[SPARK-18300][SQL] Do not apply foldable propagation with expand as a child.

## What changes were proposed in this pull request?
The `FoldablePropagation` optimizer rule, pulls foldable values out from under an `Expand`. This breaks the `Expand` in two ways:

- It rewrites the output attributes of the `Expand`. We explicitly define output attributes for `Expand`, these are (unfortunately) considered as part of the expressions of the `Expand` and can be rewritten.
- Expand can actually change the column (it will typically re-use the attributes or the underlying plan). This means that we cannot safely propagate the expressions from under an `Expand`.

This PR fixes this and (hopefully) other issues by explicitly whitelisting allowed operators.

## How was this patch tested?
Added tests to `FoldablePropagationSuite` and to `SQLQueryTestSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #15857 from hvanhovell/SPARK-18300.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/expressions.scala

@@ -428,43 +428,49 @@ object FoldablePropagation extends Rule[LogicalPlan] {
       }
       case _ => Nil
     })
+    val replaceFoldable: PartialFunction[Expression, Expression] = {
+      case a: AttributeReference if foldableMap.contains(a) => foldableMap(a)
+    }
 
     if (foldableMap.isEmpty) {
       plan
     } else {
       var stop = false
       CleanupAliases(plan.transformUp {
-        case u: Union =>
-          stop = true
-          u
-        case c: Command =>
-          stop = true
-          c
-        // For outer join, although its output attributes are derived from its children, they are
-        // actually different attributes: the output of outer join is not always picked from its
-        // children, but can also be null.
+        // A leaf node should not stop the folding process (note that we are traversing up the
+        // tree, starting at the leaf nodes); so we are allowing it.
+        case l: LeafNode =>
+          l
+
+        // Whitelist of all nodes we are allowed to apply this rule to.
+        case p @ (_: Project | _: Filter | _: SubqueryAlias | _: Aggregate | _: Window |
+                  _: Sample | _: GlobalLimit | _: LocalLimit | _: Generate | _: Distinct |
+                  _: AppendColumns | _: AppendColumnsWithObject | _: BroadcastHint |
+                  _: RedistributeData | _: Repartition | _: Sort | _: TypedFilter) if !stop =>
+          p.transformExpressions(replaceFoldable)
+
+        // Allow inner joins. We do not allow outer join, although its output attributes are
+        // derived from its children, they are actually different attributes: the output of outer
+        // join is not always picked from its children, but can also be null.
         // TODO(cloud-fan): It seems more reasonable to use new attributes as the output attributes
         // of outer join.
-        case j @ Join(_, _, LeftOuter | RightOuter | FullOuter, _) =>
-          stop = true
-          j
+        case j @ Join(_, _, Inner, _) =>
+          j.transformExpressions(replaceFoldable)
 
-        // These 3 operators take attributes as constructor parameters, and these attributes
-        // can't be replaced by alias.
-        case m: MapGroups =>
+        // We can fold the projections an expand holds. However expand changes the output columns
+        // and often reuses the underlying attributes; so we cannot assume that a column is still
+        // foldable after the expand has been applied.
+        // TODO(hvanhovell): Expand should use new attributes as the output attributes.
+        case expand: Expand if !stop =>
+          val newExpand = expand.copy(projections = expand.projections.map { projection =>
+            projection.map(_.transform(replaceFoldable))
+          })
           stop = true
-          m
-        case f: FlatMapGroupsInR =>
-          stop = true
-          f
-        case c: CoGroup =>
-          stop = true
-          c
+          newExpand
 
-        case p: LogicalPlan if !stop => p.transformExpressions {
-          case a: AttributeReference if foldableMap.contains(a) =>
-            foldableMap(a)
-        }
+        case other =>
+          stop = true
+          other
       })
     }
   }

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/FoldablePropagationSuite.scala

