Oracle RAC Explain Plan Statistics Test

The EXPLAIN PLAN statement displays execution plans chosen by the Oracle optimizer for SELECT, UPDATE, INSERT, and DELETE statements. A statement's execution plan is the sequence of operations Oracle performs to run the statement.

The Oracle optimizer plays a key role in query processing and decides the most optimal explain plan for the queries that wanted to execute. The optimizer estimates the resource consumption cost of the explain plan candidates and chooses the effective and cheap plan in terms of I/O, memory, CPU, and time. In the process, the Oracle optimizer also changes the plans during query processing when any of the following things happen on a running instance:

  • Re-run of dbms_stats for CBO statistics,
  • Alter table commands
  • Adding/dropping an index
  • Using dynamic sampling
  • Parallelism is enabled

Sometimes, query processing may be delayed when the explain plan decided by the Oracle Optimizer is used for execution. This may lead to query processing bottlenecks. To avoid such bottlenecks, it is necessary for the administrators to keep track on the explain plans that were changed during query execution. The Oracle RAC Explain Plan Statistics test helps administrators in this regard!

This test periodically monitors each database instance on the Oracle Cluster and, for each instance, reports the number of times the explain plans were changed during query execution. The detailed diagnosis reported by this test reveals the details on plan changes for each query, name of all the plans used for each query, etc.

Target of the test : An Oracle Cluster

Agent deploying the test : An internal agent

Outputs of the test : One set of results for each instance on the Oracle Cluster that is being monitored

Configurable parameters for the test
Parameters Description

Test period

How often should the test be executed. By default, this is set to 60 minutes.

Host

The IP address of the Oracle server.

Port

The port number through which the Oracle server communicates. The default port is 1521.

OraSID

The variable name of the oracle instance.

ServiceName

A ServiceName exists for the entire Oracle RAC system. When clients connect to an Oracle cluster using the ServiceName, then the cluster routes the request to any available database instance in the cluster. By default, this is set to none. In this case, the test connects to the cluster using the OraaSID and pulls out the metrics from that database instance which corresponds to that OraSID. If a valid service name is specified instead, then, the test will connect to the cluster using that service name, and will be able to pull out metrics from any available database instance in the cluster.

To know the ServiceName of a cluster, execute the following query on any node in the target cluster:

select name, value from v$parameter where name =’service_names’

User

In order to monitor an Oracle RAC, a special database user account has to be User – In order to monitor an Oracle database server, a special database user account has to be created in every Oracle database instance that requires monitoring. A Click here hyperlink is available in the test configuration page, using which a new oracle database user can be created. Alternatively, you can manually create the special database user. When doing so, ensure that this user is vested with the select_catalog_role and create session privileges.

The sample script we recommend for user creation (in Oracle database server versions before 12c) for eG monitoring is:

create user oraeg identified by oraeg create role oratest;

grant create session to oratest;

grant select_catalog_role to oratest;

grant oratest to oraeg;

The sample script we recommend for user creation (in Oracle database server 12c) for eG monitoring is:

alter session set container=<Oracle_service_name>;

create user <user_name>identified by <user_password> container=current default tablespace <name_of_default_tablespace> temporary tablespace <name_of_temporary_tablespace>;

Grant create session to <user_name>;                                 

Grant select_catalog_role to <user_name>;

The name of this user has to be specified here.

Password

Specify the password corresponding to the mentioned user.

Confirm Password

Confirm the password by retyping it.

DD Frequency

Refers to the frequency with which detailed diagnosis measures are to be generated for this test. The default is 1:1. This indicates that, by default, detailed measures will be generated every time this test runs, and also every time the test detects a problem. You can modify this frequency, if you so desire. Also, if you intend to disable the detailed diagnosis capability for this test, you can do so by specifying none against DD frequency.

Detailed Diagnosis

To make diagnosis more efficient and accurate, the eG Enterprise embeds an optional detailed diagnostic capability. With this capability, the eG agents can be configured to run detailed, more elaborate tests as and when specific problems are detected. To enable the detailed diagnosis capability of this test for a particular server, choose the On option. To disable the capability, click on the Off option.

The option to selectively enable/disable the detailed diagnosis capability will be available only if the following conditions are fulfilled:

  • The eG manager license should allow the detailed diagnosis capability
  • Both the normal and abnormal frequencies configured for the detailed diagnosis measures should not be 0.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

Explain plan changed count

Indicates the number of times the explain plans were changed during the last measurement period.

Number

The detailed diagnosis of this measure reveals the session ID, user name, database name, host name, count of unique plans, name of old and new explain plans and brief description about the query.

Rate of explain plan change

Indicates the rate at which explain plans were changed during the last measurement period.

Plans/sec