[SPARK-12077][SQL] change the default plan for single distinct

Use try to match the behavior for single distinct aggregation with Spark 1.5, but that's not scalable, we should be robust by default, have a flag to address performance regression for low cardinality aggregation.

cc yhuai nongli

Author: Davies Liu <davies@databricks.com>

Closes #10075 from davies/agg_15.

2 files changed, 3 insertions, 3 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/SQLConf.scala b/sql/core/src/main/scala/org/apache/spark/sql/SQLConf.scala
index 5ef3a48c56..58adf64e49 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/SQLConf.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/SQLConf.scala
@@ -451,7 +451,7 @@ private[spark] object SQLConf {
 
   val SPECIALIZE_SINGLE_DISTINCT_AGG_PLANNING =
     booleanConf("spark.sql.specializeSingleDistinctAggPlanning",
-      defaultValue = Some(true),
+      defaultValue = Some(false),
       isPublic = false,
       doc = "When true, if a query only has a single distinct column and it has " +
         "grouping expressions, we will use our planner rule to handle this distinct " +
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/PlannerSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/PlannerSuite.scala
index dfec139985..a4626259b2 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/PlannerSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/PlannerSuite.scala
@@ -44,10 +44,10 @@ class PlannerSuite extends SharedSQLContext {
         fail(s"Could query play aggregation query $query. Is it an aggregation query?"))
     val aggregations = planned.collect { case n if n.nodeName contains "Aggregate" => n }
 
-    // For the new aggregation code path, there will be three aggregate operator for
+    // For the new aggregation code path, there will be four aggregate operator for
     // distinct aggregations.
     assert(
-      aggregations.size == 2 || aggregations.size == 3,
+      aggregations.size == 2 || aggregations.size == 4,
       s"The plan of query $query does not have partial aggregations.")
   }