Enqueues are sophisticated locks for managing access to shared resources like tables, rows, jobs, and redo threads. An enqueue can be requested in different levels/mode: null, row share, row exclusive, share, share row exclusive or exclusive. This wait event indicates a wait for a lock that is held by another session (or sessions) in an incompatible mode to the requested mode. Isolating contention: Once an enqueue resource contention problem has been identified with Ignite, one can quickly isolate the SQL and sessions that are suffering. Often, the SQL is a good clue to what objects have locking contention. During a period of time when locking is typically a problem, use the following query to find out what session is requesting a lock, the type and mode of the requested lock and the session that is blocking. SELECT DECODE(request,0,'Holder: ','Waiter: ')||sid sess, id1, id2, lmode, request, type FROM V$LOCK WHERE (id1, id2, type) IN (SELECT id1, id2, type FROM V$LOCK WHERE request>0) ORDER BY id1, request; Solutions There are many types of locks in Oracle and each has a unique contention remedy. The following are the most common sources of contention: TX

本文转自maclean_007 51CTO博客，原文链接:http://blog.51cto.com/maclean/1277924