[SPARK-17114][SQL] Fix aggregates grouped by literals with empty input

## What changes were proposed in this pull request?
This PR fixes an issue with aggregates that have an empty input, and use a literals as their grouping keys. These aggregates are currently interpreted as aggregates **without** grouping keys, this triggers the ungrouped code path (which aways returns a single row).

This PR fixes the `RemoveLiteralFromGroupExpressions` optimizer rule, which changes the semantics of the Aggregate by eliminating all literal grouping keys.

## How was this patch tested?
Added tests to `SQLQueryTestSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #15101 from hvanhovell/SPARK-17114-3.

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

@@ -1098,9 +1098,16 @@ object ReplaceExceptWithAntiJoin extends Rule[LogicalPlan] {
  */
 object RemoveLiteralFromGroupExpressions extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transform {
-    case a @ Aggregate(grouping, _, _) =>
+    case a @ Aggregate(grouping, _, _) if grouping.nonEmpty =>
       val newGrouping = grouping.filter(!_.foldable)
-      a.copy(groupingExpressions = newGrouping)
+      if (newGrouping.nonEmpty) {
+        a.copy(groupingExpressions = newGrouping)
+      } else {
+        // All grouping expressions are literals. We should not drop them all, because this can
+        // change the return semantics when the input of the Aggregate is empty (SPARK-17114). We
+        // instead replace this by single, easy to hash/sort, literal expression.
+        a.copy(groupingExpressions = Seq(Literal(0, IntegerType)))
+      }
   }
 }
 

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

@@ -28,7 +28,7 @@ import org.apache.spark.sql.catalyst.plans.logical.{LocalRelation, LogicalPlan}
 import org.apache.spark.sql.catalyst.rules.RuleExecutor
 
 class AggregateOptimizeSuite extends PlanTest {
-  val conf = new SimpleCatalystConf(caseSensitiveAnalysis = false)
+  val conf = SimpleCatalystConf(caseSensitiveAnalysis = false, groupByOrdinal = false)
   val catalog = new SessionCatalog(new InMemoryCatalog, EmptyFunctionRegistry, conf)
   val analyzer = new Analyzer(catalog, conf)
 
@@ -49,6 +49,14 @@ class AggregateOptimizeSuite extends PlanTest {
     comparePlans(optimized, correctAnswer)
   }
 
+  test("do not remove all grouping expressions if they are all literals") {
+    val query = testRelation.groupBy(Literal("1"), Literal(1) + Literal(2))(sum('b))
+    val optimized = Optimize.execute(analyzer.execute(query))
+    val correctAnswer = analyzer.execute(testRelation.groupBy(Literal(0))(sum('b)))
+
+    comparePlans(optimized, correctAnswer)
+  }
+
   test("Remove aliased literals") {
     val query = testRelation.select('a, Literal(1).as('y)).groupBy('a, 'y)(sum('b))
     val optimized = Optimize.execute(analyzer.execute(query))

sql/core/src/test/resources/sql-tests/inputs/group-by.sql

+-- Temporary data.
+create temporary view myview as values 128, 256 as v(int_col);
+
+-- group by should produce all input rows,
+select int_col, count(*) from myview group by int_col;
+
+-- group by should produce a single row.
+select 'foo', count(*) from myview group by 1;
+
+-- group-by should not produce any rows (whole stage code generation).
+select 'foo' from myview where int_col == 0 group by 1;
+
+-- group-by should not produce any rows (hash aggregate).
+select 'foo', approx_count_distinct(int_col) from myview where int_col == 0 group by 1;
+
+-- group-by should not produce any rows (sort aggregate).
+select 'foo', max(struct(int_col)) from myview where int_col == 0 group by 1;

sql/core/src/test/resources/sql-tests/results/group-by.sql.out

+-- Automatically generated by SQLQueryTestSuite
+-- Number of queries: 6
+
+
+-- !query 0
+create temporary view myview as values 128, 256 as v(int_col)
+-- !query 0 schema
+struct<>
+-- !query 0 output
+
+
+
+-- !query 1
+select int_col, count(*) from myview group by int_col
+-- !query 1 schema
+struct<int_col:int,count(1):bigint>
+-- !query 1 output
+128	1
+256	1
+
+
+-- !query 2
+select 'foo', count(*) from myview group by 1
+-- !query 2 schema
+struct<foo:string,count(1):bigint>
+-- !query 2 output
+foo	2
+
+
+-- !query 3
+select 'foo' from myview where int_col == 0 group by 1
+-- !query 3 schema
+struct<foo:string>
+-- !query 3 output
+
+
+
+-- !query 4
+select 'foo', approx_count_distinct(int_col) from myview where int_col == 0 group by 1
+-- !query 4 schema
+struct<foo:string,approx_count_distinct(int_col):bigint>
+-- !query 4 output
+
+
+
+-- !query 5
+select 'foo', max(struct(int_col)) from myview where int_col == 0 group by 1
+-- !query 5 schema
+struct<foo:string,max(struct(int_col)):struct<int_col:int>>
+-- !query 5 output
+