大家好，我是考100分的小小码 ，祝大家学习进步，加薪顺利呀。今天说一说oracle怎么更新索引_mysql索引失效,希望您对编程的造诣更进一步.
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MySQL使用存储的键分布基数来确定表连接顺序
在决定对查询中的特定表使用哪些索引时，也会使用使用键分布基数
ANALYZE TABLE 表名 可以更新表的索引基数，使其更接近非重复的记录数，记录数可以使用show index from 表 来查询cardinality字段
mysql> show index from index_test; +------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | index_test | 0 | PRIMARY | 1 | id | A | 5 | NULL | NULL | | BTREE | | | | index_test | 1 | score_index | 1 | score | A | 2 | NULL | NULL | | BTREE | | | | index_test | 1 | name_gid_age_index | 1 | name | A | 3 | NULL | NULL | | BTREE | | | | index_test | 1 | name_gid_age_index | 2 | gid | A | 3 | NULL | NULL | | BTREE | | | | index_test | 1 | name_gid_age_index | 3 | age | A | 6 | NULL | NULL | | BTREE | | | +------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 5 rows in set (0.00 sec) mysql> select * from index_test; +----+------------+-----+-----+-------+ | id | name | gid | age | score | +----+------------+-----+-----+-------+ | 1 | taoshihan | 2 | 0 | 0 | | 2 | taoshihan1 | 2 | 0 | 0 | | 3 | taoshihan2 | 3 | 10 | 10 | | 4 | taoshihan | 2 | 1 | 0 | | 5 | taoshihan | 2 | 2 | 0 | | 6 | taoshihan | 2 | 3 | 0 | +----+------------+-----+-----+-------+ 6 rows in set (0.03 sec) mysql> ANALYZE TABLE index_test; +--------------------+---------+----------+----------+ | Table | Op | Msg_type | Msg_text | +--------------------+---------+----------+----------+ | my_test.index_test | analyze | status | OK | +--------------------+---------+----------+----------+ 1 row in set (0.13 sec) mysql> show index from index_test; +------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | index_test | 0 | PRIMARY | 1 | id | A | 6这里变了 | NULL | NULL | | BTREE | | | | index_test | 1 | score_index | 1 | score | A | 2 | NULL | NULL | | BTREE | | | | index_test | 1 | name_gid_age_index | 1 | name | A | 3 | NULL | NULL | | BTREE | | | | index_test | 1 | name_gid_age_index | 2 | gid | A | 3 | NULL | NULL | | BTREE | | | | index_test | 1 | name_gid_age_index | 3 | age | A | 6 | NULL | NULL | | BTREE | | | +------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 5 rows in set (0.07 sec)
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