[TOC] # 分布式执行计划调度 分布式执行计划的简单调度模型如下： 在执行计划生成的最后阶段，以 EXCHANGE 节点为界，拆分成多个子计划，每个子计划被封装成为一个 DFO，在并行度大于 1 的场景下，会一次调度两个 DFO，依次完成 DFO 树的遍历执行；在并行度等于 1 的场景下，每个 DFO 会将产生的数据存入中间结果管理器，按照后序遍历的形式完成整个 DFO 树的遍历执行。 ## 单 DFO 调度 示例：在并行度为 1 的场景下，对于查询计划执行单 DFO 调度。 ~~~ ====================================================================================== |ID|OPERATOR |NAME |EST. ROWS |COST | -------------------------------------------------------------------------------------- |0 |LIMIT | |10 |6956829987| |1 | PX COORDINATOR MERGE SORT | |10 |6956829985| |2 | EXCHANGE OUT DISTR |:EX10002 |10 |6956829976| |3 | LIMIT | |10 |6956829976| |4 | TOP-N SORT | |10 |6956829975| |5 | HASH GROUP BY | |454381562 |5815592885| |6 | HASH JOIN | |500918979 |5299414557| |7 | EXCHANGE IN DISTR | |225943610 |2081426759| |8 | EXCHANGE OUT DISTR (PKEY) |:EX10001 |225943610 |1958446695| |9 | MATERIAL | |225943610 |1958446695| |10| HASH JOIN | |225943610 |1480989849| |11| JOIN FILTER CREATE | |30142669 |122441311 | |12| PX PARTITION ITERATOR | |30142669 |122441311 | |13| TABLE SCAN |CUSTOMER |30142669 |122441311 | |14| EXCHANGE IN DISTR | |731011898 |900388059 | |15| EXCHANGE OUT DISTR (PKEY)|:EX10000 |731011898 |614947815 | |16| JOIN FILTER USE | |731011898 |614947815 | |17| PX BLOCK ITERATOR | |731011898 |614947815 | |18| TABLE SCAN |ORDERS |731011898 |614947815 | |19| PX PARTITION ITERATOR | |3243094528|1040696710| |20| TABLE SCAN |LINEITEM(I_L_Q06_001)|3243094528|1040696710| ====================================================================================== ~~~ 如下图所示，DFO 树除 ROOT DFO 外，在垂直方向上被分别划分为 0、1、2 号 DFO, 从而后序遍历调度的顺序为 0->1->2，即可完成整个计划树的迭代。  ## 两 DFO 调度 示例：对于并行度大于 1 的计划, 对于查询计划执行两 DFO 调度。 ~~~ Query Plan ============================================================================= |ID|OPERATOR |NAME |EST. ROWS|COST | ----------------------------------------------------------------------------- |0 |PX COORDINATOR MERGE SORT | |9873917 |692436562| |1 | EXCHANGE OUT DISTR |:EX10002|9873917 |689632565| |2 | SORT | |9873917 |689632565| |3 | SUBPLAN SCAN |VIEW5 |9873917 |636493382| |4 | WINDOW FUNCTION | |29621749 |629924873| |5 | HASH GROUP BY | |29621749 |624266752| |6 | HASH JOIN | |31521003 |591048941| |7 | JOIN FILTER CREATE | |407573 |7476793 | |8 | EXCHANGE IN DISTR | |407573 |7476793 | |9 | EXCHANGE OUT DISTR (BROADCAST) |:EX10001|407573 |7303180 | |10| HASH JOIN | |407573 |7303180 | |11| JOIN FILTER CREATE | |1 |53 | |12| EXCHANGE IN DISTR | |1 |53 | |13| EXCHANGE OUT DISTR (BROADCAST)|:EX10000|1 |53 | |14| PX BLOCK ITERATOR | |1 |53 | |15| TABLE SCAN |NATION |1 |53 | |16| JOIN FILTER USE | |10189312 |3417602 | |17| PX BLOCK ITERATOR | |10189312 |3417602 | |18| TABLE SCAN |SUPPLIER|10189312 |3417602 | |19| JOIN FILTER USE | |803481600|276540086| |20| PX PARTITION ITERATOR | |803481600|276540086| |21| TABLE SCAN |PARTSUPP|803481600|276540086| ============================================================================= ~~~ 如下图所示，DFO 树除 ROOT DFO 外，被划分为 3 个 DFO，调度时会先调 0 和 1 对应的 DFO，待 0 号 DFO 执行完毕后，会再调度 1 号和 2 号 DFO，依次迭代完成执行。