The MySQL EXPLAIN statement is a tool that helps in optimizing queries by providing critical information about how MySQL executes the query. It returns the execution plan of a SELECT, DELETE, INSERT, or UPDATE statement.
When you execute an EXPLAIN statement for a query, MySQL displays information on how it intends to execute the query. Specifically, it shows how it will access and combine rows in the selected tables. It also reveals whether MySQL can optimize the query, for example by using an index to access rows or by avoiding sorting operations, which could significantly speed up query execution.
The syntax for the EXPLAIN statement in MySQL is as follows:
EXPLAIN SELECT select_list FROM table_list [WHERE where_condition] [GROUP BY col_list] [ORDER BY col_list] [LIMIT rows]The output of the EXPLAIN statement contains the following columns:
- id: A sequential number that represents the order in which MySQL executes the query.
- select_type: The type of SELECT statement that MySQL uses to execute the query. Examples include SIMPLE, SUBQUERY, UNION, and DERIVED.
- table: The name of the table that MySQL accesses to retrieve data. If a query involves multiple tables, the table listed first is typically the one that MySQL accesses first.
- partitions: If the table is partitioned, this column will show the partitioning type and any partitioning expressions used.
- type: The type of access method that MySQL uses to access data from the table. Examples include ALL, UNIQUE_INDEX, RANGE, and REF.
- possible_keys: The list of indexes that MySQL can use to execute the query.
- key: The index that MySQL selects to execute the query. If the value of this column is NULL, it means that MySQL cannot use any index to execute the query.
- key_len: The length of the index that MySQL uses to execute the query.
- ref: The columns or constants that MySQL compares to the index.
- rows: The number of rows that MySQL examines to execute the query.
- filtered: The percentage of rows that MySQL filters out using the WHERE clause or other conditions.
- Extra: Additional information about how MySQL executes the query, such as whether it uses a temporary table or a file sort.
By studying the output of the EXPLAIN statement, you can identify potential performance bottlenecks and optimize the query accordingly. For example, you can use the information from the output to make sure that you have the appropriate indexes in place, or to identify whether MySQL performs a full table scan rather than using an index.
In summary, the MySQL EXPLAIN statement provides valuable information about how MySQL executes a query, which can help you identify performance bottlenecks and optimize your queries for maximum efficiency.