# 执行计划中的算子 表连接类算子（E）

+ `Index Hash Join`
  + `mysql> EXPLAIN SELECT /*+ INL_HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;`

```7
+-----------------------------+----------+-----------+------------------------+--------------------------------------------------------------------------------+
| id                          | estRows  | task      | access object          | operator info                                                                  |
+-----------------------------+----------+-----------+------------------------+--------------------------------------------------------------------------------+
| IndexHashJoin_18            | 12487.50 | root      |                        | inner join, inner:IndexReader_10, outer key:test.t1.id, inner key:test.t2.id   |
| ├─IndexReader_31(Build)     | 9990.00  | root      |                        | index:IndexFullScan_30                                                         |
| │ └─IndexFullScan_30        | 9990.00  | cop[tikv] | table:t1, index:id(id) | keep order:false, stats:pseudo                                                 |
| └─IndexReader_10(Probe)     | 1.00     | root      |                        | index:Selection_9                                                              |
|   └─Selection_9             | 1.00     | cop[tikv] |                        | not(isnull(test.t2.id))                                                        |
|     └─IndexRangeScan_8      | 1.00     | cop[tikv] | table:t2, index:id(id) | range: decided by [eq(test.t2.id, test.t1.id)], keep order:false, stats:pseudo |
+-----------------------------+----------+-----------+------------------------+--------------------------------------------------------------------------------+
```
-------------------------------------------------------------

普通的 Hash Join 是在表上完成扫描与连接的，Index Hash Join 是将连接的对象从表换成索引。所以在某些情况下，Index Hash Join 既保留了 Index Join 的优点，也保留的 Hash Join 的优点。