LP:736358 - Unexpected increased timestamp resolution with UNION

When executed in a UNION context, datetime operations return a millisecond precision, even though no fractional seconds were used anywhere in the expression.

Test case:

CREATE TABLE t1 (f1 timestamp(6));
INSERT INTO t1 VALUES ('2002-07-15 21:00:00');

SELECT TIME(f1) FROM t1;

returns:

21:00:00

( SELECT TIME(f1) FROM t1) UNION ALL ( SELECT TIME(f1) FROM t1);

returns
21:00:00.000000
21:00:00.000000