xmlconcat performance

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

On Fri, Feb 22, 2013 at 3:21 AM, Davide Berra <d(dot)berra(at)esitelsrl(dot)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

On Fri, Mar 1, 2013 at 2:18 AM, Davide Berra <d(dot)berra(at)esitelsrl(dot)it> wrote:
> Il 28/02/2013 18:48, Merlin Moncure ha scritto:
>
>> On Fri, Feb 22, 2013 at 3:21 AM, Davide Berra <d(dot)berra(at)esitelsrl(dot)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
>>
> Thank you for the reply Merlin but i don't fully get what you mean. (sorry,
> i'm not a PostgreSQL expert)
> How would you change the above example function in order to improve
> performance?
> What do you mean with "manipulate through arrays"?

well arrays, or simple aggregation. for example:
select string_agg(v, '') from (select 'aaaaaaa'::text as v from
generate_series(1,50000)) q;

runs in ~ 30 ms.

merlin