Re: LIMIT problem

On 2010-04-30, silly sad <sad(at)bankir(dot)ru> wrote:
> suppose i request
>
> SELECT foo(t.x) FROM t LIMIT 1;
>
> Whither it DEFINED how many times foo() will be executed?

foo will be executed repeatedly until it returns a result or all the
rows in t are exhausted.

> May anyone rely on it?

not sure

> Or we have to avoid this non SQLish trick?

This will execute it once (or not at all where t has no rows)

SELECT foo(x) FROM (SELECT x FROM t LIMIT 1) as bar;

But may return a number of records differing from 1 in the case where
foo is a set-returning function.

jasen=# select a from foo;
a
---
1
4
7

6
3
6 rows)

jasen=# select generate_series(1,a),a from foo limit 1;
generate_series | a
-----------------+---
1 | 1
(1 row)

the first row jas 1 and the first row from
generate_series(1,1) is returned

jasen=# select generate_series(5,a),a from foo limit 1;
generate_series | a
-----------------+---
5 | 7
(1 row)

the 1st row has 1 and generate_series(5,1) returns 0 rows
the 2nd row has 4 and generate_series(5,4) returns 0 rows
the 3rd row has 7 and generate_series(5,7) returns 3 rows

And the first of those is returned.