mysql5.6, mysql5.7什么时候该创建索引？

# 若CARDINALITY / TABLE_ROWS小于10%（经验值），那么表示数据重复率较高，通常需要考虑是否有必要创建该索引 


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# mysql 5.6
目前5.6的STATISTICS表中关于Cardinality的统计是错误的！！！具体可见MySQL bugs #78066。
 但是，表innodb_index_stats中关于Cardinality值得统计依然是正确的  
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SELECT  

     t.TABLE_SCHEMA, 

     t.TABLE_NAME,INDEX_NAME, 

     s.CARDINALITY,  

     t.TABLE_ROWS,  

     s.CARDINALITY/t.TABLE_ROWS AS SELECTIVITY 

FROM 

    information_schema.TABLES t, 

    (SELECT database_name,table_name,index_name,stat_value AS CARDINALITY 

     FROM mysql.innodb_index_stats  

     WHERE (database_name,table_name,index_name,stat_name) IN  

           (SELECT table_schema,table_name,index_name,CONCAT('n_diff_pfx0',MAX(seq_in_index)) 

              FROM information_schema.STATISTICS 

          -- where table_name='xxxxx'  

          GROUP BY table_schema , table_name , index_name )) s 

WHERE t.table_schema = s.database_name  

  AND t.table_name = s.table_name  

  AND t.table_rows != 0 

  AND t.table_schema NOT IN ( 'mysql','performance_schema','information_schema')  

ORDER BY SELECTIVITY;

以下部分出自： http://mp.weixin.qq.com/s?__biz=MjM5MjIxNDA4NA==&mid=401131835&idx=1&sn=37c5fd9d3d8670fb379a1e0565e50eeb&scene=0#wechat_redirect

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# mysql 5.7   
表STATISTICS记录了各个索引的CARDINALITY值 

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SELECT  

     t.TABLE_SCHEMA,t.TABLE_NAME,INDEX_NAME, CARDINALITY,  

    TABLE_ROWS, CARDINALITY/TABLE_ROWS AS SELECTIVITY 

FROM 

    information_schema.TABLES t, 

 ( 

  SELECT table_schema,table_name,index_name,cardinality 

  FROM information_schema.STATISTICS  

  WHERE (table_schema,table_name,index_name,seq_in_index) IN ( 

  SELECT table_schema,table_name,index_name,MAX(seq_in_index) 

  FROM information_schema.STATISTICS 

  GROUP BY table_schema , table_name , index_name ) 

 ) s 

WHERE 

    t.table_schema = s.table_schema  

        AND t.table_name = s.table_name AND t.table_rows != 0 

        AND t.table_schema NOT IN ( 'mysql','performance_schema','information_schema')  

ORDER BY SELECTIVITY;