Running the following MTR test case

CREATE TABLE t1(f1 YEAR(4));

INSERT INTO t1 VALUES (0000),(2001);

--enable_metadata

(SELECT MAX(f1) FROM t1) UNION (SELECT MAX(f1) FROM t1);

--disable_metadata

DROP TABLE t1;

leads to test failure in case it is run with the option --ps-protocol with the following diagnostic message:

@@ -2,7 +2,7 @@

 INSERT INTO t1 VALUES (0000),(2001);

 (SELECT MAX(f1) FROM t1) UNION (SELECT MAX(f1) FROM t1);

 Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr

-def				MAX(f1)	MAX(f1)	13	4	4	Y	32864	0	63

+def				MAX(f1)	MAX(f1)	13	4	6	Y	32864	0	63

 MAX(f1)

 2001

 DROP TABLE t1;

mysqltest: Result content mismatch

Running above mentioned test under debugger shows that resultset metadata in Server Response packet is set correctly.

Discussion of this issue with Georg discovered surprising thing - Max column size sent in Response packet is ignored and actual value is calculated based on constants initialized in the function mysql_init_ps_subsystem().

It seems such implementation is good candidate for rewriting.