@@ -116,16 +116,35 @@ class FoldablePropagationSuite extends PlanTest {
   test("Propagate in subqueries of Union queries") {
     val query = Union(
       Seq(
-        testRelation.select(Literal(1).as('x), 'a).select('x + 'a),
-        testRelation.select(Literal(2).as('x), 'a).select('x + 'a)))
+        testRelation.select(Literal(1).as('x), 'a).select('x, 'x + 'a),
+        testRelation.select(Literal(2).as('x), 'a).select('x, 'x + 'a)))
       .select('x)
     val optimized = Optimize.execute(query.analyze)
     val correctAnswer = Union(
       Seq(
-        testRelation.select(Literal(1).as('x), 'a).select((Literal(1).as('x) + 'a).as("(x + a)")),
-        testRelation.select(Literal(2).as('x), 'a).select((Literal(2).as('x) + 'a).as("(x + a)"))))
+        testRelation.select(Literal(1).as('x), 'a)
+          .select(Literal(1).as('x), (Literal(1).as('x) + 'a).as("(x + a)")),
+        testRelation.select(Literal(2).as('x), 'a)
+          .select(Literal(2).as('x), (Literal(2).as('x) + 'a).as("(x + a)"))))
       .select('x).analyze
+    comparePlans(optimized, correctAnswer)
+  }
 
+  test("Propagate in expand") {
+    val c1 = Literal(1).as('a)
+    val c2 = Literal(2).as('b)
+    val a1 = c1.toAttribute.withNullability(true)
+    val a2 = c2.toAttribute.withNullability(true)
+    val expand = Expand(
+      Seq(Seq(Literal(null), 'b), Seq('a, Literal(null))),
+      Seq(a1, a2),
+      OneRowRelation.select(c1, c2))
+    val query = expand.where(a1.isNotNull).select(a1, a2).analyze
+    val optimized = Optimize.execute(query)
+    val correctExpand = expand.copy(projections = Seq(
+      Seq(Literal(null), c2),
+      Seq(c1, Literal(null))))
+    val correctAnswer = correctExpand.where(a1.isNotNull).select(a1, a2).analyze
     comparePlans(optimized, correctAnswer)
   }
 }

sql/core/src/test/resources/sql-tests/inputs/group-by.sql

@@ -32,3 +32,6 @@ SELECT a + 1 + 1, COUNT(b) FROM testData GROUP BY a + 1;
 -- Aggregate with nulls.
 SELECT SKEWNESS(a), KURTOSIS(a), MIN(a), MAX(a), AVG(a), VARIANCE(a), STDDEV(a), SUM(a), COUNT(a)
 FROM testData;
+
+-- Aggregate with foldable input and multiple distinct groups.
+SELECT COUNT(DISTINCT b), COUNT(DISTINCT b, c) FROM (SELECT 1 AS a, 2 AS b, 3 AS c) GROUP BY a;

sql/core/src/test/resources/sql-tests/results/group-by.sql.out

@@ -1,5 +1,5 @@
 -- Automatically generated by SQLQueryTestSuite
--- Number of queries: 14
+-- Number of queries: 15
 
 
 -- !query 0
@@ -131,3 +131,11 @@ FROM testData
 struct<skewness(CAST(a AS DOUBLE)):double,kurtosis(CAST(a AS DOUBLE)):double,min(a):int,max(a):int,avg(a):double,var_samp(CAST(a AS DOUBLE)):double,stddev_samp(CAST(a AS DOUBLE)):double,sum(a):bigint,count(a):bigint>
 -- !query 13 output
 -0.2723801058145729	-1.5069204152249134	1	3	2.142857142857143	0.8095238095238094	0.8997354108424372	15	7
+
+
+-- !query 14
+SELECT COUNT(DISTINCT b), COUNT(DISTINCT b, c) FROM (SELECT 1 AS a, 2 AS b, 3 AS c) GROUP BY a
+-- !query 14 schema
+struct<count(DISTINCT b):bigint,count(DISTINCT b, c):bigint>
+-- !query 14 output
+1	1