In MySQL, histograms are statistical representations of the distribution of data in a table column. They provide information about the distribution of values in a column, which can be used by the query optimizer to make more informed decisions about query execution plans.
There are two types of histograms available in MySQL:
- Equi-height Histograms: Equi-height histograms divide the data into buckets of roughly equal size, which makes them more representative of the actual data distribution compared to other types of histograms.
- Singleton Histograms: Singleton histograms are created for columns with a large number of unique values, and each value in the column is treated as a separate bucket.
In MySQL, histograms are created by the query optimizer when it analyzes the data in a table. The optimizer uses the histograms to estimate the selectivity of a query, which is a measure of how selective a predicate is in filtering data. The optimizer uses this information to estimate the number of rows that will be returned by a query and to choose the best execution plan for the query.
It’s important to note that histograms are not automatically created for all columns in MySQL, and that the query optimizer must analyze the data in a table to create them. Additionally, the accuracy of histograms can be impacted by the size and distribution of data in the column, so it’s important to periodically analyze the data and update the histograms as needed.
Monitoring Histograms in MySQL with Performance Schema
The Performance Schema in MySQL provides a set of tables and views that can be used to monitor various aspects of MySQL performance, including histograms.
To monitor histograms in MySQL using the Performance Schema, you can use the following tables and views:
- performance_schema.histograms table: This table provides information about histograms that have been created by the query optimizer.
- performance_schema.histogram_buckets table: This table provides information about the individual buckets in each histogram, including the range of values covered by each bucket and the number of rows in each bucket.
- performance_schema.histogram_integer_values table: This table provides information about the integer values in each histogram bucket.
- performance_schema.histogram_double_values table: This table provides information about the double-precision floating-point values in each histogram bucket.
By using these tables and views, you can monitor the distribution of data in the columns of your tables and see how the query optimizer is using histograms to make decisions about query execution plans. You can also use this information to identify potential performance issues and make changes to your data and queries to improve performance.
It’s important to note that the Performance Schema is an optional component of MySQL, and that it must be enabled and configured properly in order to use it to monitor histograms. Additionally, the Performance Schema can have an impact on MySQL performance, so it’s important to monitor its performance and make adjustments as needed.