expression: Switch on the pushdown for first_value() and last_value

expression/aggregation/window_func.go

@@ -131,7 +131,8 @@ func (s *WindowFuncDesc) CanPushDownToTiFlash(ctx sessionctx.Context) bool {
 	}
 	// window functions
 	switch s.Name {
-	case ast.WindowFuncRowNumber, ast.WindowFuncRank, ast.WindowFuncDenseRank, ast.WindowFuncLead, ast.WindowFuncLag:
+	case ast.WindowFuncRowNumber, ast.WindowFuncRank, ast.WindowFuncDenseRank, ast.WindowFuncLead, ast.WindowFuncLag,
+		ast.WindowFuncFirstValue, ast.WindowFuncLastValue:
 		return true
 		// TODO: support aggregate functions
 		//case ast.AggFuncSum, ast.AggFuncCount, ast.AggFuncAvg, ast.AggFuncMax, ast.AggFuncMin:

planner/core/exhaust_physical_plans.go

@@ -2775,6 +2775,9 @@ func (lw *LogicalWindow) tryToGetMppWindows(prop *property.PhysicalProperty) []P
 					"MPP mode may be blocked because window function frame can't be pushed down, because " + err.Error())
 				return nil
 			}
+			lw.SCtx().GetSessionVars().RaiseWarningWhenMPPEnforced(
+				"MPP mode may be blocked because window function frame can't be pushed down, because TiFlash does not support range frame type yet.")
+			return nil
 		}
 	}