Re: Multiple SRF right after SELECT

On Wed, 19 Mar 2008, "Nikolay Samokhvalov" <samokhvalov@gmail.com> writes:
> I wonder, if the following is correct and provides expected result:
>
> test=# select generate_series(1, 2), generate_series(1, 4);
>  generate_series | generate_series
> -----------------+-----------------
>                1 |               1
>                2 |               2
>                1 |               3
>                2 |               4
> (4 rows)
>
>
>  1. Is it correct at all to use SRF in select list, w/o explicit FROM?
> Why then we do not allow using subselects that return multiple rows?
> I'd rather expect that these two things work in similar manner.
>  2. Why the query above provides 4 rows, not 2*4=8? Actually, that's
> interesting -- I can use this query to find l.c.m. But it's defenetely
> not that I'd expect before my try...

From PL/scheme sources:

/** There're 2 ways to return from an SRF:** 1. Value-per-call Mode*    You return each tuple one by one via
SRF_RETURN_NEXT()macro. But*    PG_RETURN_DATUM() calls in the macro, makes it quite*    impracticble. OTOH, this
methodgives opportunity to call SRFs in*    a fashion like "SELECT mysrf();"** 2. Materialize Mode*    In this mode,
youcollect all tuples in a single set and return*    that set. When compared to previos method, it's not possible to*
use SRF of materialize mode like "SELECT my_materialized_srf();",*    instead, you need to access it as a simple table:
"SELECT* FROM*    my_materialized_srf();".** ...*/
 

And I conclude to that generate_series() is written as a SRF function of
value-per-call mode. (Also you may want to check Returning Sets[1]
chapter at PostgreSQL manual.)

[1] http://www.postgresql.org/docs/current/static/xfunc-c.html#XFUNC-C-RETURN-SET


Regards.