When aliasing a function, I do not see composite types - postgresql

Question

I create table gs:
foo=>create table gs as select generate_subscripts('{{1,2,3},{4,5,6}}'::integer[],2);
I alias the table and the column:
foo=> select s.a from gs s(a);
a
---
1
2
3
(3 rows)
If I only alias the table, I see composite types:
foo=> select s from gs s;
s
-----
(1)
(2)
(3)
(3 rows)
But when I only alias a function as if it was a table, I do not see composite types, but it is as if I had aliased a table and column:
foo=> select s from generate_subscripts('{{1,2,3},{4,5,6}}'::integer[],2) s;
s
---
1
2
3
(3 rows)
I do not understand why I do not see composite types instead.

Answer 1

Set returning functions (or table functions, SRF) are treated differently than actual relations (tables, views, etc.). Especially, when they return only one column (a base type):
If no table_alias is specified, the function name is used as the table name; in the case of a ROWS FROM() construct, the first function's name is used.
If column aliases are not supplied, then for a function returning a base data type, the column name is also the same as the function name. For a function returning a composite type, the result columns get the names of the individual attributes of the type.
What is not covered though, is the case, when you supply a table alias, but not a column alias (for a single-columned SRF). In that case, the column alias will be the same as the table alias, so you can't access the function's row-type (composite type) explicitly (its reference is hidden by the column alias).
select s from generate_subscripts('{{1,2,3},{4,5,6}}'::integer[],2) s;
-- "s" is both a column alias and a table alias here, so:
select s.s from generate_subscripts('{{1,2,3},{4,5,6}}'::integer[],2) s;
-- is also valid
More intriguing however, is that when you use an explicit table alias and an explicit column alias for a single-columned SRF, the table alias also becomes a column alias (its type will be the base type, not a row-type -- composite type).
select s, a from generate_subscripts('{{1,2,3},{4,5,6}}'::integer[],2) s(a);
+---+---+
| s | a |
+---+---+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+---+---+
I'm not sure though if it's a bug, or just an undocumented "feature".
http://rextester.com/VJOBI47962