[PATCH] Generate column names for subquery expressions

Hi list,

In current PostgreSQL versions, subquery expressions in the SELECT list
always generate columns with name "?column?"

postgres=# select (select 1 as foo);
?column?
--------
1

This patch improves on that:
select (SELECT 1 AS foo) => foo
select exists(SELECT 1) => exists
select array(SELECT 1) => array

The "array" one is now consistent with an array literal: select array[1];

Other subquery types (=ALL(), =ANY() and row comparison) don't change
because they act more like operators.

I guess it's fairly unlikely that users rely on column names being
"?column?", but it does change the name of some expressions, for example:
select (select 1 foo)::int;
select case when true then 1 else (select 1 as foo) end;

Previously these returned column names "int4" and "case", now they would
return "foo". Personally I prefer it this way, but if it is considered a
compatibility problem, lowering the strength of subquery names in
FigureColnameInternal would resort to the old behavior.

How this affects different queries can be seen from the regression diffs.

Does this sound like a good idea?
Should I submit this to the CommitFest?

Regards,
Marti Raudsepp