测试数据
- t1 表
bash
select * from t1;
+--------+----------+---------+--------+
| t1.id | t1.name | t1.age | t1.dt |
+--------+----------+---------+--------+
| 1 | aa | 12 | 01 |
| 1 | aa | 12 | 02 |
| 2 | aa | 14 | 01 |
| 2 | bb | 14 | 02 |
| 3 | cc | 16 | 02 |
| NULL | aa | 12 | 01 |
+--------+----------+---------+--------+- t2 表
bash
select * from t2;
+--------+----------+---------+--------+
| t2.id | t2.name | t2.age | t2.dt |
+--------+----------+---------+--------+
| 1 | 1 | aa | 12 |
| 2 | 1 | aa | 12 |
| 1 | NULL | aa | 12 |
| 1 | 2 | aa | 14 |
| 2 | 2 | bb | 14 |
| 2 | 3 | cc | 16 |
+--------+----------+---------+--------+关联查询
- t1 left join t2
bash
select * from t1 a left join t2 b on a.id=b.id;
+-------+---------+--------+-------+-------+---------+--------+-------+
| a.id | a.name | a.age | a.dt | b.id | b.name | b.age | b.dt |
+-------+---------+--------+-------+-------+---------+--------+-------+
| 3 | cc | 16 | 02 | NULL | NULL | NULL | NULL |
| NULL | aa | 12 | 01 | NULL | NULL | NULL | NULL |
| 1 | aa | 12 | 01 | 1 | NULL | aa | 12 |
| 1 | aa | 12 | 01 | 1 | 1 | aa | 12 |
| 1 | aa | 12 | 01 | 1 | 2 | aa | 14 |
| 1 | aa | 12 | 02 | 1 | NULL | aa | 12 |
| 1 | aa | 12 | 02 | 1 | 1 | aa | 12 |
| 1 | aa | 12 | 02 | 1 | 2 | aa | 14 |
| 2 | aa | 14 | 01 | 2 | 3 | cc | 16 |
| 2 | aa | 14 | 01 | 2 | 1 | aa | 12 |
| 2 | aa | 14 | 01 | 2 | 2 | bb | 14 |
| 2 | bb | 14 | 02 | 2 | 3 | cc | 16 |
| 2 | bb | 14 | 02 | 2 | 1 | aa | 12 |
| 2 | bb | 14 | 02 | 2 | 2 | bb | 14 |
+-------+---------+--------+-------+-------+---------+--------+-------+- t1 left join t2 on a.id=b.id where a.dt ='01'，保留表谓词下推，map端提前过滤
bash
select * from t1 a left join t2 b on a.id=b.id where a.dt ='01';
+-------+---------+--------+-------+-------+---------+--------+-------+
| a.id | a.name | a.age | a.dt | b.id | b.name | b.age | b.dt |
+-------+---------+--------+-------+-------+---------+--------+-------+
| NULL | aa | 12 | 01 | NULL | NULL | NULL | NULL |
| 1 | aa | 12 | 01 | 1 | NULL | aa | 12 |
| 1 | aa | 12 | 01 | 1 | 1 | aa | 12 |
| 1 | aa | 12 | 01 | 1 | 2 | aa | 14 |
| 2 | aa | 14 | 01 | 2 | 3 | cc | 16 |
| 2 | aa | 14 | 01 | 2 | 1 | aa | 12 |
| 2 | aa | 14 | 01 | 2 | 2 | bb | 14 |
+-------+---------+--------+-------+-------+---------+--------+-------+- t1 left join t2 on a.id=b.id and a.dt ='01'，保留表非谓词下推，reduce端过滤
bash
select * from t1 a left join t2 b on a.id=b.id and a.dt ='01';
+-------+---------+--------+-------+-------+---------+--------+-------+
| a.id | a.name | a.age | a.dt | b.id | b.name | b.age | b.dt |
+-------+---------+--------+-------+-------+---------+--------+-------+
| 1 | aa | 12 | 02 | NULL | NULL | NULL | NULL |
| 2 | bb | 14 | 02 | NULL | NULL | NULL | NULL |
| 3 | cc | 16 | 02 | NULL | NULL | NULL | NULL |
| NULL | aa | 12 | 01 | NULL | NULL | NULL | NULL |
| 1 | aa | 12 | 01 | 1 | 2 | aa | 14 |
| 1 | aa | 12 | 01 | 1 | NULL | aa | 12 |
| 1 | aa | 12 | 01 | 1 | 1 | aa | 12 |
| 2 | aa | 14 | 01 | 2 | 2 | bb | 14 |
| 2 | aa | 14 | 01 | 2 | 3 | cc | 16 |
| 2 | aa | 14 | 01 | 2 | 1 | aa | 12 |
+-------+---------+--------+-------+-------+---------+--------+-------+说明：保留表应用谓词下推，提前过滤，会把不符合条件的数据提前过滤掉；保留表不应用谓词下推，不提前过滤，只能在join发生时，不符合条件的数据不参与关联计算；
- t1 left join t2 on a.id=b.id and b.dt ='12'，空表谓词下推，map端过滤
bash
select * from t1 a left join t2 b on a.id=b.id and b.dt ='12';
+-------+---------+--------+-------+-------+---------+--------+-------+
| a.id | a.name | a.age | a.dt | b.id | b.name | b.age | b.dt |
+-------+---------+--------+-------+-------+---------+--------+-------+
| 2 | aa | 14 | 01 | 2 | 1 | aa | 12 |
| 2 | bb | 14 | 02 | 2 | 1 | aa | 12 |
| 3 | cc | 16 | 02 | NULL | NULL | NULL | NULL |
| NULL | aa | 12 | 01 | NULL | NULL | NULL | NULL |
| 1 | aa | 12 | 01 | 1 | NULL | aa | 12 |
| 1 | aa | 12 | 01 | 1 | 1 | aa | 12 |
| 1 | aa | 12 | 02 | 1 | NULL | aa | 12 |
| 1 | aa | 12 | 02 | 1 | 1 | aa | 12 |
+-------+---------+--------+-------+-------+---------+--------+-------+- t1 left join t2 on a.id=b.id where b.dt ='12'，空表非谓词下推，reduce端过滤
bash
select * from t1 a left join t2 b on a.id=b.id where b.dt ='12';
+-------+---------+--------+-------+-------+---------+--------+-------+
| a.id | a.name | a.age | a.dt | b.id | b.name | b.age | b.dt |
+-------+---------+--------+-------+-------+---------+--------+-------+
| 2 | aa | 14 | 01 | 2 | 1 | aa | 12 |
| 2 | bb | 14 | 02 | 2 | 1 | aa | 12 |
| 1 | aa | 12 | 01 | 1 | NULL | aa | 12 |
| 1 | aa | 12 | 01 | 1 | 1 | aa | 12 |
| 1 | aa | 12 | 02 | 1 | NULL | aa | 12 |
| 1 | aa | 12 | 02 | 1 | 1 | aa | 12 |
+-------+---------+--------+-------+-------+---------+--------+-------+说明：空表应用谓词下推，提前过滤，会把不符合条件的数据提前过滤掉；空表不应用谓词下推，不提前过滤，只能在join完成时，过滤不符合条件的数据；
总结 : 是否应用谓词下推，最后产生的结果往往不同，这里需要特别注意。