1. Introduction
In MySql, the "EXPLAIN" statement is used to obtain the execution plan and other details about how MySql executes a query. When running the "EXPLAIN" statement, the result contains several columns, including "filtered". The value in the "filtered" column represents the estimated percentage of rows that will be examined for a particular table in a query result. This article will discuss the meaning and significance of the "filtered" value in the "EXPLAIN" result in MySql.
2. Understanding the 'Filtered' Column
The "filtered" column in the "EXPLAIN" result provides an estimate of the percentage of rows that will be examined for a specific table in the execution of a query. In other words, it indicates the selectivity of the conditions and filters applied in the query. The value in the "filtered" column ranges from 0 to 1, with 1 indicating that all rows for the table will be examined and 0 indicating that no rows will be examined.
2.1 The Meaning of a 'Filtered' Value of 1
A "filtered" value of 1 means that all rows in the table will be examined during the execution of the query. This typically occurs when there are no conditions or filters applied to the table in the query. For example:
EXPLAIN SELECT * FROM table_name;In the above query, the "filtered" value for the table will be 1 because no conditions are applied to the table, and hence all rows need to be examined.
2.2 The Meaning of a 'Filtered' Value of 0
A "filtered" value of 0 means that no rows in the table will be examined during the execution of the query. This can occur when there is a condition that excludes all rows from the table. For example:
EXPLAIN SELECT * FROM table_name WHERE column_name = 'nonexistent_value';In the above query, if 'nonexistent_value' does not exist in the column "column_name", the "filtered" value for the table will be 0 because there are no rows that satisfy the condition. Therefore, no rows need to be examined.
3. Significance of the 'Filtered' Value
The "filtered" value in the "EXPLAIN" result provides important information about the efficiency and optimization of a query. It helps to determine whether the query is using indexes effectively and if the conditions are selective enough. It can be used to identify potential performance issues and optimize the query accordingly.
3.1 High 'Filtered' Value
A high "filtered" value, close to 1, suggests that the conditions and filters in the query are highly selective. This means that a small percentage of rows need to be examined, resulting in faster query execution. It indicates that the query is likely to make efficient use of indexes and can be considered well-optimized.
3.2 Low 'Filtered' Value
A low "filtered" value, close to 0, indicates that the conditions and filters in the query are not selective enough. This means that a large percentage of rows need to be examined, resulting in slower query execution. It suggests that the query may not be utilizing indexes effectively and could benefit from optimization.
3.3 Intermediate 'Filtered' Value
An intermediate "filtered" value, between 0 and 1, suggests that the query is selective to some extent, but not highly selective. This means that a moderate percentage of rows need to be examined. While it is not as efficient as a high "filtered" value, it is also not as inefficient as a low "filtered" value. Optimization techniques such as creating appropriate indexes or rewriting the query can be explored to improve performance.
4. Conclusion
The "filtered" value in the "EXPLAIN" result in MySql provides insight into the selectivity of conditions and filters applied in a query. It helps to determine the efficiency and optimization of a query by indicating the percentage of rows that need to be examined. A high "filtered" value suggests a well-optimized query, while a low "filtered" value indicates potential performance issues that need to be addressed through optimization. By understanding the significance of the "filtered" value, developers and database administrators can improve the performance of MySql queries.