73dd6cf9b5
## What changes were proposed in this pull request?
Currently the set operations INTERSECT, UNION and EXCEPT are assigned the same precedence. This PR fixes the problem by giving INTERSECT higher precedence than UNION and EXCEPT. UNION and EXCEPT operators are evaluated in the order in which they appear in the query from left to right.
This results in change in behavior because of the change in order of evaluations of set operators in a query. The old behavior is still preserved under a newly added config parameter.
Query `:`
```
SELECT * FROM t1
UNION
SELECT * FROM t2
EXCEPT
SELECT * FROM t3
INTERSECT
SELECT * FROM t4
```
Parsed plan before the change `:`
```
== Parsed Logical Plan ==
'Intersect false
:- 'Except false
: :- 'Distinct
: : +- 'Union
: : :- 'Project [*]
: : : +- 'UnresolvedRelation `t1`
: : +- 'Project [*]
: : +- 'UnresolvedRelation `t2`
: +- 'Project [*]
: +- 'UnresolvedRelation `t3`
+- 'Project [*]
+- 'UnresolvedRelation `t4`
```
Parsed plan after the change `:`
```
== Parsed Logical Plan ==
'Except false
:- 'Distinct
: +- 'Union
: :- 'Project [*]
: : +- 'UnresolvedRelation `t1`
: +- 'Project [*]
: +- 'UnresolvedRelation `t2`
+- 'Intersect false
:- 'Project [*]
: +- 'UnresolvedRelation `t3`
+- 'Project [*]
+- 'UnresolvedRelation `t4`
```
## How was this patch tested?
Added tests in PlanParserSuite, SQLQueryTestSuite.
Please review http://spark.apache.org/contributing.html before opening a pull request.
Author: Dilip Biswal <dbiswal@us.ibm.com>
Closes #21941 from dilipbiswal/SPARK-24966.
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