Re: [PERFORM] Performance issue with castings args of the function

[Андрей Хозов]

Thanks all for explain!

On Mon, Jan 2, 2017 at 9:36 PM, Tom Lane  wrote:

> =?UTF-8?B?0JDQvdC00YDQtdC5INCl0L7Qt9C+0LI=?=  writes:
> > create table t1 (id serial, str char(32));
>
> > create function f1(line text) returns void as $$
> > begin
> >   perform * from t1 where str = line;
> > end;
> > $$ language plpgsql;
>
> This query is specifying a text comparison (text = text operator).
> Since the table column isn't text, a char-to-text conversion must
> happen at each line.
>
> > create function f2(line char) returns void as $$
> > begin
> >   perform * from t1 where str = line;
> > end;
> > $$ language plpgsql;
>
> This query is specifying a char(n) comparison (char = char operator).
> No type conversion step needed, so it's faster.
>
> regards, tom lane
>



-- 
​Andrey Khozov

Re: [PERFORM] Performance issue with castings args of the function

[Tom Lane]

=?UTF-8?B?0JDQvdC00YDQtdC5INCl0L7Qt9C+0LI=?=  writes:
> create table t1 (id serial, str char(32));

> create function f1(line text) returns void as $$
> begin
>   perform * from t1 where str = line;
> end;
> $$ language plpgsql;

This query is specifying a text comparison (text = text operator).
Since the table column isn't text, a char-to-text conversion must
happen at each line.

> create function f2(line char) returns void as $$
> begin
>   perform * from t1 where str = line;
> end;
> $$ language plpgsql;

This query is specifying a char(n) comparison (char = char operator).
No type conversion step needed, so it's faster.

regards, tom lane


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Re: [PERFORM] Performance issue with castings args of the function

[Pavel Stehule]

Hi

2017-01-02 15:34 GMT+01:00 Андрей Хозов :

> 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, 100)
> 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('2b00042f7481c7b056c4b410d2
> 8f33cf'::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
>

[PERFORM] Performance issue with castings args of the function

[Андрей Хозов]

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, 100)
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('2b00042f7481c7b056c4b410d28f33
cf'::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)

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