Fixed DISTINCT if all columns are constant [#CLICKHOUSE-3505].

dbms/src/DataStreams/DistinctBlockInputStream.cpp

@@ -31,6 +31,9 @@ Block DistinctBlockInputStream::readImpl()
     /// a block with some new records will be gotten.
     while (1)
     {
+        if (no_more_rows)
+            return Block();
+
         /// Stop reading if we already reach the limit.
         if (limit_hint && data.getTotalRowCount() >= limit_hint)
             return Block();
@@ -41,7 +44,13 @@ Block DistinctBlockInputStream::readImpl()
 
         const ColumnRawPtrs column_ptrs(getKeyColumns(block));
         if (column_ptrs.empty())
+        {
+            /// Only constants. We need to return single row.
+            no_more_rows = true;
+            for (auto & elem : block)
+                elem.column = elem.column->cut(0, 1);
             return block;
+        }
 
         if (data.empty())
             data.init(SetVariants::chooseMethod(column_ptrs, key_sizes));
@@ -54,12 +63,12 @@ Block DistinctBlockInputStream::readImpl()
         {
             case SetVariants::Type::EMPTY:
                 break;
-    #define M(NAME) \
+        #define M(NAME) \
             case SetVariants::Type::NAME: \
                 buildFilter(*data.NAME, column_ptrs, filter, rows, data); \
                 break;
-        APPLY_FOR_SET_VARIANTS(M)
-    #undef M
+            APPLY_FOR_SET_VARIANTS(M)
+        #undef M
         }
 
         /// Just go to the next block if there isn't any new record in the current one.
@@ -86,9 +95,8 @@ Block DistinctBlockInputStream::readImpl()
             }
         }
 
-        size_t all_columns = block.columns();
-        for (size_t i = 0; i < all_columns; ++i)
-            block.safeGetByPosition(i).column = block.safeGetByPosition(i).column->filter(filter, -1);
+        for (auto & elem : block)
+            elem.column = elem.column->filter(filter, -1);
 
         return block;
     }

dbms/src/DataStreams/DistinctBlockInputStream.h

@@ -46,6 +46,8 @@ private:
     Sizes key_sizes;
     size_t limit_hint;
 
+    bool no_more_rows = false;
+
     /// Restrictions on the maximum size of the output data.
     size_t max_rows;
     size_t max_bytes;