Oracle Internals: An Introduction

Enqueue Locks

Many of Oracle’s locks are called enqueue locks. To enqueue a lock request is to place that request on the queue for its resource. So although the word “enqueue” is strictly speaking a verb, it is used adjectivally in the term enqueue lock. It is also used as a noun when referring to a particular enqueue resource, such as the CF (control file) enqueue.

Oracle uses two classes of local locks—those for which the lock and resource data structures are dynamically allocated in the shared pool, and those that use fixed arrays for the lock and resource data structures. Although almost all types of lock requests may be enqueued, the term enqueue should be taken to refer exclusively to those locks that use the fixed arrays for the lock and resource data structures, unless otherwise qualified.

Enqueue Resources

The fixed array for enqueue resources is sized by the ENQUEUE_RESOURCES parameter. The number of slots in this array that are in use varies from time to time, and these can be seen in V$RESOURCE . Each row in V$RESOURCE represents a resource that is currently locked in any mode by one or more sessions. These resources are not persistent in that they are no longer defined once all locks on the resource have been released.

Rows in V$RESOURCE are identified by a two-character code representing the type of resource, and two numeric fields used to encode either the resource identity or the activities protected by locks on the resource, depending on the resource type.