transactions - MySQL InnoDB: Difference Between `FOR UPDATE` and `LOCK IN SHARE MODE`

Question

What is the exact difference between the two locking read clauses:

SELECT ... FOR UPDATE

and

SELECT ... LOCK IN SHARE MODE 

And why would you need to use one over the other?

Answer 1

I have been trying to understand the difference between the two. I'll document what I have found in hopes it'll be useful to the next person.

Both LOCK IN SHARE MODE and FOR UPDATE ensure no other transaction can update the rows that are selected. The difference between the two is in how they treat locks while reading data.

LOCK IN SHARE MODE does not prevent another transaction from reading the same row that was locked.

FOR UPDATE prevents other locking reads of the same row (non-locking reads can still read that row; LOCK IN SHARE MODE and FOR UPDATE are locking reads).

This matters in cases like updating counters, where you read value in 1 statement and update the value in another. Here using LOCK IN SHARE MODE will allow 2 transactions to read the same initial value. So if the counter was incremented by 1 by both transactions, the ending count might increase only by 1 - since both transactions initially read the same value.

Using FOR UPDATE would have locked the 2nd transaction from reading the value till the first one is done. This will ensure the counter is incremented by 2.