# 执行计划中的算子 - 汇聚数据类算子（B）

+ Hash Aggregate 示例
  + `mysql> EXPLAIN SELECT /*+ HASH_AGG() */ count(*) FROM t;`
```
+---------------------------+----------+-----------+---------------+---------------------------------+
| id                        | estRows  | task      | access object | operator info                   |
+---------------------------+----------+-----------+---------------+---------------------------------+
| HashAgg_9                 | 1.00     | root      |               | funcs:count(Column#4)->Column#3 |
| └─TableReader_10          | 1.00     | root      |               | data:HashAgg_5                  |
|   └─HashAgg_5             | 1.00     | cop[tikv] |               | funcs:count(1)->Column#4        |
|     └─TableFullScan_8     | 10055.00 | cop[tikv] | table:t       | keep order:false                |
+---------------------------+----------+-----------+---------------+---------------------------------+
4 rows in set (0.01 sec)
```
-------------------------------------------------------------
Hash Aggregate 示例
Hash Aggregate 算子
Hash Aggregation 算子采用多线程并发优化，执行速度快，但会消耗较多内存。通过 HASH_AGG hint 我们可以强制执行计划走 hash aggregate 算子。
```
mysql> EXPLAIN SELECT /*+ HASH_AGG() */ count(*) FROM t;
+---------------------------+----------+-----------+---------------+---------------------------------+
| id                        | estRows  | task      | access object | operator info                   |
+---------------------------+----------+-----------+---------------+---------------------------------+
| HashAgg_9                 | 1.00     | root      |               | funcs:count(Column#4)->Column#3 |
| └─TableReader_10          | 1.00     | root      |               | data:HashAgg_5                  |
|   └─HashAgg_5             | 1.00     | cop[tikv] |               | funcs:count(1)->Column#4        |
|     └─TableFullScan_8     | 10055.00 | cop[tikv] | table:t       | keep order:false                |
+---------------------------+----------+-----------+---------------+---------------------------------+
4 rows in set (0.01 sec)
```