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在排查所有查詢語句效率的過程中 , 發(fā)現(xiàn)了join關聯(lián)表的時候 , 被驅動表沒有走索引而是進行的全表掃描 實際的sql語句如下: explain select a.* from audit_rules a left join audit_rules_detail b on a.id=b.rule_id where a.ent_id=23684 輸出: +----+-------------+-------+------+---------------+------------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------------+---------+-------+------+-------+ | 1 | SIMPLE | a | ref | idx_ent_id | idx_ent_id | 4 | const | 12 | | | 1 | SIMPLE | b | ALL | NULL | NULL | NULL | NULL | 35 | | +----+-------------+-------+------+---------------+------------+---------+-------+------+-------+ 看到表b是全表掃描 , 這是因為b的字段rule_id沒有索引 增加上索引以后 +----+-------------+-------+------+---------------+-------------+---------+--------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+-------------+---------+--------------+------+-------------+ | 1 | SIMPLE | a | ref | idx_ent_id | idx_ent_id | 4 | const | 12 | | | 1 | SIMPLE | b | ref | idx_rule_id | idx_rule_id | 4 | sinanet.a.id | 1 | Using index | MySQL是只支持一種JOIN算法Nested-Loop Join（嵌套循環(huán)鏈接） 沒有索引時:
有索引時
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