Hi, I have tested the correlated and uncorrelated subqueries in Hive CDH 5.13 and you are right.
CORRELATED:
First the data and test in Impala:
[10.197.0.0:21000] > create table work.tbla ( id int, s string );
[10.197.0.0:21000] > create table work.tblb ( id int, s string );
[10.197.0.0:21000] > insert into work.tbla select 100, '2008-01-01' union select 200, '2009-12-01';
Modified 2 row(s) in 3.69s
[10.197.0.0:21000] > insert into work.tblb select 100, '2008-01-01' union select 200, '2009-12-01';
Modified 2 row(s) in 3.63s
[10.197.0.0:21000] > select * from work.tbla a where exists ( select null from work.tblb b where a.id = b.id and b.s > '2009-10-01' );
+-----+------------+
| id | s |
+-----+------------+
| 200 | 2009-12-01 |
+-----+------------+
Now in Hive:
0: jdbc:hive2://xxx.com> explain select * from work.tbla a where exists ( select null from work.tblb b where a.id = b.id and b.s > '2009-10-01' );
..
INFO : OK
+----------------------------------------------------+--+
| Explain |
+----------------------------------------------------+--+
| STAGE DEPENDENCIES: |
| Stage-4 is a root stage |
| Stage-3 depends on stages: Stage-4 |
| Stage-0 depends on stages: Stage-3 |
| |
| STAGE PLANS: |
| Stage: Stage-4 |
| Map Reduce Local Work |
| Alias -> Map Local Tables: |
| sq_1:b |
| Fetch Operator |
| limit: -1 |
| Alias -> Map Local Operator Tree: |
| sq_1:b |
| TableScan |
| alias: b |
| filterExpr: ((s > '2009-10-01') and id is not null) (type: boolean) |
| Statistics: Num rows: 1 Data size: 30 Basic stats: COMPLETE Column stats: NONE |
| Filter Operator |
| predicate: ((s > '2009-10-01') and id is not null) (type: boolean) |
| Statistics: Num rows: 1 Data size: 30 Basic stats: COMPLETE Column stats: NONE |
| Select Operator |
| expressions: id (type: int) |
| outputColumnNames: _col1 |
| Statistics: Num rows: 1 Data size: 30 Basic stats: COMPLETE Column stats: NONE |
| Group By Operator |
| keys: _col1 (type: int) |
| mode: hash |
| outputColumnNames: _col0 |
| Statistics: Num rows: 1 Data size: 30 Basic stats: COMPLETE Column stats: NONE |
| HashTable Sink Operator |
| keys: |
| 0 id (type: int) |
| 1 _col0 (type: int) |
| |
| Stage: Stage-3 |
| Map Reduce |
| Map Operator Tree: |
| TableScan |
| alias: a |
| filterExpr: id is not null (type: boolean) |
| Statistics: Num rows: 1 Data size: 30 Basic stats: COMPLETE Column stats: NONE |
| Filter Operator |
| predicate: id is not null (type: boolean) |
| Statistics: Num rows: 1 Data size: 30 Basic stats: COMPLETE Column stats: NONE |
| Map Join Operator |
| condition map: |
| Left Semi Join 0 to 1 |
| keys: |
| 0 id (type: int) |
| 1 _col0 (type: int) |
| outputColumnNames: _col0, _col1 |
| Statistics: Num rows: 1 Data size: 33 Basic stats: COMPLETE Column stats: NONE |
| File Output Operator |
| compressed: false |
| Statistics: Num rows: 1 Data size: 33 Basic stats: COMPLETE Column stats: NONE |
| table: |
| input format: org.apache.hadoop.mapred.TextInputFormat |
| output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat |
| serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe |
| Local Work: |
| Map Reduce Local Work |
| |
| Stage: Stage-0 |
| Fetch Operator |
| limit: -1 |
| Processor Tree: |
| ListSink |
| |
+----------------------------------------------------+--+
69 rows selected (0.311 seconds)
UNCORRELATED - Hive fails:
0: jdbc:hive2://xxxx.com> explain select * from work.tbla a where exists ( select null from work.tblb b where b.s > '2009-10-01' );
Error: Error while compiling statement: FAILED: SemanticException Line 1:65 Invalid SubQuery expression ''2009-10-01'' in definition of SubQuery sq_1 [
exists ( select null from work.tblb b where b.s > '2009-10-01' )
] used as sq_1 at Line 1:39: For Exists/Not Exists operator SubQuery must be Correlated. (state=42000,code=40000)
Impala ok
[10.197.0.0:21000] > select * from work.tbla a where exists ( select null from work.tblb b where b.s > '2009-10-01' );
+-----+------------+
| id | s |
+-----+------------+
| 100 | 2008-01-01 |
| 200 | 2009-12-01 |
+-----+------------+
Fetched 2 row(s) in 0.19s
