GREATEST() does not always return same signness of argument types.

[Antony T Curtis]

Description:
There is a problem on all platforms; to varying degrees.

Windows 32bit and Windows 64bit:
  When supplied BIGINT UNSIGNED, the return type is a BIGINT SIGNED,

Others:
  The return type appears to be valid however for BIGINT values, the comparison is always
performed as if the values were signed.

Workaround would be to use IF(x>y,x,y) function instead.

How to repeat:
To reproduce this bug, create this simple table.

DROP TABLE `cma`;
CREATE TABLE `cma` (
  `a` binary(16) NOT NULL,
  `b` bigint(20) unsigned NOT NULL,
  PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- Execute the following statement twice

INSERT INTO cma
(a, b)
VALUES ('foobar',13836376518955650385)
ON DUPLICATE KEY UPDATE
b=GREATEST(b,VALUES(b));