Thanks for the info. I guess my big lesson here is: (plpg)sql is much more statically typed than I thought.
I was hoping for a function like:
CREATE FUNCTION field_for_composite_value(record, text) RETURNS any AS $$ return record[text] $$ LANGUAGE plpython3u IMMUTABLE STRICT;
but pretty much all of the "pseudo-types" are unavailable to all four of the built-in extension languages. (In fact, "any" is so unsupported that it's a syntax error here!) I guess I need to figure out C extensions for that.
The bigger picture is that I wanted to write something kind of like to_json() which dumps composite values recursively, but with a different format. (There are also *_to_xml() functions, but unlike JSON, they output composite values as plain strings like (a,,,b) -- and also only operate on cursor/table/query/database, not individual values.) I'm still playing with these, and I'm not sure if I can use them to fake a `field_for_composite_value()` function or not.
