expression, planner: exclude concat_ws from aggresive folding constant

expression/constant_fold.go

@@ -178,7 +178,12 @@ func foldConstant(expr Expression) (Expression, bool) {
 			}
 		}
 		if !allConstArg {
-			if !hasNullArg || !sc.InNullRejectCheck || x.FuncName.L == ast.NullEQ {
+			// try to optimize on the situation when not all arguments are const
+			// for most functions, if one of the arguments are NULL, the result can be a constant (NULL or something else)
+			//
+			// NullEQ and ConcatWS are excluded, because they could have different value when the non-constant value is
+			// 1 or NULL. For example, concat_ws(NULL, NULL) gives NULL, but concat_ws(1, NULL) gives ''
+			if !hasNullArg || !sc.InNullRejectCheck || x.FuncName.L == ast.NullEQ || x.FuncName.L == ast.ConcatWS {
 				return expr, isDeferredConst
 			}
 			constArgs := make([]Expression, len(args))

expression/constant_test.go

@@ -217,6 +217,15 @@ func TestConstantFolding(t *testing.T) {
 			condition: newFunction(ast.LT, newColumn(0), newFunction(ast.Plus, newColumn(1), newFunction(ast.Plus, newLonglong(2), newLonglong(1)))),
 			result:    "lt(Column#0, plus(Column#1, 3))",
 		},
+		{
+			condition: func() Expression {
+				expr := newFunction(ast.ConcatWS, newColumn(0), NewNull())
+				function := expr.(*ScalarFunction)
+				function.GetCtx().GetSessionVars().StmtCtx.InNullRejectCheck = true
+				return function
+			}(),
+			result: "concat_ws(cast(Column#0, var_string(20)), <nil>)",
+		},
 	}
 	for _, tt := range tests {
 		newConds := FoldConstant(tt.condition)

planner/core/integration_test.go

@@ -8309,6 +8309,19 @@ func TestAutoIncrementCheckWithCheckConstraint(t *testing.T) {
 	)`)
 }
 
+// https://github.com/pingcap/tidb/issues/36888.
+func TestIssue36888(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("CREATE TABLE t0(c0 INT);")
+	tk.MustExec("CREATE TABLE t1(c0 INT);")
+
+	tk.MustExec("INSERT INTO t0 VALUES (NULL);")
+	tk.MustQuery("SELECT t0.c0 FROM t0 LEFT JOIN t1 ON t0.c0>=t1.c0 WHERE (CONCAT_WS(t0.c0, t1.c0) IS NULL);").Check(testkit.Rows("<nil>"))
+}
+
+// https://github.com/pingcap/tidb/issues/40285.
 func TestIssue40285(t *testing.T) {
 	store := testkit.CreateMockStore(t)
 	tk := testkit.NewTestKit(t, store)