[SPARK-12770][SQL] Implement rules for branch elimination for CaseWhen

The three optimization cases are:

1. If the first branch's condition is a true literal, remove the CaseWhen and use the value from that branch.
2. If a branch's condition is a false or null literal, remove that branch.
3. If only the else branch is left, remove the CaseWhen and use the value from the else branch.

Author: Reynold Xin <rxin@databricks.com>

Closes #10827 from rxin/SPARK-12770.

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

@@ -635,6 +635,24 @@ object SimplifyConditionals extends Rule[LogicalPlan] with PredicateHelper {
     case q: LogicalPlan => q transformExpressionsUp {
       case If(TrueLiteral, trueValue, _) => trueValue
       case If(FalseLiteral, _, falseValue) => falseValue
+
+      case e @ CaseWhen(branches, elseValue) if branches.exists(_._1 == FalseLiteral) =>
+        // If there are branches that are always false, remove them.
+        // If there are no more branches left, just use the else value.
+        // Note that these two are handled together here in a single case statement because
+        // otherwise we cannot determine the data type for the elseValue if it is None (i.e. null).
+        val newBranches = branches.filter(_._1 != FalseLiteral)
+        if (newBranches.isEmpty) {
+          elseValue.getOrElse(Literal.create(null, e.dataType))
+        } else {
+          e.copy(branches = newBranches)
+        }
+
+      case e @ CaseWhen(branches, _) if branches.headOption.map(_._1) == Some(TrueLiteral) =>
+        // If the first branch is a true literal, remove the entire CaseWhen and use the value
+        // from that. Note that CaseWhen.branches should never be empty, and as a result the
+        // headOption (rather than head) added above is just a extra (and unnecessary) safeguard.
+        branches.head._2
     }
   }
 }

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

@@ -23,6 +23,7 @@ import org.apache.spark.sql.catalyst.expressions.Literal.{FalseLiteral, TrueLite
 import org.apache.spark.sql.catalyst.plans.PlanTest
 import org.apache.spark.sql.catalyst.plans.logical._
 import org.apache.spark.sql.catalyst.rules._
+import org.apache.spark.sql.types.IntegerType
 
 
 class SimplifyConditionalSuite extends PlanTest with PredicateHelper {
@@ -37,6 +38,10 @@ class SimplifyConditionalSuite extends PlanTest with PredicateHelper {
     comparePlans(actual, correctAnswer)
   }
 
+  private val trueBranch = (TrueLiteral, Literal(5))
+  private val normalBranch = (NonFoldableLiteral(true), Literal(10))
+  private val unreachableBranch = (FalseLiteral, Literal(20))
+
   test("simplify if") {
     assertEquivalent(
       If(TrueLiteral, Literal(10), Literal(20)),
@@ -47,4 +52,36 @@ class SimplifyConditionalSuite extends PlanTest with PredicateHelper {
       Literal(20))
   }
 
+  test("remove unreachable branches") {
+    // i.e. removing branches whose conditions are always false
+    assertEquivalent(
+      CaseWhen(unreachableBranch :: normalBranch :: unreachableBranch :: Nil, None),
+      CaseWhen(normalBranch :: Nil, None))
+  }
+
+  test("remove entire CaseWhen if only the else branch is reachable") {
+    assertEquivalent(
+      CaseWhen(unreachableBranch :: unreachableBranch :: Nil, Some(Literal(30))),
+      Literal(30))
+
+    assertEquivalent(
+      CaseWhen(unreachableBranch :: unreachableBranch :: Nil, None),
+      Literal.create(null, IntegerType))
+  }
+
+  test("remove entire CaseWhen if the first branch is always true") {
+    assertEquivalent(
+      CaseWhen(trueBranch :: normalBranch :: Nil, None),
+      Literal(5))
+
+    // Test branch elimination and simplification in combination
+    assertEquivalent(
+      CaseWhen(unreachableBranch :: unreachableBranch:: trueBranch :: normalBranch :: Nil, None),
+      Literal(5))
+
+    // Make sure this doesn't trigger if there is a non-foldable branch before the true branch
+    assertEquivalent(
+      CaseWhen(normalBranch :: trueBranch :: normalBranch :: Nil, None),
+      CaseWhen(normalBranch :: trueBranch :: normalBranch :: Nil, None))
+  }
 }