MySQL5.6中，使用union all相当于创建一张临时表，这在执行大的联合查询时候会增加I/O开销，降低查询速度。
例如执行以下SQL语句：
(select id from accessLog order by id) union all (select id from access_test order by id);
在MySQL5.6环境：

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mysql> select version();

| version() |

| 5.6.14-log |

1 row in set (0.00 sec)

mysql> explain (select id from accessLog order by id) union all (select id from access_test order by id);

id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

| 1 | PRIMARY | accessLog | index | NULL | loginuserId | 9 | NULL | 535513 | Using index |

| 2 | UNION | access_test | index | NULL | idx_loginuid | 9 | NULL | 477248 | Using index |

| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |

可以看到执行计划中提现到了创建的临时表。
在MySQL5.7环境：
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mysql> select version();

| version() |

| 5.7.18-log |

1 row in set (0.00 sec)

mysql> explain (select id from accessLog order by id) union all (select id from access_test order by id);

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

| 1 | PRIMARY | accessLog | NULL | index | NULL | loginuserId | 9 | NULL | 586090 | 100.00 | Using index |

| 2 | UNION | access_test | NULL | ALL | NULL | NULL | NULL | NULL | 571023 | 100.00 | NULL |

整个查询过程没有创建临时表，按照顺序，accessLog表的查询结果首先传输到客户端，然后access_test表的查询结果再传输到客户端。
注意：此项优化对union和在最外层用order by无效，如下：
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mysql> select version();

| version() |

| 5.7.18-log |

1 row in set (0.00 sec)

mysql> explain (select id from accessLog order by id) union all (select id from access_test order by id) order by id;

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

| 1 | PRIMARY | accessLog | NULL | index | NULL | loginuserId | 9 | NULL | 586090 | 100.00 | Using index |

| 2 | UNION | access_test | NULL | ALL | NULL | NULL | NULL | NULL | 571023 | 100.00 | NULL |

| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary; Using filesort |