 rows)
>
> pang=# explain analyze select test_function(15000);
>                                         QUERY PLAN
> ------------------------------------------------------------------------------------------
>  Result  (cost=0.00..0.26 rows=1 width=0) (actual time=4044.903..4044.904
> rows=1 loops=1)
>  Total runtime: 4044.928 ms
> (2 rows)
>
> pang=# explain analyze select test_function(50000);
>                                          QUERY PLAN
> --------------------------------------------------------------------------------------------
>  Result  (cost=0.00..0.26 rows=1 width=0) (actual time=94994.337..94994.369
> rows=1 loops=1)
>  Total runtime: 94994.396 ms
> (2 rows)
>
> I already tried to update to 8.3.23 service version but i didn't see any
> improvement.
>
> Do you have any suggestion about how to increase the performance of
> xmlconcat?
>
> My need is to use stored procedures that calls xmlconcat more than 50000
> times, but it is unacceptable 94 seconds to complete the job.
>
> Thanks in advance

typically for high performance string manipulation you have to do
things on more purely textual level and manipulate through arrays to
get really good performance.  iterative string concatenation is
typically wrong approach -- you have to think in set terms.

also your database version is obsolete -- time to start thinking about upgrade.

merlin