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_posts/2019-05-09-MySQL_中的索引.md

@@ -192,6 +192,53 @@ INDEX (col1, col2)
 
 不同存储引擎对其支持情况不一，详见 [Spatial Indexes](https://dev.mysql.com/doc/refman/8.0/en/create-index.html#create-index-spatial)。
 
+## 索引的查看
+
+通过如下语句查看有哪些索引：
+
+```sql
+SHOW INDEX FROM table_name;
+```
+
+e.g.:
+
+以 [employees 示例数据库](https://github.com/wayou/wayou.github.io/issues/82)中 titles 表为例，先创建索引
+
+```sql
+mysql> CREATE INDEX part_of_name ON titles (title(10));
+```
+
+查看索引
+
+```sql
+mysql> show index from titles;
++--------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
+| Table  | Non_unique | Key_name     | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
++--------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
+| titles |          0 | PRIMARY      |            1 | emp_no      | A         |      300519 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+| titles |          0 | PRIMARY      |            2 | title       | A         |      442783 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+| titles |          0 | PRIMARY      |            3 | from_date   | A         |      442783 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+| titles |          1 | part_of_name |            1 | title       | A         |           6 |       10 |   NULL |      | BTREE      |         |               | YES     | NULL       |
++--------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
+4 rows in set (0.01 sec)
+```
+
+## 查看索引是否命中
+
+索引创建后，可通过 [`explain`](https://github.com/wayou/wayou.github.io/issues/100) 语句分析查询是否命中索引。
+
+```sql
+mysql> EXPLAIN SELECT * FROM titles WHERE title = 'Engineer';
++----+-------------+--------+------------+------+---------------+--------------+---------+-------+--------+----------+-----------------------+
+| id | select_type | table  | partitions | type | possible_keys | key          | key_len | ref   | rows   | filtered | Extra                 |
++----+-------------+--------+------------+------+---------------+--------------+---------+-------+--------+----------+-----------------------+
+|  1 | SIMPLE      | titles | NULL       | ref  | part_of_name  | part_of_name | 42      | const | 221391 |   100.00 | Using index condition |
++----+-------------+--------+------------+------+---------------+--------------+---------+-------+--------+----------+-----------------------+
+1 row in set, 1 warning (0.01 sec)
+```
+
+其中 `possible_keys` 为可选的索引，`key` 为真实命中的索引。
+
 
 ## 相关资源