Oracle Lock

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Types of Locks
Oracle automatically uses different types of locks to control concurrent access to data and to prevent destructive interaction between users. Oracle automatically locks a resource on behalf of a transaction to prevent other transactions from doing something also requiring exclusive access to the same resource. The lock is released automatically when some event occurs so that the transaction no longer requires the resource.
Throughout its operation, Oracle automatically acquires different types of locks at different levels of restrictiveness depending on the resource being locked and the operation being performed.
Oracle locks fall into one of three general categories.
LockDescription
DML locks (data locks)
DML locks protect data. For example, table locks lock entire tables, row locks lock selected rows.
DDL locks (dictionary locks)
DDL locks protect the structure of schema objects--for example, the definitions of tables and views.
Internal locks and latches
Internal locks and latches protect internal database structures such as datafiles. Internal locks and latches are entirely automatic.
The following sections discuss DML locks, DDL locks, and internal locks.
DML Locks
The purpose of a DML (data) lock is to guarantee the integrity of data being accessed concurrently by multiple users. DML locks prevent destructive interference of simultaneous conflicting DML or DDL operations. For example, Oracle DML locks guarantee that a specific row in a table can be updated by only one transaction at a time and that a table cannot be dropped if an uncommitted transaction contains an insert into the table.
DML operations can acquire data locks at two different levels: for specific rows and for entire tables.
Row Locks (TX)
The only DML locks Oracle acquires automatically are row-level locks. There is no limit to the number of row locks held by a statement or transa