Can CURRENT_TIMESTAMP change within an INSERT or UPDATE command? - mysql

Question

I have a table with a few dates (among other things) that look like this:
CREATE TABLE message (
created_on DATEIMTE NOT NULL DEFAULT CURRENT_TIMESTAMP,
send_on DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
modified_on DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
status CHAR NOT NULL DEFAULT 'V',
status_date DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
...
);
When I do an INSERT INTO, I generally do not define the modified_on field since the default makes sense. I may change the send_on value if I do not want my message to be sent immediately.
So an insertion could look like this:
INSERT INTO message
(send_on)
VALUES ('2017-08-31 14:31:22');
Later, the user may ask to change the send_on field. At that point, I run an UPDATE statement as follow:
UPDATE message
SET send_on = <user-defined-date>,
modified_on = CURRENT_TIMESTAMP
WHERE id = 123;
This case is fine since I modified only one date.
Now, at the time the user makes a modification, I may have to also change the status field. That would translate into something like this:
UPDATE message
SET send_on = <user-defined-date>,
modified_on = CURRENT_TIMESTAMP,
status = <new-status>,
status_date = CURRENT_TIMESTAMP
WHERE id = 123;
In that last UPDATE statement and less obvious, in the INSERT INTO statement above, I am setting more than one DATETIME field to the value CURRENT_TIMESTAMP (2 in that UPDATE, 3 in the INSERT INTO through the DEFAULT definitions).
My question is: will all the columns assigned the value CURRENT_TIMESTAMP always receive the exact same value whenever it gets used more than once? Or will each column determine the value of CURRENT_TIMESTAMP once it gets executed and hence, could be a second later (if we were a few ms away from the next second, it can easily happen.)
So far I have not seen a problem, but that does not mean it could not happen. Is that specific case properly documented somewhere in the MySQL reference?

Answer 1

According to the documentation,
NOW() returns a constant time that indicates the time at which the statement began to execute. (Within a stored function or trigger, NOW() returns the time at which the function or triggering statement began to execute.) This differs from the behavior for SYSDATE(), which returns the exact time at which it executes.
So current_timestamp (which is a synonym for now()) will have the same value even if the query runs for hours. It is so relevant that it will actually even keep that value if the query gets replicated to a different server.