text: #include <Functions/IFunction.h>
  #include <Functions/FunctionFactory.h>
  #include <DataTypes/DataTypesNumber.h>
  
  
  namespace DB
  {
  
  
  /** The `indexHint` function takes any number of any arguments and always returns one.
    *
    * This function has a special meaning (see ExpressionAnalyzer, KeyCondition)
    * - the expressions inside it are not evaluated;
    * - but when analyzing the index (selecting ranges for reading), this function is treated the same way,
    *   as if instead of using it the expression itself would be.
    *
    * Example: WHERE something AND indexHint(CounterID = 34)
    * - do not read or calculate CounterID = 34, but select ranges in which the CounterID = 34 expression can be true.
    *
    * The function can be used for debugging purposes, as well as for (hidden from the user) query conversions.
    */
  class FunctionIndexHint : public IFunction
  {
  public:
      static constexpr auto name = "indexHint";
      static FunctionPtr create(ContextPtr)
      {
          return std::make_shared<FunctionIndexHint>();
      }
  
      bool isVariadic() const override
      {
          return true;
      }
      size_t getNumberOfArguments() const override
      {
          return 0;
      }
  
      bool useDefaultImplementationForNulls() const override { return false; }
  
      bool isSuitableForConstantFolding() const override { return false; }
  
      String getName() const override
      {
          return name;
      }
      DataTypePtr getReturnTypeImpl(const DataTypes & /*arguments*/) const override
      {
          return std::make_shared<DataTypeUInt8>();
      }
  
      ColumnPtr executeImpl(const ColumnsWithTypeAndName &, const DataTypePtr &, size_t input_rows_count) const override
      {
          return DataTypeUInt8().createColumnConst(input_rows_count, 1u);
      }
  
      ColumnPtr getResultIfAlwaysReturnsConstantAndHasArguments(const ColumnsWithTypeAndName &) const override
      {
          return DataTypeUInt8().createColumnConst(1, 1u);
      }
  };
  
  
  void registerFunctionIndexHint(FunctionFactory & factory)
  {
      factory.registerFunction<FunctionIndexHint>();
  }
  
  }
  