Oracle Wait Events Introduction

An event can be defined as a particular function, or a task, that the Oracle kernel performs on behalf of the user session or its own background process. Tasks such as reading and writing data blocks to and from data files, waiting for latch acquisitions before accessing or manipulating data in the memory, inter-process postings and communications, are known as database events, and they have specific names.

Why are these events called Wait Events? All sessions accessing Oracle database instance need resources to perform their tasks. A resource may be a data buffer, a latch, an enqueue (lock), a pin, or a database object. Whenever a session has to wait for something, the wait time is tracked and charged to the event that is associated with that wait. For example, a session that needs an index block that is not in the SGA makes a read call to the operating system and waits for the delivery of the block. The wait time is charged to the db file sequential read event. Another session may have completed the last instruction and is now idle waiting for user input. In this case, the wait time is charged to the SQL*Net message from client event. When a session is not using the CPU, it may be waiting for a resource, an action to complete or simply for more work. For this reason events that are associated with all waits are known as Wait Events.

Wait Interface Components

SELECT *
FROM V$EVENT_NAME;