MySQL 5.6快速定位不合理索引

# 若CARDINALITY / TABLE_ROWS小於10%（經驗值），那麼表示資料重複率較高，通常需要考慮是否有必要建立該索引


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# mysql 5.6
目前5.6的STATISTICS表中關於Cardinality的統計是錯誤的！！！具體可見MySQL bugs #78066。
但是，表innodb_index_stats中關於Cardinality值得統計依然是正確的  
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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記錄了各個索引的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;