Re: [GENERAL] Problem with volatile function

Artacus wrote:
> So my understanding of volatile functions is that volatile functions can
> return different results given the same input.
>
> I have a function random(int, int) that returns a random value between
> $1 and $2. I want to use it in a query to generate values. But it only
> evaluates once per query and not once per row like I need it to.
>
> -- This always returns the same value
> SELECT ts.sis_id, bldg_id, f_name.name, l_name.name
> FROM tmp_students ts
> JOIN names AS f_name ON
> ts.gender = f_name.gender
> WHERE f_name.counter = random(1,300)
>
> --As does this
> SELECT ts.sis_id, bldg_id, f_name.name, l_name.name
> FROM tmp_students ts
> JOIN names AS f_name ON
> ts.gender = f_name.gender
> AND ts.counter = random(1,100)
>
> -- This generates different numbers
> SELECT random(1,100), s.*
> FROM usr_students s
>
Are you sure it's volatile? (as opposed to stable)

postgres=# create or replace function random(int,int) returns int as
$$select round($2*random()*$1)::int;$$ language sql volatile;
CREATE FUNCTION
postgres=# select generate_series from generate_series(1,10,1) where
random(1,100) > 50;
generate_series
-----------------
1
3
4
6
7
9
(6 rows)

postgres=# select generate_series from generate_series(1,10,1) where
random(1,100) > 50;
generate_series
-----------------
3
7
8
9
(4 rows)

postgres=# select random(1,100), generate_series from
generate_series(1,10,1);
random | generate_series
--------+-----------------
56 | 1
23 | 2
80 | 3
57 | 4
16 | 5
99 | 6
9 | 7
41 | 8
90 | 9
88 | 10
(10 rows)

postgres=# create or replace function random(int,int) returns int as
$$select round($2*random()*$1)::int;$$ language sql stable;
CREATE FUNCTION
postgres=# select generate_series from generate_series(1,10,1) where
random(1,100) > 50;
generate_series
-----------------
1
2
3
4
5
6
7
8
9
10
(10 rows)

postgres=# select generate_series from generate_series(1,10,1) where
random(1,100) > 50;
generate_series
-----------------
(0 rows)

postgres=# select random(1,100), generate_series from
generate_series(1,10,1);
random | generate_series
--------+-----------------
72 | 1
90 | 2
53 | 3
47 | 4
53 | 5
33 | 6
10 | 7
56 | 8
78 | 9
87 | 10
(10 rows)

postgres=#

--
Klint Gore
Database Manager
Sheep CRC
A.G.B.U.
University of New England
Armidale NSW 2350

Ph: 02 6773 3789
Fax: 02 6773 3266
EMail: kgore4@une.edu.au

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