[GENERAL] Why do subselects in target lists behave differently wrt implicit casts?

[Mike Mascari]

This is a really trivial question, but I'm curious. Why do 
subselects in target lists behave differently than simple queries?

Ex:
[lexus] create temporary table bar (key varchar(32) not null);
CREATE TABLE
[lexus] insert into bar select '';
INSERT 1319585 1
[lexus] insert into bar select (select '') as key;
ERROR:  failed to find conversion function from "unknown" to 
character varying
[lexus] insert into bar select (select ''::text) as key;
INSERT 1319586 1

Just curious,
Mike Mascari

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