Returning upsert test (#10195)

Author: ssmike

ydb/core/kqp/ut/opt/kqp_returning_ut.cpp

@@ -229,6 +229,17 @@ Y_UNIT_TEST(ReturningSerial) {
         CompareYson(R"([[2;[4];[3]]])", FormatResultSetYson(result.GetResultSet(0)));
     }
 
+    {
+        const auto query = Q_(R"(
+            --!syntax_v1
+            UPSERT INTO ReturningTableExtraValue (value, value2) VALUES (4, 5) RETURNING key;
+        )");
+
+        auto result = session.ExecuteDataQuery(query, TTxControl::BeginTx().CommitTx()).GetValueSync();
+        UNIT_ASSERT(result.IsSuccess());
+        CompareYson(R"([[3]])", FormatResultSetYson(result.GetResultSet(0)));
+    }
+
     {
         const auto query = Q_(R"(
             --!syntax_v1