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The database system allows competing transactions to access the same database objects. Locks are required to guarantee that parallel processes are synchronized and individual transactions isolated.

Lock Types

Name

System Name

Description

Objects that Can Be Locked

Shared lock

SHARE LOCK

Other transactions retain read access to the database object and can continue to set shared locks for this database object.

Tables, rows, database catalog

Exclusive lock

EXCLUSIVE LOCK

Other transactions have neither read nor write access to the database object.

All SQL statements that change data, such as INSERT, UPDATE and DELETE statements, always request an exclusive lock.

Tables, rows, database catalog

Optimistic lock

-

Keeps track of changes to a database object

Other transactions can continue to set shared, exclusive or optimistic locks for the database object.

Rows

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