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再检查执行计划：子查询物化后（select_type=DERIVED)参与 JOIN。虽然估算行扫描仍然为90万，但是利用了索引以及 LIMIT 子句后，实际执行时间变得很小。
- +----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+
- | 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 15 | Using temporary; Using filesort |
- | 1 | PRIMARY | u | eq_ref | PRIMARY | PRIMARY | 4 | o.uid | 1 | NULL |
- | 1 | PRIMARY | p | ALL | PRIMARY | NULL | NULL | NULL | 6 | Using where; Using join buffer (Block Nested Loop) |
- | 2 | DERIVED | o | index | NULL | idx_1 | 5 | NULL | 909112 | Using where |
- +----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+
8、中间结果集下推 （编辑：鄂州站长网）
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