Monday, July 10, 2006

Locking Chapter 1

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
resource.

* 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.

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