有时候,一个 select 语句中包含多个窗口函数,它们的窗口定义(OVER 子句)可能相同、也可能不同。
对于相同的窗口,完全没必要再做一次分区和排序,我们可以将它们合并成一个 Window 算子。
比如 spark、hive中窗口函数实现原理复盘 中的案例:
select id, sq, cell_type, rank, row_number() over(partition by id order by rank ) naturl_rank, rank() over(partition by id order by rank) as r, dense_rank() over(partition by cell_type order by id) as dr from window_test_table group by id,sq,cell_type,rank;
row_number() rank() 的窗口一样,可以放在一次分区和排序中完成,这一块hive sql与spark sql的表现是一致的。
但对于另外一种情况:
select id, rank, row_number() over(partition by id order by rank ) naturl_rank, sum(rank) over(partition by id) as snum from window_test_table
虽然这 2 个窗口并非完全一致,但是 sum(rank) 不关心分区内的顺序,完全可以复用 row_number() 的窗口。
从下面执行计划可以看出,spark sql sum(rank) 和row_number() 复用了同一个窗口,而hive sql没有。
spark sql的执行计划:
spark-sql> explain select id,rank,row_number() over(partition by id order by rank ) naturl_rank,sum(rank) over(partition by id) as snum from window_test_table; == Physical Plan ==*(3) Project [id#13, rank#16, naturl_rank#8, snum#9L]+- Window [row_number() windowspecdefinition(id#13, rank#16 ASC NULLS FIRST, specifiedwindowframe(RowFrame, unboundedpreceding$(), currentrow$())) AS naturl_rank#8], [id#13], [rank#16 ASC NULLS FIRST] +- *(2) Sort [id#13 ASC NULLS FIRST, rank#16 ASC NULLS FIRST], false, 0 +- Window [sum(cast(rank#16 as bigint)) windowspecdefinition(id#13, specifiedwindowframe(RowFrame, unboundedpreceding$(), unboundedfollowing$())) AS snum#9L], [id#13] +- *(1) Sort [id#13 ASC NULLS FIRST], false, 0 +- Exchange hashpartitioning(id#13, 200) +- Scan hive tmp.window_test_table [id#13, rank#16], HiveTableRelation `tmp`.`window_test_table`, org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe, [id#13, sq#14, cell_type#15, rank#16]Time taken: 0.278 seconds, Fetched 1 row(s)
hive sql执行计划:
hive> explain select id,rank,row_number() over(partition by id order by rank ) naturl_rank,sum(rank) over(partition by id) as snum from window_test_table;
OKSTAGE DEPENDENCIES: Stage-1 is a root stage Stage-2 depends on stages: Stage-1 Stage-0 depends on stages: Stage-2
STAGE PLANS: Stage: Stage-1 Map Reduce Map Operator Tree: TableScan alias: window_test_table Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE Reduce Output Operator key expressions: id (type: int), rank (type: int) sort order: ++ Map-reduce partition columns: id (type: int) Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE Reduce Operator Tree: Select Operator expressions: KEY.reducesinkkey0 (type: int), KEY.reducesinkkey1 (type: int) outputColumnNames: _col0, _col3 Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE PTF Operator Function definitions: Input definition input alias: ptf_0 output shape: _col0: int, _col3: int type: WINDOWING Windowing table definition input alias: ptf_1 name: windowingtablefunction order by: _col3 ASC NULLS FIRST partition by: _col0 raw input shape: window functions: window function definition alias: row_number_window_0 name: row_number window function: GenericUDAFRowNumberEvaluator window frame: PRECEDING(MAX)~FOLLOWING(MAX) isPivotResult: true Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE Select Operator expressions: _col0 (type: int), _col3 (type: int), row_number_window_0 (type: int) outputColumnNames: _col0, _col3, row_number_window_0 Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE File Output Operator compressed: false table: input format: org.apache.hadoop.mapred.SequenceFileInputFormat output format: org.apache.hadoop.hive.ql.io.HiveSequenceFileOutputFormat serde: org.apache.hadoop.hive.serde2.lazybinary.LazyBinarySerDe
Stage: Stage-2 Map Reduce Map Operator Tree: TableScan Reduce Output Operator key expressions: _col0 (type: int) sort order: + Map-reduce partition columns: _col0 (type: int) Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE value expressions: row_number_window_0 (type: int), _col3 (type: int) Reduce Operator Tree: Select Operator expressions: VALUE._col0 (type: int), KEY.reducesinkkey0 (type: int), VALUE._col3 (type: int) outputColumnNames: _col0, _col1, _col4 Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE PTF Operator Function definitions: Input definition input alias: ptf_0 output shape: _col0: int, _col1: int, _col4: int type: WINDOWING Windowing table definition input alias: ptf_1 name: windowingtablefunction order by: _col1 ASC NULLS FIRST partition by: _col1 raw input shape: window functions: window function definition alias: sum_window_1 arguments: _col4 name: sum window function: GenericUDAFSumLong window frame: PRECEDING(MAX)~FOLLOWING(MAX) Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE Select Operator expressions: _col1 (type: int), _col4 (type: int), _col0 (type: int), sum_window_1 (type: bigint) outputColumnNames: _col0, _col1, _col2, _col3 Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE File Output Operator compressed: false Statistics: Num rows: 13 Data size: 104 Basic stats: COMPLETE Column stats: NONE table: input format: org.apache.hadoop.mapred.SequenceFileInputFormat output format: org.apache.hadoop.hive.ql.io.HiveSequenceFileOutputFormat serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe
Stage: Stage-0 Fetch Operator limit: -1 Processor Tree: ListSink
Time taken: 0.244 seconds, Fetched: 106 row(s)