Question

Background: I have a performance-critical query I’d like to run and I don’t care about dirty reads.

My question is; If I’m using joins, do I have to specify the NOLOCK hint on those as well?

For instance; is:

SELECT * FROM table1 a WITH (NOLOCK)
INNER JOIN table2 b WITH (NOLOCK) ON a.ID = b.ID

Equivalent to:

SELECT * FROM table1 a WITH (NOLOCK)
INNER JOIN table2 b ON a.ID = b.ID

Or will I need to specify the (NOLOCK) hint on the join to ensure I’m not locking the joined table?

Answer 1

I won’t address the READ UNCOMMITTED argument, just your original question.

Yes, you need WITH(NOLOCK) on each table of the join. No, your queries are not the same.

Try this exercise. Begin a transaction and insert a row into table1 and table2. Don’t commit or rollback the transaction yet. At this point your first query will return successfully and include the uncommitted rows; your second query won’t return because table2 doesn’t have the WITH(NOLOCK) hint on it.