# Overview of the Performance Schema for Performance Insights on Amazon RDS for MariaDB or MySQL
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The Performance Schema is an optional feature for monitoring Amazon RDS for MariaDB or MySQL runtime performance at a low level of detail. The Performance Schema is designed to have minimal impact on database performance. Performance Insights is a separate feature that you can use with or without the Performance Schema.

**Topics**
+ [Overview of the Performance Schema](#USER_PerfInsights.EnableMySQL.overview)
+ [Performance Insights and the Performance Schema](#USER_PerfInsights.effect-of-pfs)
+ [Automatic management of the Performance Schema by Performance Insights](#USER_PerfInsights.EnableMySQL.options)
+ [Effect of a reboot on the Performance Schema](#USER_PerfInsights.EnableMySQL.reboot)
+ [Determining whether Performance Insights is managing the Performance Schema](USER_PerfInsights.EnableMySQL.determining-status.md)
+ [Turn on the Performance Schema for Amazon RDS for MariaDB or MySQL](USER_PerfInsights.EnableMySQL.RDS.md)

## Overview of the Performance Schema
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The Performance Schema monitors events in MariaDB and MySQL databases. An *event* is a database server action that consumes time and has been instrumented so that timing information can be collected. Examples of events include the following:
+ Function calls
+ Waits for the operating system
+ Stages of SQL execution
+ Groups of SQL statements

The `PERFORMANCE_SCHEMA` storage engine is a mechanism for implementing the Performance Schema feature. This engine collects event data using instrumentation in the database source code. The engine stores events in memory-only tables in the `performance_schema` database. You can query `performance_schema` just as you can query any other tables. For more information, see [MySQL Performance Schema](https://dev.mysql.com/doc/refman/8.0/en/performance-schema.html) in the *MySQL Reference Manual*.

## Performance Insights and the Performance Schema
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Performance Insights and the Performance Schema are separate features, but they are connected. The behavior of Performance Insights for Amazon RDS for MariaDB or MySQL depends on whether the Performance Schema is turned on, and if so, whether Performance Insights manages the Performance Schema automatically. The following table describes the behavior.


| Performance Schema turned on | Performance Insights management mode | Performance Insights behavior | 
| --- | --- | --- | 
|  Yes  |  Automatic  |  [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PerfInsights.EnableMySQL.html)  | 
|  Yes  |  Manual  |  [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PerfInsights.EnableMySQL.html)  | 
|  No  |  N/A  |  [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PerfInsights.EnableMySQL.html)  | 

## Automatic management of the Performance Schema by Performance Insights
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When you create an Amazon RDS for MariaDB or MySQL DB instance with Performance Insights turned on, the Performance Schema is also turned on. In this case, Performance Insights automatically manages your Performance Schema parameters. This is the recommended configuration.

When Performance Insights manages the Performance Schema automatically, the **Source** of `performance_schema` is `System default`.

**Note**  
Automatic management of the Performance Schema isn't supported for the t4g.medium instance class.

If you change the `performance_schema` parameter value manually, and then later want to change to automatic management, see [Turn on the Performance Schema for Amazon RDS for MariaDB or MySQL](USER_PerfInsights.EnableMySQL.RDS.md).

**Important**  
When Performance Insights turns on the Performance Schema, it doesn't change the parameter group values. However, the values are changed on the DB instances that are running. The only way to see the changed values is to run the `SHOW GLOBAL VARIABLES` command.

## Effect of a reboot on the Performance Schema
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Performance Insights and the Performance Schema differ in their requirements for DB instance reboots:

**Performance Schema**  
To turn this feature on or off, you must reboot the DB instance.

**Performance Insights**  
To turn this feature on or off, you don't need to reboot the DB instance.

If the Performance Schema isn't currently turned on, and you turn on Performance Insights without rebooting the DB instance, the Performance Schema won't be turned on.