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From hvanhovell <...@git.apache.org>
Subject [GitHub] spark pull request #16046: [SPARK-18582][SQL] Whitelist LogicalPlan operator...
Date Thu, 01 Dec 2016 19:39:16 GMT
Github user hvanhovell commented on a diff in the pull request:

    https://github.com/apache/spark/pull/16046#discussion_r90521074
  
    --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
---
    @@ -1120,47 +1173,54 @@ class Analyzer(
               } else {
                 a
               }
    -        case w : Window =>
    -          failOnOuterReference(w)
    -          failOnNonEqualCorrelatedPredicate(foundNonEqualCorrelatedPred, w)
    -          w
    -        case j @ Join(left, _, RightOuter, _) =>
    -          failOnOuterReference(j)
    -          failOnOuterReferenceInSubTree(left, "a RIGHT OUTER JOIN")
    -          j
    -        // SPARK-18578: Do not allow any correlated predicate
    -        // in a Full (Outer) Join operator and its descendants
    -        case j @ Join(_, _, FullOuter, _) =>
    -          failOnOuterReferenceInSubTree(j, "a FULL OUTER JOIN")
    -          j
    -        case j @ Join(_, right, jt, _) if !jt.isInstanceOf[InnerLike] =>
    -          failOnOuterReference(j)
    -          failOnOuterReferenceInSubTree(right, "a LEFT (OUTER) JOIN")
    +
    +        // Join can host correlated expressions.
    +        case j @ Join(left, right, joinType, _) =>
    +          joinType match {
    +            // Inner join, like Filter, can be anywhere.
    +            // LeftSemi is a special case of Inner join which returns
    +            // only the first matched row to the right table.
    +            case _: InnerLike | LeftSemi =>
    --- End diff --
    
    It makes sense to extract predicates from the right hand side of a `LeftSemi` join. My
problem with this is far more practical. `Join` with a `LeftSemi` join type does not output
any right hand side attributes (see: https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala#L273),
the plan **breaks** are as soon as you extract a correlated predicate from the right hand
side of the join and try to rewrite the tree using that predicate. That is all.
    
    It would - fortunately - be quite simple to support this. Just rewrite every LeftSemi
join with underlying predicates into an Inner join. I am not entirely sure if we should support
this, technically the query is incorrect. Lets defer this to a follow-up PR.
    
    The example you give is different. I do think we should support that. Please note that
the example you give will not have any left semi joins during analysis, the left semi joins
are introduced during optimization; this makes it relatively straight forward to detect such
a nested case.


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