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An exclusive lock can be imposed to a page or row only if there is no other shared or exclusive lock imposed already on the target.
This practically means that only one exclusive lock can be imposed to a page or row, and once imposed no other lock can be imposed on locked resources Shared lock (S) – this lock type, when imposed, will reserve a page or row to be available only for reading, which means that any other transaction will be prevented to modify the locked record as long as the lock is active.
Essentially, locks are in-memory structures which have owners, types, and the hash of the resource that it should protect.
A lock as an in-memory structure is 96 bytes in size.
Only one shared with intent exclusive lock (SIX) can be acquired on a table at a time and it will block other transactions from making updates, but it will not prevent other transactions to read the lower hierarchy resources they can acquire the intent shared (IS) lock on the table Shared with intent update (SIU) – this is a bit more specific lock as it is a combination of the shared (S) and intent update (IU) locks.
A typical example of this lock is when a transaction is using a query executed with the PAGELOCK hint and query, then the update query.However, a shared lock can be imposed by several transactions at the same time over the same page or row and in that way several transactions can the ability for data reading since the reading process itself will not affect anyhow the actual page or row data.In addition, a shared lock will allow write operations, but no DDL changes will be allowed Update lock (U) – this lock is similar to an exclusive lock but is designed to be more flexible in a way.You need to speed up a SQL Server app, but you only want to make index changes.You don’t want to buy hardware, change SQL Server, or change the code.While the update lock can be imposed on a record that has the shared lock, the shared lock cannot be imposed on the record that already has the update lock Intent locks (I) – this lock is a means used by a transaction to inform another transaction about its to acquire a lock.