The rules given in the preceding sections will result in assignment of non-
unknowndata types to all expressions in a SQL query, except for unspecified-type literals that appear as simple output columns of a
SELECTcommand. For example, in
SELECT 'Hello World';
there is nothing to identify what type the string literal should be taken as. In this situationPostgreSQLwill fall back to resolving the literal's type as
SELECTis one arm of a
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
UNIONarm in the first case, or from the destination column in the second case.
RETURNINGlists are treated the same as
SELECToutput lists for this purpose.
Prior toPostgreSQL10, this rule did not exist, and unspecified-type literals in a
SELECToutput list were left as type
unknown. That had assorted bad consequences, so it's been changed.