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 constructs 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.
typcategory in the pg_type system catalog indicates the data type category. typispreferred indicates whether a type is preferred in typcategory.
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.
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.
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.
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. Fail in the ORA type.
-- In Oracle mode, create the oracle_1 database compatible with Oracle: CREATE DATABASE oracle_1 dbcompatibility = 'ORA'; -- Switch to the oracle_1 database: \c oracle_1 -- Create the t1 table: oracle_1=# CREATE TABLE t1(a int, b varchar(10)); -- View the execution plan of the query statement that uses the coalesce parameter to enter data of int and varchar types: oracle_1=# EXPLAIN SELECT coalesce (a, b) FROM t1; ERROR: COALESCE types integer and character varying cannot be matched CONTEXT: referenced column: coalesce -- Delete the tables. oracle_1=# DROP TABLE t1; -- Switch to the postgres database: oracle_1=# \c postgres -- In TD mode, create the td_1 database compatible with Teradata: CREATE DATABASE td_1 dbcompatibility = 'TD'; -- Switch to the td_1 database: \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((t1.a)::character varying, t1.b)) Node/s: All datanodes Remote query: SELECT COALESCE(a::character varying, b) AS "coalesce" FROM public.t1 (4 rows) -- Delete the tables. td_1=# DROP TABLE t2; -- Switch to the postgres database: td_1=# \c postgres -- Delete databases in Oracle and TD mode: DROP DATABASE oracle_1; DROP DATABASE td_1;