Re: Performance issue with castings args of the function

[Pavel Stehule <pavel.stehule@xxxxxxxxx>]

Hi

2017-01-02 15:34 GMT+01:00 Андрей Хозов <avkhozov@xxxxxxxxx>:

Hello there!

I have an performance issue with functions and args type.

Table and data:

create table t1 (id serial, str char(32));

insert into t1 (str) select md5(s::text) from generate_series(1, 1000000) as s;

And simple functions:

create function f1(line text) returns void as $$

begin

  perform * from t1 where str = line;

end;

$$ language plpgsql;

create function f2(line char) returns void as $$

begin

  perform * from t1 where str = line;

end;

$$ language plpgsql;

​Query:

test=> explain analyze select f2('2b00042f7481c7b056c4b410d28f33cf');

                                       QUERY PLAN                                       

----------------------------------------------------------------------------------------

 Result  (cost=0.00..0.26 rows=1 width=0) (actual time=189.008..189.010 rows=1 loops=1)

 Planning time: 0.039 ms

 Execution time: 189.039 ms

(3 rows)

Time: 189,524 ms

test=> explain analyze select f1('2b00042f7481c7b056c4b410d28f33cf');

                                       QUERY PLAN                                       

----------------------------------------------------------------------------------------

 Result  (cost=0.00..0.26 rows=1 width=0) (actual time=513.734..513.735 rows=1 loops=1)

 Planning time: 0.024 ms

 Execution time: 513.757 ms

(3 rows)

Time: 514,125 ms

test=> explain analyze select f1('2b00042f7481c7b056c4b410d28f33cf'::char(32));

                                       QUERY PLAN                                       

----------------------------------------------------------------------------------------

 Result  (cost=0.00..0.26 rows=1 width=0) (actual time=513.507..513.509 rows=1 loops=1)

 Planning time: 0.074 ms

 Execution time: 513.535 ms

(3 rows)

This explain shows nothing - you need to use nested explain

look on auto-explain  https://www.postgresql.org/docs/current/static/auto-explain.html

Maybe index was not used due different types.

Regards

Pavel

Time: 514,104 ms

test=> 

​

​Seems that casting param from text to char(32) needs to be done only once and​ f1 and f2 must be identical on performance. But function f2 with text param significantly slower, even with casting arg while pass it to function.

Tested postgresql versions 9.5.5 and 9.6.1 on Ubuntu 16.04. It's normal behavior or it's can be fixed?

--

​Andrey Khozov