Hello
I propose enhance current syntax that allows to specify label for any
function parameter:
fcename(expr [as label], ...)
fcename(colname, ...)
I would to allow same behave of custom functions like xmlforest function:
postgres=# select xmlforest(a) from foo;xmlforest
-----------<a>10</a>
(1 row)
postgres=# select xmlforest(a as b) from foo;xmlforest
-----------<b>10</b>
(1 row)
Actually I am not sure what is best way for PL languages for acces to
these info. Using some system variables needed new column in pg_proc,
because collecting these needs some time and in 99% cases we don't
need it. So I prefere some system function that returns labels for
outer function call. Like
-- test
create function getlabels() returns varchar[] as $$select '{name,
age}'::varchar[]$$ language sql immutable;
create or replace function json(variadic varchar[])
returns varchar as $$
select '[' || array_to_string( array( select (getlabels())[i]|| ':' || $1[i] from
generate_subscripts($1,1)g(i)) ,',') || ']'
$$ language sql immutable strict;
postgres=# select json('Zdenek' as name,'30' as age); json
----------------------[name:Zdenek,age:30]
(1 row)
postgres=# select json(name, age) from person; json
----------------------[name:Zdenek,age:30]
(1 row)
There are two possibilities a) collect labels in parse time b) collect labels in executor time
@a needs info in pg_proc, but it is simpler, @b is little bit
difficult, but doesn't need any changes in system catalog. I thinking
about b now.
Necessary changes:
=================
labels are searched in parse tree fcinfo->flinfo->fn_expr. I need
insert label into parse tree, so I it needs special node
labeled_param, For getting column reference I need to put current
exprstate to fcinfo. Function getlabels() should take code from
ExecEvalVar function.
Any notes, ideas?
Pavel Stehule
