#include #include #include 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(ContextConstPtr) { return std::make_shared(); } 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(); } ColumnPtr executeImpl(const ColumnsWithTypeAndName &, const DataTypePtr &, size_t input_rows_count) const override { return DataTypeUInt8().createColumnConst(input_rows_count, 1u); } }; void registerFunctionIndexHint(FunctionFactory & factory) { factory.registerFunction(); } }