UNION, CASE, and Related Constructs

SQL UNION constructs 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 INTERSECT and EXCEPT construct resolve dissimilar types in the same way as UNION. The CASE, ARRAY, VALUES, GREATEST and LEAST constructs use the identical algorithm to match up their component expressions and select a result data type.

Type Resolution for UNION, CASE, and Related Constructs

• If all inputs are of the same type, and it is not unknown, resolve as that type.

• If all inputs are of type unknown, resolve as type text (the preferred type of the string category). Otherwise, unknown inputs are ignored.

• If the inputs are not all of the same type category, a failure will be resulted. (Type unknown is not included in this case.)

• If the inputs are all of the same type category, choose the top preferred type in that category. (Exception: The UNION operation regards the type of the first branch as the selected type.)

  NOTE: typcategory in the pg_type system catalog indicates the data type category. typispreferred indicates whether a type is preferred in typcategory.

• Convert all inputs to the selected type. (Retain the original lengths of strings). Fail if there is not an implicit conversion from a given input to the selected type.

• If the input contains the json, txid_snapshot, sys_refcursor, or geometry type, UNION cannot be performed.

Type Resolution for CASE and COALESCE in TD Compatibility Type

• If all inputs are of the same type, and it is not unknown, resolve as that type.
• If all inputs are of type unknown, resolve as type text.
• If inputs are of the string type (including unknown which is resolved as type text) and digit type, resolve as the string type. If the inputs are not of the two types, an error will be reported.
• If the inputs are all of the same type category, choose the top preferred type in that category.
• Convert all inputs to the selected type. Fail if there is not an implicit conversion from a given input to the selected type.

Example

Example 1: Use type resolution with underspecified types in a union as the first example. Here, the unknown-type literal 'b' will be resolved to type text.

mogdb=# SELECT text 'a' AS "text" UNION SELECT 'b';
 text
------
 a
 b
(2 rows)

Example 2: Use type resolution in a simple union as the second example. The literal 1.2 is of type numeric, and the integer value 1 can be cast implicitly to numeric, so that type is used.

mogdb=# SELECT 1.2 AS "numeric" UNION SELECT 1;
 numeric
---------
       1
     1.2
(2 rows)

Example 3: Use type resolution in a transposed union as the third example. Since type real cannot be implicitly cast to integer, but integer can be implicitly cast to real, the union result type is resolved as real.

mogdb=# SELECT 1 AS "real" UNION SELECT CAST('2.2' AS REAL);
 real
------
    1
  2.2
(2 rows)

Example 4: In the TD type, if input parameters for COALESCE are of int and varchar types, resolve as type varchar. In the A type, an error is reported.

-- In the A type, create the a_1 database compatible with A.
mogdb=# CREATE DATABASE a_1 dbcompatibility = 'A';

-- Switch to the a_1 database.
mogdb=# \c a_1

-- Create the t1 table.
a_1=# CREATE TABLE t1(a int, b varchar(10));

-- Show the execution plan of a statement for querying the types int and varchar of input parameters for COALESCE.
a_1=# EXPLAIN SELECT coalesce(a, b) FROM t1;
ERROR:  COALESCE types integer and character varying cannot be matched
LINE 1: EXPLAIN SELECT coalesce(a, b) FROM t1;
                                   ^
CONTEXT:  referenced column: coalesce

-- Delete the table.
a_1=# DROP TABLE t1;

-- Switch to the mogdb database:
a_1=# \c mogdb

-- In the TD type, create the td_1 database compatible with Teradata.
mogdb=# CREATE DATABASE td_1 dbcompatibility = 'C';

-- Switch to the td_1 database.
mogdb=# \c td_1

-- Create the t2 table.
td_1=# CREATE TABLE t2(a int, b varchar(10));

-- Show the execution plan of a statement for querying the types int and varchar of input parameters for COALESCE.
td_1=# EXPLAIN VERBOSE select coalesce(a, b) from t2;
                                      QUERY PLAN
---------------------------------------------------------------------------------------
 Data Node Scan  (cost=0.00..0.00 rows=0 width=0)
   Output: (COALESCE((t2.a)::character varying, t2.b))
   Node/s: All dbnodes
   Remote query: SELECT COALESCE(a::character varying, b) AS "coalesce" FROM public.t2
(4 rows)

-- Delete the table.
td_1=# DROP TABLE t2;

-- Switch to the mogdb database:
td_1=# \c mogdb

-- Delete databases in A and TD types.
mogdb=# DROP DATABASE a_1;
mogdb=# DROP DATABASE td_1;