When updating a row in a system-versioned table and updating a column's value with the exact current value, a new historical row is written, that has the same ROW_START value as well as the same value for any other column as the previous one, while a ON UPDATE CURRENT_TIMESTAMP() column is not updated on the current row.

I also experienced that issue to not exist when updating a ENUM column with the exact current value, while existing when updating a INT column with the exact current value. However, in the example below the columns all behave the same.

CREATE TABLE Foobar (

  id INT(11) NOT NULL,

  a INT(11) NOT NULL,

  b ENUM('foo', 'bar') NOT NULL,

  c VARCHAR(255) NOT NULL,

  ts DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP() ON UPDATE CURRENT_TIMESTAMP(),

  PRIMARY KEY (id)

) WITH SYSTEM VERSIONING;

INSERT INTO Foobar (id, a, b, c) VALUES (1, 1, 'foo', 'hello');

-- 1 row

SELECT *, ROW_START, ROW_END FROM Foobar FOR SYSTEM_TIME ALL WHERE id = 1;

UPDATE Foobar SET b = 'foo' WHERE id = 1;

-- 2 rows with the exact same values except for ROW_END

SELECT *, ROW_START, ROW_END FROM Foobar FOR SYSTEM_TIME ALL WHERE id = 1;

UPDATE Foobar SET a = 1 WHERE id = 1;

-- 3 rows with the exact same values except for ROW_END

SELECT *, ROW_START, ROW_END FROM Foobar FOR SYSTEM_TIME ALL WHERE id = 1;

SELECT *, ROW_START, ROW_END FROM Foobar FOR SYSTEM_TIME AS OF '<row_start_value>' WHERE id = 1 will also return 3 rows, while I would expect only 1.

I would expect that a historical row is only written when data in the row really change, just like a ON UPDATE column behaves.