In-depth explanation of Mysql’s Explain

Explain is a command in MySQL that is used to analyze and optimize query statements, providing information about the execution plan of the query. By executing the Explain command, you can understand how MySQL executes the query statement and which indexes and optimization strategies are being used. The syntax for Explain is as follows:

EXPLAIN SELECT * FROM table_name WHERE condition;

The output of the Explain command includes the following columns:
1. id: indicates the sequence of the query, with multiple ids if it’s a composite query (such as with subqueries), increasing sequentially.
2. select_type: indicates the type of query, commonly seen types include SIMPLE (simple query), PRIMARY (main query), SUBQUERY (subquery), DERIVED (derived table query), UNION (union query), etc.
3. table: indicates the name of the table being queried.
4. partitions: indicates information about the table partitions being queried.
5. type: indicates the type of index being used in the query, with common types being ALL, index, range, ref, eq_ref, const, system. In general, the order of index types from worst to best is: ALL > index > range > ref > eq_ref > const > system.
6. possible_keys: shows a list of indexes that could potentially be used in the query.
7. key: shows the actual index being used in the query.
8. key_len: indicates the length of the index in bytes.
9. ref: shows the index column or constant being used in the query.
10. rows: indicates the number of rows that need to be scanned in the query.
11. filtered: represents the filtering rate of the query results, i.e., the percentage of the returned results.
12. Extra: provides additional information, commonly seen entries include Using index, Using where, Using temporary, Using filesort.
By analyzing the output of Explain, one can evaluate and optimize the performance of query statements. For example, it can help determine if appropriate indexes are being used, if there are full table scans, or if the query statement needs to be optimized. In conclusion, Explain is a very useful tool in MySQL that can assist developers and DBAs in optimizing query statements and improving database performance.