> 官網：https://dev.mysql.com/doc/refman/8.0/en/explain-output.html **EXPLAIN** 命令，它可以對`SELECT`語句進行分析，并輸出`SELECT`執行的詳細信息，以供開發人員針對性優化。 ~~~ mysql> EXPLAIN SELECT * from users where id = 1; +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ | 1 | SIMPLE | users | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL | +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ 1 row in set (0.02 sec) ~~~ | Column | JSON Name | Meaning | | --- | --- |--- | | id | 1 | `SELECT`標識符 | | select_type | SIMPLE | `SELECT`類型 | | table | users | 查詢的表 | | partitions | NULL | 匹配的分區 | | type | const | 聯接類型 | | possible_keys | PRIMARY | 可能選擇的索引 | | key | PRIMARY | 實際選擇的索引 | | key_len | 8 | 所選密鑰的長度 | | ref | const | 與索引比較的列 | | rows | 1 | 估計要檢查的行數 | | filtered | 100.00 | 按表條件過濾的行百分比 | | Extra | NULL | 附加信息 |