10.6. 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.
Note
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.