# 执行计划中的算子 汇聚数据类算子（C） + Stream Aggregate 示例 + `EXPLAIN SELECT /*+ STREAM_AGG() */ col1, count(*) FROM t2 GROUP BY col1;` ``` +------------------------------------+------------+------------+-------------------+-----------------------------------------------------------------------------------------------+ | id | estRows | task | access object | operator info | +------------------------------------+------------+------------+-------------------+-----------------------------------------------------------------------------------------------+ | Projection_4 | 8000.00 | root | | test.t2.col1, Column#3 | | └─StreamAgg_8 | 8000.00 | root | | group by:test.t2.col1, funcs:count(1)->Column#3, funcs:firstrow(test.t2.col1)->test.t2.col1 | | └─Sort_13 | 10000.00 | root | | test.t2.col1 | | └─TableReader_12 | 10000.00 | root | | data:TableFullScan_11 | | └─TableFullScan_11 | 10000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo | +------------------------------------+------------+------------+------------ ------+-----------------------------------------------------------------------------------------------+ 5 rows in set (0.00 sec) ``` ------------------------------------------------------------- Stream Aggregate 示例 Stream Aggregate 算子 Stream Aggregation 算子通常会比 Hash Aggregate 占用更少的内存，有些场景中也会比 Hash Aggregate 执行得更快。当数据量太大或者系统内存不足时，可以试试 Stream Aggregate 算子。通过 STREAM_AGG hint 我们可以强制执行计划走 stream aggregate 算子。