SELECT Output ColumnsThe 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.
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