expression: Support to_seconds function push down for tiflash (#34627)

close pingcap/tidb#34625

expression/expr_to_pb_test.go

@@ -1025,6 +1025,11 @@ func TestExprPushDownToFlash(t *testing.T) {
 	require.NoError(t, err)
 	exprs = append(exprs, function)
 
+	// ToSeconds: supported
+	function, err = NewFunction(mock.NewContext(), ast.ToSeconds, types.NewFieldType(mysql.TypeLonglong), datetimeColumn)
+	require.NoError(t, err)
+	exprs = append(exprs, function)
+
 	// regexpUTF8: supported
 	function, err = NewFunction(mock.NewContext(), ast.Regexp, types.NewFieldType(mysql.TypeLonglong), stringColumn, stringColumn)
 	require.NoError(t, err)

expression/expression.go

@@ -1056,7 +1056,7 @@ func scalarExprSupportedByFlash(function *ScalarFunction) bool {
 		ast.Concat, ast.ConcatWS,
 		ast.Date, ast.Year, ast.Month, ast.Day, ast.Quarter, ast.DayName, ast.MonthName,
 		ast.DateDiff, ast.TimestampDiff, ast.DateFormat, ast.FromUnixTime,
-		ast.DayOfWeek, ast.DayOfMonth, ast.DayOfYear, ast.LastDay, ast.WeekOfYear,
+		ast.DayOfWeek, ast.DayOfMonth, ast.DayOfYear, ast.LastDay, ast.WeekOfYear, ast.ToSeconds,
 
 		ast.Sqrt, ast.Log, ast.Log2, ast.Log10, ast.Ln, ast.Exp, ast.Pow, ast.Sign,
 		ast.Radians, ast.Degrees, ast.Conv, ast.CRC32,