Fixed string type codegen.

Author: rxin

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

@@ -254,12 +254,12 @@ abstract class BinaryComparison extends BinaryExpression with Predicate {
       case dt: NumericType if ctx.isNativeType(dt) => defineCodeGen (ctx, ev, {
         (c1, c3) => s"$c1 $symbol $c3"
       })
-      case DateType | TimestampType => defineCodeGen (ctx, ev, {
-        (c1, c3) => s"$c1 $symbol $c3"
-      })
-      case other => defineCodeGen (ctx, ev, {
-        (c1, c2) => s"$c1.compare($c2) $symbol 0"
-      })
+      case StringType =>
+        defineCodeGen (ctx, ev, (c1, c2) => s"$c1.compareTo($c2) $symbol 0")
+      case DateType | TimestampType =>
+        defineCodeGen (ctx, ev, (c1, c3) => s"$c1 $symbol $c3")
+      case _ =>
+        defineCodeGen (ctx, ev, (c1, c2) => s"$c1.compare($c2) $symbol 0")
     }
   }