Github user hvanhovell commented on a diff in the pull request:
https://github.com/apache/spark/pull/12720#discussion_r61382939
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala
---
@@ -337,6 +337,16 @@ case class PrettyAttribute(
override def nullable: Boolean = true
}
+/**
+ * A place holder used to hold a reference that has been resolved to a
field outside of the current
+ * plan. This is used for correlated subqueries.
+ */
+case class OuterReference(e: NamedExpression) extends LeafExpression with
Unevaluable {
+ override def dataType: DataType = e.dataType
+ override def nullable: Boolean = e.nullable
+ override def prettyName: String = "outer"
--- End diff --
It will look like `outer(some ref)`, for example:
```
16/04/28 09:15:11 INFO SessionState$$anon$1:
=== Applying Rule
org.apache.spark.sql.catalyst.analysis.Analyzer$ResolveSubquery ===
'Project [*] 'Project [*]
+- 'Filter exists#20 +- 'Filter exists#20
! : +- 'SubqueryAlias exists#20 : +- SubqueryAlias
exists#20
! : +- 'Project ['id] : +- Project
[id#12L]
! : +- 'Filter ('id = 'a.id) : +- Filter
(id#12L = outer(id#0L))
! : +- 'UnresolvedRelation `d`, None : +-
SubqueryAlias d
! +- Join LeftOuter, Some((id#8L = id#4L)) : +-
Range 3, 12, 1, 8, [id#12L]
! :- Join Inner, Some((id#0L = id#4L)) +- Join LeftOuter,
Some((id#8L = id#4L))
! : :- SubqueryAlias a :- Join Inner,
Some((id#0L = id#4L))
! : : +- Range 0, 10, 1, 8, [id#0L] : :-
SubqueryAlias a
! : +- SubqueryAlias b : : +- Range 0,
10, 1, 8, [id#0L]
! : +- Range 5, 15, 1, 8, [id#4L] : +-
SubqueryAlias b
! +- SubqueryAlias c : +- Range 5,
15, 1, 8, [id#4L]
! +- Range 7, 25, 1, 8, [id#8L] +- SubqueryAlias c
! +- Range 7, 25,
1, 8, [id#8L]
```
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