UNIONconstructs must match up possibly dissimilar types to become a single result set. The resolution algorithm is applied separately to each output column of a union query. The
EXCEPTconstructs resolve dissimilar types in the same way as
LEASTconstructs use the identical algorithm to match up their component expressions and select a result data type.
Type Resolution for
CASE, and Related Constructs
If all inputs are of the same type, and it is not
unknown, resolve as that type.
If any input is of a domain type, treat it as being of the domain's base type for all subsequent steps.
If all inputs are of type
unknown, resolve as type
text(the preferred type of the string category). Otherwise,
unknowninputs are ignored for the purposes of the remaining rules.
If the non-unknown inputs are not all of the same type category, fail.
Choose the first non-unknown input type which is a preferred type in that category, if there is one.
Otherwise, choose the last non-unknown input type that allows all the preceding non-unknown inputs to be implicitly converted to it. (There always is such a type, since at least the first type in the list must satisfy this condition.)
Convert all inputs to the selected type. Fail if there is not a conversion from a given input to the selected type.
Some examples follow.
Example 10.9. Type Resolution with Underspecified Types in a Union
SELECT text 'a' AS "text" UNION SELECT 'b'; text ------ a b (2 rows)
Here, the unknown-type literal
'b'will be resolved to type
Example 10.10. Type Resolution in a Simple Union
SELECT 1.2 AS "numeric" UNION SELECT 1; numeric --------- 1 1.2 (2 rows)
1.2is of type
numeric, and the
1can be cast implicitly to
numeric, so that type is used.
Example 10.11. Type Resolution in a Transposed Union
SELECT 1 AS "real" UNION SELECT CAST('2.2' AS REAL); real ------ 1 2.2 (2 rows)
Here, since type
realcannot be implicitly cast to
integercan be implicitly cast to
real, the union result type is resolved as
Somewhat like the treatment of domain inputs for operators and functions, this behavior allows a domain type to be preserved through a
UNIONor similar construct, so long as the user is careful to ensure that all inputs are implicitly or explicitly of that exact type. Otherwise the domain's base type will be preferred.