MySQL CARDINALITY确认索引选择性

SELECT
t.TABLE_SCHEMA,t.TABLE_NAME,INDEX_NAME, CARDINALITY, TABLE_ROWS,
CARDINALITY/TABLE_ROWS AS SELECTIVITY -- 得到选择性
FROM
TABLES t, -- 查询的表一，TABLES
(
SELECT
table_schema,
table_name,
index_name,
cardinality
FROM STATISTICS
WHERE (table_schema,table_name,index_name,seq_in_index) IN (
SELECT
table_schema,
table_name,
index_name,
MAX(seq_in_index)
FROM
STATISTICS
GROUP BY table_schema , table_name , index_name )
) s -- 查询的表二，就是上面3.2的查询结果
WHERE
t.table_schema = s.table_schema -- 通过库关联
AND t.table_name = s.table_name -- 再通过表变量
AND t.table_schema = 'employees' -- 指定某一个库名
ORDER BY SELECTIVITY;

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