How many rows will be locked by SELECT … ORDER BY xxx LIMIT 1 FOR UPDATE?

Question

I have a query with the following structure:

SELECT ..... WHERE status = \'QUEUED\' ORDER BY position ASC LIMIT 1 FOR UPDATE;

It\'s a singl

Answer 1

I've made tests. Created the following table:

id  data1   data2
1   1   2
2   2   1
5   2   2
6   3   3
3   3   4
4   4   3

Then I created first connection with transaction:

SELECT id FROM test ORDER BY data1 LIMIT 1 FOR UPDATE;

result was the row with id=1;

Then I created second transaction from another connection without commiting first:

SELECT id FROM test WHERE data1=2 FOR UPDATE;

It didn't block. And it blocked only when I tried to select the very row selected by the first transaction. I tried the following with changing ORDER BY to DESC one, it works also.

Conclusion: MySQL blocks only the rows it actually selected when using ORDER BY and LIMIT clauses. See @Morgan answer for gap locking explanation.

My MySQL version is 5.0.45