OceanBase 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。
