# 执行计划中的算子 表连接类算子（B）

+ `Hash Join`  示例
  + `EXPLAIN SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;`
```
+-----------------------------+-----------+-----------+---------------+------------------------------------------------+
| id                          | estRows   | task      | access object | operator info                                  |
+-----------------------------+-----------+-----------+---------------+------------------------------------------------+
| HashJoin_27                 | 142020.00 | root      |               | inner join, equal:[eq(test.t1.id, test.t2.id)] |
| ├─TableReader_29(Build)     | 142020.00 | root      |               | data:TableFullScan_28                          |
| │ └─TableFullScan_28        | 142020.00 | cop[tikv] | table:t1      | keep order:false                               |
| └─TableReader_31(Probe)     | 180000.00 | root      |               | data:TableFullScan_30                          |
|   └─TableFullScan_30        | 180000.00 | cop[tikv] | table:t2      | keep order:false                               |
+-----------------------------+-----------+-----------+---------------+------------------------------------------------+
5 rows in set (0.00 sec)
```

-------------------------------------------------------------
Hash Join 示例
```
EXPLAIN SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;

 
+-----------------------------+-----------+-----------+---------------+------------------------------------------------+
| id                          | estRows   | task      | access object | operator info                                  |
+-----------------------------+-----------+-----------+---------------+------------------------------------------------+
| HashJoin_27                 | 142020.00 | root      |               | inner join, equal:[eq(test.t1.id, test.t2.id)] |
| ├─TableReader_29(Build)     | 142020.00 | root      |               | data:TableFullScan_28                          |
| │ └─TableFullScan_28        | 142020.00 | cop[tikv] | table:t1      | keep order:false                               |
| └─TableReader_31(Probe)     | 180000.00 | root      |               | data:TableFullScan_30                          |
|   └─TableFullScan_30        | 180000.00 | cop[tikv] | table:t2      | keep order:false                               |
+-----------------------------+-----------+-----------+---------------+------------------------------------------------+
5 rows in set (0.00 sec)
```
Hash Join 算子
Hash Join 算子采用多线程进行优化，执行速度较快，但会消耗较多的内存。在 Hash Join 中国，会将 build 段的数据缓存在内存中，然后根据这些数据构造出一个 hash table，随后读取 probe 段的数据，使用 probe 段的数据来探测 build 段构造出来的 hash table。与其他数据库产品相同，由于对数据进行了 hash 计算，所以 hash join 只能存在等值查询中。