MySQL 优化之前缀索引
--查看选择率 select count(distinct(fileid))/count(*) AS Selectivity from t_file_info;select count(distinct left(fileid,32))/count(*) from t_file_info;
(root@localhost) > select count(distinct(fileid))/count(*) from t_file_info;
+----------------------------------+
| count(distinct(fileid))/count(*) |
+----------------------------------+
| 1.0000 |
+----------------------------------+
1 row in set (0.17 sec)
(root@localhost) > select count(distinct left(fileid,32))/count(*) from t_file_info;
+------------------------------------------+
| count(distinct left(fileid,32))/count(*) |
+------------------------------------------+
| 0.9999 |
+------------------------------------------+
1和0.9999几乎可以等同，其实这里因为点特殊情况，正常应该都是1才对的。
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