问题
1:
I was trying this and it was working fine:
start transaction;
select * from orders where id = 21548 LOCK IN SHARE MODE;
update orders set amount = 1500 where id = 21548;
commit;
According to the definition of LOCK IN SHARE MODE , it locks the table with IS lock and lock the selected rows with S lock.
When a row is locked with S lock.How can it be modified without releasing lock? It needs X lock to modify it.Right? Or is it valid only for different connection transaction?
2:
//session1
start transaction;
select * from orders where id = 21548 FOR UPDATE;
Keep this session1 same and try this in the different session:
//session2
select * from orders where id = 21548; //working
update orders set amount = 2000 where id = 21548; //waiting
FOR UPDATE locks the entire table into IX mode and selected row into X mode.
As X mode is incompatible with S mode then how come select query in second session is getting executed?
One answer might be that select query is not asking for S lock that's why it's running successfully.But update query in the second session is also not asking for X lock , but as you execute it , it starts waiting for the lock held by session1.
I have read a lot of stuff regarding this but not able to clear my doubts.Please help.
回答1:
Bill Karwin answered this question through email.He said:
The same transaction that holds an S lock can promote the lock to an X lock. This is not a conflict.
The SELECT in session 1 with FOR UPDATE acquires an X lock. A simple SELECT query with no locking clause specified does not need to acquire an S lock.
Any UPDATE or DELETE needs to acquire an X lock. That's implicit. Those statements don't have any special locking clause for that.
For more details on IS/IX locks and FOR UPDATE/LOCK IN SHARE MODE please visit shared-and-exclusive-locks .
来源：https://stackoverflow.com/questions/31880185/how-locks-s-x-is-ix-work-in-mysql-with-queries-like-for-update-lock-in-share-m