a setting

src/Core/Settings.h

@@ -405,6 +405,7 @@ class IColumn;
     M(Bool, optimize_skip_merged_partitions, false, "Skip partitions with one part with level > 0 in optimize final", 0) \
     M(Bool, optimize_on_insert, true, "Do the same transformation for inserted block of data as if merge was done on this block.", 0) \
     M(Bool, allow_experimental_map_type, false, "Allow data type Map", 0) \
+    M(Bool, allow_experimental_window_functions, false, "Allow experimental window functions", 0) \
     \
     M(Bool, use_antlr_parser, false, "Parse incoming queries using ANTLR-generated parser", 0) \
     \

src/Interpreters/ExpressionAnalyzer.cpp

@@ -472,6 +472,15 @@ bool ExpressionAnalyzer::makeAggregateDescriptions(ActionsDAGPtr & actions)
 
 bool ExpressionAnalyzer::makeWindowDescriptions(ActionsDAGPtr & actions)
 {
+    // Convenient to check here because at least we have the Context.
+    if (windowFunctions().size() > 0 &&
+        !context.getSettingsRef().allow_experimental_window_functions)
+    {
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "Window functions are not implemented (while processing '{}')",
+            windowFunctions()[0]->formatForErrorMessage());
+    }
+
     for (const ASTFunction * function_node : windowFunctions())
     {
         assert(function_node->is_window_function);

tests/queries/0_stateless/01591_window_functions.sql

@@ -1,5 +1,7 @@
 -- { echo }
 
+set allow_experimental_window_functions = 1;
+
 -- just something basic
 select number, count() over (partition by intDiv(number, 3) order by number) from numbers(10);