Lock

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

More Information

Concepts of the Database System, Locks

Database Administration, Analyzing Database Performance

Database Administration in CCMS, SQL Locks