SELECT Output Columns #
The rules given in the preceding sections will result in assignment
of non-unknown data types to all expressions in an SQL query,
except for unspecified-type literals that appear as simple output
columns of a SELECT command. For example, in
SELECT 'Hello World';
there is nothing to identify what type the string literal should be
taken as. In this situation PostgreSQL will fall back
to resolving the literal's type as text.
When the SELECT is one arm of a UNION
(or INTERSECT or EXCEPT) construct, or when it
appears within INSERT ... SELECT, this rule is not applied
since rules given in preceding sections take precedence. The type of an
unspecified-type literal can be taken from the other UNION arm
in the first case, or from the destination column in the second case.
RETURNING lists are treated the same as SELECT
output lists for this purpose.
Prior to PostgreSQL 10, this rule did not exist, and
unspecified-type literals in a SELECT output list were
left as type unknown. That had assorted bad consequences,
so it's been changed.