Fix SortCollapseRule when inner order is DESC. (#4157)

* Fix SortCollapseRule when inner order is DESC.

* Remove unused import.

sql/src/main/java/io/druid/sql/calcite/rule/SortCollapseRule.java

@@ -21,7 +21,6 @@ package io.druid.sql.calcite.rule;
 
 import org.apache.calcite.plan.RelOptRule;
 import org.apache.calcite.plan.RelOptRuleCall;
-import org.apache.calcite.rel.RelNode;
 import org.apache.calcite.rel.core.Sort;
 import org.apache.calcite.rex.RexLiteral;
 
@@ -55,6 +54,7 @@ public class SortCollapseRule extends RelOptRule
       final int firstOffset = (first.offset != null ? RexLiteral.intValue(first.offset) : 0);
       final int secondOffset = (second.offset != null ? RexLiteral.intValue(second.offset) : 0);
 
+      final int offset = firstOffset + secondOffset;
       final int fetch;
 
       if (first.fetch == null && second.fetch == null) {
@@ -76,10 +76,13 @@ public class SortCollapseRule extends RelOptRule
         );
       }
 
-      final RelNode combined = call.builder()
-                                   .push(first.getInput())
-                                   .sortLimit(firstOffset + secondOffset, fetch, first.getChildExps())
-                                   .build();
+      final Sort combined = first.copy(
+          first.getTraitSet(),
+          first.getInput(),
+          first.getCollation(),
+          offset == 0 ? null : call.builder().literal(offset),
+          call.builder().literal(fetch)
+      );
 
       call.transformTo(combined);
     }

sql/src/test/java/io/druid/sql/calcite/CalciteQueryTest.java

@@ -2075,6 +2075,30 @@ public class CalciteQueryTest
     );
   }
 
+  @Test
+  public void testSelectDistinctWithSortAsOuterQuery4() throws Exception
+  {
+    testQuery(
+        "SELECT * FROM (SELECT DISTINCT dim2 FROM druid.foo ORDER BY dim2 DESC LIMIT 5) LIMIT 10",
+        ImmutableList.<Query>of(
+            new TopNQueryBuilder()
+                .dataSource(CalciteTests.DATASOURCE1)
+                .intervals(QSS(Filtration.eternity()))
+                .granularity(Granularities.ALL)
+                .dimension(new DefaultDimensionSpec("dim2", "d0"))
+                .metric(new InvertedTopNMetricSpec(new DimensionTopNMetricSpec(null, StringComparators.LEXICOGRAPHIC)))
+                .threshold(5)
+                .context(QUERY_CONTEXT_DEFAULT)
+                .build()
+        ),
+        ImmutableList.of(
+            new Object[]{""},
+            new Object[]{"abc"},
+            new Object[]{"a"}
+        )
+    );
+  }
+
   @Test
   public void testCountDistinct() throws Exception
   {