code: func compareConstant(ctx sessionctx.Context, fn string, c1, c2 Expression) (int64, bool, error) {
  	cmp, err := NewFunction(ctx, fn, types.NewFieldType(mysql.TypeTiny), c1, c2)
  	if err != nil {
  		return 0, false, err
  	}
  	return cmp.EvalInt(ctx, chunk.Row{})
  }
docs: // compareConstant compares two expressions. c1 and c2 should be constant with the same type.