SQL Explain Plan Statistics Test

An execution plan in SQL Server is a simple graphical representation of the operations that the query optimizer generates to calculate the most efficient way to return a set of results. Once a query is executed, the query processing engine quickly generates multiple execution plans and selects the one which returns the results with the best performance. There are two types of execution plans to be specific :

  • Estimated Execution Plan – This type of execution plan is just a guess by the query processor about how the specific steps that are to be involved while returning the results. It is often generated before the query has been executed

  • Actual Execution Plan – This plan is generated after the query has been executed. It shows the actual operations and steps involved while executing the query. This may or may not differ from the Estimated Execution Plan

The query optimizer plays a key role in query processing and decides the most optimal explain plan for the queries that should be executed. 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 query optimizer also changes the plans during query processing as the underlying costs change. The explain change happens when any of the following 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 SQL 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 SQL Explain Plan Statistics test helps administrators in this regard!

This test periodically monitors the target Microsoft SQL server and reports the number of times that 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 : A Microsoft SQL server

Agent deploying the test : An internal agent

Outputs of the test : One set of results for the Microsoft SQL Server 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.


The IP address of the Microsoft SQL server.


The port number through which the Microsoft SQL server communicates. The default port is 1433.


If a Microsoft SQL Server 7.0/2000 is monitored, then provide the name of a SQL user with the Sysadmin role in this text box. While monitoring a Microsoft SQL Server 2005 or above, provide the name of a SQL user with all of the privileges outlined in User Privileges Required for Monitoring Microsoft SQL server.


Specify the password corresponding to the mentioned user.

Confirm Password

Confirm the password by retyping it.


Enter the name of a specific Microsoft SQL instance that is to be monitored against this parameter. The default value of this parameter is “default”. To monitor a Microsoft SQL instance named “CFS”, enter this as the value of the Instance parameter.


By default, none is displayed in the Domain text box. If the ‘SQL server and Windows’ authentication has been enabled for the server being monitored, then the Domain can continue to be none. On the other hand, if ‘Windows only’ authentication has been enabled, then, in the Domain text box, specify the Windows domain in which the managed Microsoft SQL server exists. Also, in such a case, the User name and Password that you provide should be that of a user authorized to access the monitored SQL server.


In some Windows networks, NTLM (NT LAN Manager) may be enabled. NTLM is a suite of Microsoft security protocols that provides authentication, integrity, and confidentiality to users. NTLM version 2 (“NTLMv2”) was concocted to address the security issues present in NTLM. By default, the ISNTLMV2 flag is set to No, indicating that NTLMv2 is not enabled by default on the target Microsoft SQL host. Set this flag to Yes if NTLMv2 is enabled on the target host.


If the value chosen is Yes, then the Microsoft SQL server under consideration is a passive server in a SQL cluster. No alerts will be generated if the server is not running. Measures will be reported as “Not applicable" by the agent if the server is not up.


By default, the SSL flag is set to No, indicating that the target Microsoft SQL server is not SSL-enabled by default. To enable the test to connect to an SSL-enabled Microsoft SQL server, set the SSL flag to Yes.

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.


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.