[SPARK-12771][SQL] Simplify CaseWhen code generation

The generated code for CaseWhen uses a control variable "got" to make sure we do not evaluate more branches once a branch is true. Changing that to generate just simple "if / else" would be slightly more efficient.

This closes #10737.

Author: Reynold Xin <rxin@databricks.com>

Closes #10755 from rxin/SPARK-12771.

1 files changed, 35 insertions, 25 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/conditionalExpressions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/conditionalExpressions.scala
index 8cc7bc1da2..83abbcdc61 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/conditionalExpressions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/conditionalExpressions.scala
@@ -137,45 +137,55 @@ case class CaseWhen(branches: Seq[(Expression, Expression)], elseValue: Option[E
   }
 
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): String = {
-    val got = ctx.freshName("got")
-
-    val cases = branches.map { case (condition, value) =>
-      val cond = condition.gen(ctx)
-      val res = value.gen(ctx)
+    // Generate code that looks like:
+    //
+    // condA = ...
+    // if (condA) {
+    //   valueA
+    // } else {
+    //   condB = ...
+    //   if (condB) {
+    //     valueB
+    //   } else {
+    //     condC = ...
+    //     if (condC) {
+    //       valueC
+    //     } else {
+    //       elseValue
+    //     }
+    //   }
+    // }
+    val cases = branches.map { case (condExpr, valueExpr) =>
+      val cond = condExpr.gen(ctx)
+      val res = valueExpr.gen(ctx)
       s"""
-        if (!$got) {
-          ${cond.code}
-          if (!${cond.isNull} && ${cond.value}) {
-            $got = true;
-            ${res.code}
-            ${ev.isNull} = ${res.isNull};
-            ${ev.value} = ${res.value};
-          }
+        ${cond.code}
+        if (!${cond.isNull} && ${cond.value}) {
+          ${res.code}
+          ${ev.isNull} = ${res.isNull};
+          ${ev.value} = ${res.value};
         }
       """
-    }.mkString("\n")
+    }
 
-    val elseCase = {
-      if (elseValue.isDefined) {
-        val res = elseValue.get.gen(ctx)
+    var generatedCode = cases.mkString("", "\nelse {\n", "\nelse {\n")
+
+    elseValue.foreach { elseExpr =>
+      val res = elseExpr.gen(ctx)
+      generatedCode +=
         s"""
-        if (!$got) {
           ${res.code}
           ${ev.isNull} = ${res.isNull};
           ${ev.value} = ${res.value};
-        }
         """
-      } else {
-        ""
-      }
     }
 
+    generatedCode += "}\n" * cases.size
+
     s"""
-      boolean $got = false;
       boolean ${ev.isNull} = true;
       ${ctx.javaType(dataType)} ${ev.value} = ${ctx.defaultValue(dataType)};
-      $cases
-      $elseCase
+      $generatedCode
     """
   }