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Shared lock is also called read lock, used for reading data items only….Difference between Shared Lock and Exclusive Lock :
| S.No. | Shared Lock | Exclusive Lock |
|---|---|---|
| 5. | Any number of transaction can hold shared lock on an item. | Exclusive lock can be hold by only one transaction. |
| 6. | S-lock is requested using lock-S instruction. | X-lock is requested using lock-X instruction. |
What is a shared lock in SQL?
Shared locks are acquired automatically by SQL Server when data is read. Many processes can hold shared locks on the same data, but no process can acquire an exclusive lock on data that has a shared lock on it (unless the process requesting the exclusive lock is the same process as the one holding the shared lock).
What is row share lock?
Row Share Table Locks (RS)—These locks are issued when an SQL transaction has declared its intent to update the table in row share mode. This type of lock will allow other queries to update rows in the customer table. For example: lock table customer in row share mode; SELECT customer_name.
A shared ( S ) lock permits the transaction that holds the lock to read a row. An exclusive ( X ) lock permits the transaction that holds the lock to update or delete a row.
What is shared lock in SAP?
When a transaction sets a shared lock on a database object, then other transactions can continue to gain read access to the database object, and other shared locks can be set for this database object. A shared or read lock is also known as a SHARE lock.
What is shared lock and deadlock in SQL?
Here is some information about locks that SQL Server uses: Shared lock (S) is used to read data. When two transactions are waiting on each other to convert Shared locks on resources to Exclusive locks, a deadlock occurs. Update lock (U) is used to avoid deadlocks.
What is lock and types of lock in DBMS?
Locks are of two kinds − Binary Locks − A lock on a data item can be in two states; it is either locked or unlocked. Shared/exclusive − This type of locking mechanism differentiates the locks based on their uses. If a lock is acquired on a data item to perform a write operation, it is an exclusive lock.
What’s the difference between shared lock and shared lock?
A shared lock reserves a resource (page or row) for reading only. Other processes cannot modify the locked resource while the lock remains. On the other hand, several processes can hold a shared lock for a resource at the same time—that is, several processes can read the resource locked with the shared lock.
When does a transaction request a shared lock?
The graphs shows that if two transactions only read the same data object they do not conf1ict, but if one transaction writes a data object and another either read or write the same data object, then they conflict with each other. A transaction requests a shared lock on data item Q by executing the lock-S (Q) instruction.
Shared locks are placed on resources whenever a read operation (select) is performed. Multiple shared locks can be simultaneously set on a resource. Exclusive locks are placed on resources whenever a write operation (INSERT, UPDATE And DELETE) are performed. Only one exclusive lock can be placed on a resource at a time.
What happens when you lock a table in shared mode?
In your example you locked the table in shared mode. This does not prevent other sessions locking the same object in shared mode, but it does prevent them from locking it in exclusive mode so you could not drop the table (which requires an exclusive lock) while it is being updated (which has a shared lock).