UNION adds trailing zero bytes to ADDTIME() results under SET NAMES binary

Under SET NAMES binary, combining two identical ADDTIME() expressions with UNION changes the returned value by appending ten zero bytes.

SELECT DISTINCT returns the expected 16-byte value. The equivalent UNION query returns a 26-byte value. The additional bytes are visible through HEX() and OCTET_LENGTH(), and cause comparison with the directly computed value to return false.

Reproducer

SET NAMES binary;

SELECT

  HEX(col1) AS hex_value,

  OCTET_LENGTH(col1) AS byte_length,

  col1 = ADDTIME('-01:01:01.01', '-23:59:59.1') AS equals_direct

FROM (

  SELECT DISTINCT

    ADDTIME('-01:01:01.01', '-23:59:59.1') AS col1

) AS s;

SELECT

  HEX(col1) AS hex_value,

  OCTET_LENGTH(col1) AS byte_length,

  col1 = ADDTIME('-01:01:01.01', '-23:59:59.1') AS equals_direct

FROM (

  SELECT ADDTIME('-01:01:01.01', '-23:59:59.1') AS col1

  UNION

  SELECT ADDTIME('-01:01:01.01', '-23:59:59.1') AS col1

) AS s;

Actual result

The first query returns:

hex_value:     2D32353A30313A30302E313130303030

byte_length:   16

equals_direct: 1

The second query returns:

hex_value:     2D32353A30313A30302E31313030303000000000000000000000

byte_length:   26

equals_direct: 0

The second result contains ten trailing 0x00 bytes after the expected string '-25:01:00.110000'.

Expected result

Both queries should return:

hex_value:     2D32353A30313A30302E313130303030

byte_length:   16

equals_direct: 1

Both UNION branches evaluate the same ADDTIME() expression. Combining their results should not append bytes or change the value.

Additional observations

• The difference is reproducible through server-side HEX(), OCTET_LENGTH(), and equality comparison.
• The same query pair returns matching results with SET NAMES utf8mb4 or SET NAMES latin1.

Related report

MySQL Bug #40217: UNION ALL query results present faulty internal datetime

MySQL Bug #40217 is closed. The developer comment states that it was fixed in MySQL 5.6.40 and later.