Enable spark.sql.useAggregate2 by default for testing purpose.

Author: yhuai

sql/core/src/main/scala/org/apache/spark/sql/SQLConf.scala

@@ -403,7 +403,7 @@ private[spark] object SQLConf {
     isPublic = false)
 
   val USE_SQL_AGGREGATE2 = booleanConf("spark.sql.useAggregate2",
-    defaultValue = Some(false), doc = "<TODO>")
+    defaultValue = Some(true), doc = "<TODO>")
 
   val USE_SQL_SERIALIZER2 = booleanConf(
     "spark.sql.useSerializer2",