1.Implicit locking occurs for all SQL statements except SELECT.
2.When users lock data manually then it's called explicit locking.
Two lock modes:
Exclusive : Locks out other users
Shared: Allows other user to access the server
High level of data concurrency:
- DML: table share row exclusive
- Queries: No locks reqd.
-DDL: Prtects object definitions
Locks held until commit or rollback.
* A shared lock is autometically obtained at the level during DML operations.
With share lock mode,several transactions can aquire share locks on the same
* An exclusive lock is aquired autometically for each row modified by a DML.
Exclusive locks prevent the row from being changed by other transantions until the transactionis comitted or rolled back.
Note: DDL locks occurs when a database object, a table for example, is modified.