[GitHub] [calcite] jinxing64 opened a new pull request #1495: [CALCITE-3399] RelFieldTrimmer trim fields for UNION, INTERSECT, INTE…

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jinxing64 opened a new pull request #1495: [CALCITE-3399] RelFieldTrimmer trim fields for UNION, INTERSECT, INTE…
URL: https://github.com/apache/calcite/pull/1495
 
 
   …RSECT ALL, EXCEPT, EXCEPT ALL (Jin Xing)
   
   `RelFieldTrimmer#trimFields` provides functionality to trim fields for  `UNION, UNION ALL, INTERSECT, INTERSECT ALL, EXCEPT, EXCEPT ALL`;
   
   But `UNION ALL, INTERSECT, INTERSECT ALL, EXCEPT, EXCEPT ALL` works by checking duplication. Column pruning on inputs of `SetOp` might lead to different semantics.
   
   Take below example for illustration
   ```
   emp0:
   name, deptno
    "A",       0
    "B",       1
   
   emp1:
   name, deptno
    "C",      0
    "D",      2
   
   select deptno from
   (select name, deptno from emp0
   intersect
   select name, deptno from emp1)
   ```
   
   Run above Sql on `emp0` and `emp1`, result is not empty if trim fields on inputs of `INTERSECT`, but result is empty if DO NOT trim fields on inputs of `INTERSECT`
   
   This PR proposes to trim fields only for `UNION ALL`

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