YQL-20280: Warn on ASSUME ORDER BY usage after UNION

commit_hash:a9c400c784ed04924390abb11509664e6576a3b7

1 files changed, 10 insertions, 0 deletions

diff --git a/yql/essentials/sql/v1/sql_select.cpp b/yql/essentials/sql/v1/sql_select.cpp
index b2ae4916924..a846dff5483 100644
--- a/yql/essentials/sql/v1/sql_select.cpp
+++ b/yql/essentials/sql/v1/sql_select.cpp
@@ -1392,6 +1392,16 @@ TSourcePtr TSqlSelect::BuildStmt(const TRule& node, TPosition& pos) {
         .Label = extra.Last.Settings.Label,
     };
 
+    if (assumeOrderBy) {
+        YQL_ENSURE(!orderBy.empty());
+
+        Ctx_.Warning(orderBy[0]->OrderExpr->GetPos(), TIssuesIds::WARNING)
+            << "ASSUME ORDER BY is used, "
+            << "but UNION, INTERSECT and EXCEPT "
+            << "operators have no ordering guarantees, "
+            << "therefore consider using ORDER BY";
+    }
+
     if (orderBy) {
         TVector<TNodePtr> groupByExpr;
         TVector<TNodePtr> groupBy;