SQL Locks

SQL Server has more than 20 different lock types . let us focus on common locks

• Shared locks (S). Those locks acquired by readers during read operations such as SELECT. I’d like to mention that it happens in most part of the cases but not all the time. There are some cases when readers don’t acquire (S) locks. We will talk about it later.
• Exclusive locks (X). Those locks acquired by writers during data modification operators such as Insert, Update or Delete. Those locks prevent one object to be modified by the different sessions. Those locks are always acquired and held till end of transaction
• Update locks (U). Those locks are the mix between shared and exclusive locks. SQL Server uses them with data modification statements while searching for the rows need to be modified. For example, if you issue the statement like: “update MyTable set Column1 = 0 where Column1 is null” SQL Server acquires update lock for every row it processes while searching for Column1 is null. When eligible row found, SQL Server converts (U) lock to (X).
• Intent locks (IS, IX, IU, etc). Those locks indicate locks on the child objects. For example, if row has (X) lock, it would introduce (IX) locks on page, table and database level. Main purpose of those locks is optimization. This about situation when you need to have exclusive access to the database (i.e. (X) lock on database level). If SQL Server did not have intent locks, it would have to scan all rows in the all objects and see if there are any low level locks acquired.