الموضوع الاول : Read Consistency
Database users access the database in two ways:
• Read operations (SELECT statement)
• Write operations (INSERT, UPDATE, DELETE statements)
You need read consistency so that the following occur:
• The database reader and writer are ensured a consistent view of the data.
• Readers do not view data that is in the process of being changed.
• Writers are ensured that the changes to the database are done in a consistent way.
• Changes made by one writer do not disrupt or conflict with changes another writer is making.
The purpose of read consistency is to ensure that each user sees data as it existed at the last commit,
before a DML operation started.
الموضوع الثاني : Locking
In an Oracle database, locks:
• Prevent destructive interaction between
• Require no user action
• Automatically use the lowest level of
• Are held for the duration of the transaction
• Are of two types: explicit locking and implicit
What Are Locks?
Locks are mechanisms that prevent destructive interaction between transactions accessing the same
resource, either a user object (such as tables or rows) or a system object not visible to users (such as
shared data structures and data dictionary rows).
How the Oracle Database Locks Data
Oracle locking is performed automatically and requires no user action. Implicit locking occurs for
SQL statements as necessary, depending on the action requested. Implicit locking occurs for all SQL
statements except SELECT.
The users can also lock data manually, which is called explicit locking.
وشكراً جزيلاً لكم
تم التعديل بواسطة omar-alreyati, 17 November 2005 - 09:16 AM.