Does a transaction with no net effect touch the database on-disk? - mysql

Question

I have a circumstance where I need to update some table rows, marking the ones that do not appear in an external data-source as disabled (i.e. update active=0). The straight-forwards solution is to BEGIN a transaction, UPDATE every row to active=0, and then scan the remote data, doing an UPDATE for each entry that should be active=1 to put it back. I have around 1k rows, so this should be a relatively quick operation, even if there is a lot of inefficient query parsing.
However, this data will often not change at all. Hence, in the majority of cases, the net effects of the transaction will be zero change. If the database engine will resolve the whole thing, detect that nothing is changing, and not change anything as a result, that would be ideal. However, if it is going to go through and actually update every row, every time, I would rather find another solution.

Answer 1

Here's a demo. I created a table with just a simple integer in a row.
mysql> create table t ( i int );
mysql> insert into t set i = 42;
I check the current number of log writes.
mysql> show status like 'innodb_log_write_requests';
+---------------------------+---------+
| Variable_name | Value |
+---------------------------+---------+
| Innodb_log_write_requests | 5432152 |
+---------------------------+---------+
Then change the value in the row with an UPDATE and confirm it resulted in a log write:
mysql> update t set i = 43;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> show status like 'innodb_log_write_requests';
+---------------------------+---------+
| Variable_name | Value |
+---------------------------+---------+
| Innodb_log_write_requests | 5432153 |
+---------------------------+---------+
Next, make an UPDATE that has no net effect.
mysql> update t set i = 43;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 0
Notice Changed: 0.
Look at the log writes, it is also unchanged:
mysql> show status like 'innodb_log_write_requests';
+---------------------------+---------+
| Variable_name | Value |
+---------------------------+---------+
| Innodb_log_write_requests | 5432153 |
+---------------------------+---------+

Answer 2

I think it has pretty much been concluded that there is disk I/O for your no-op. Let's discuss the task at hand:
Instead of actually modifying the database, can you keep in memory a list of the items that might be disabled? After you have made the scan, if there are any to disable, then proceed to disable all in a single UPDATE ... WHERE id IN (...)
On another topic... If you are actually doing this
BEGIN;
UPDATE a=0; -- for all rows
COMMIT;
-- all are disabled briefly
BEGIN;
UPDATE a=1 WHERE id = ... -- one row at a time
COMMIT;
Then you have a window where everthing is disabled. You probably don't want that.