+----+-------------+---------+------+---------------------------+---------------------+---------+------------------+------+--------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------------------+---------------------+---------+------------------+------+--------------------------------------+
| 1 | SIMPLE | v | ref | PRIMARY,cand_visitor_omit | cand_visitor_omit | 4 | const | 1 | Using where |
| 1 | SIMPLE | current | ref | visitorid_visittime | visitorid_visittime | 8 | test.v.visitorid | 2 | Using where; Using index |
| 1 | SIMPLE | earlier | ref | visitorid_visittime | visitorid_visittime | 8 | test.v.visitorid | 2 | Using where; Using index; Not exists |
+----+-------------+---------+------+---------------------------+---------------------+---------+------------------+------+--------------------------------------+
以这种方式更改索引也会使访问者(visitorid)上的其他单列索引变得多余,因此您可以删除该索引.
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