Re: Set returning aggregate? - Mailing list pgsql-general

Hello

2013/12/8 Bborie Park <dustymugs@gmail.com>

I'm wondering if an aggregate function can return a set of records? 

No, final function cannot returns set. It is disallowed.

Theoretically, it should be possible - it is explicitly prohibited. But if it will be allowed, you can get same problems like using SRF function in target list.

postgres=# select generate_series(1,2),generate_series(1,2);
 generate_series │ generate_series
─────────────────┼─────────────────
               1 │               1
               2 │               2
(2 rows)

Time: 49.332 ms
postgres=# select generate_series(1,2),generate_series(1,3);
 generate_series │ generate_series
─────────────────┼─────────────────
               1 │               1
               2 │               2
               1 │               3
               2 │               1
               1 │               2
               2 │               3
(6 rows)

Time: 0.445 ms

It will be hard defined a expected behaviour when somebody use more these aggregates in same query and returns different number of rows.

Regards

Pavel

 

Say I have a table with a column of type raster (PostGIS). I want to get the number of times the pixel values 1, 3 and 4 occur in that raster column. I am hoping to build an aggregrate function that returns the following...

value | count

--------+--------

1       | 12

--------+--------

2       | 12

--------+--------

3       | 12

Is it possible for an aggregate function to return a set? I've written some test cases and it looks like the answer is No but I'd like confirmation.

Thanks,

Bborie Park

PostGIS Steering Committee