1#include <Functions/IFunctionImpl.h>
2#include <Functions/FunctionFactory.h>
3#include <DataTypes/DataTypesNumber.h>
4
5
6namespace DB
7{
8
9
10/** The `indexHint` function takes any number of any arguments and always returns one.
11 *
12 * This function has a special meaning (see ExpressionAnalyzer, KeyCondition)
13 * - the expressions inside it are not evaluated;
14 * - but when analyzing the index (selecting ranges for reading), this function is treated the same way,
15 * as if instead of using it the expression itself would be.
16 *
17 * Example: WHERE something AND indexHint(CounterID = 34)
18 * - do not read or calculate CounterID = 34, but select ranges in which the CounterID = 34 expression can be true.
19 *
20 * The function can be used for debugging purposes, as well as for (hidden from the user) query conversions.
21 */
22class FunctionIndexHint : public IFunction
23{
24public:
25 static constexpr auto name = "indexHint";
26 static FunctionPtr create(const Context &)
27 {
28 return std::make_shared<FunctionIndexHint>();
29 }
30
31 bool isVariadic() const override
32 {
33 return true;
34 }
35 size_t getNumberOfArguments() const override
36 {
37 return 0;
38 }
39
40 bool useDefaultImplementationForNulls() const override { return false; }
41
42 String getName() const override
43 {
44 return name;
45 }
46 DataTypePtr getReturnTypeImpl(const DataTypes & /*arguments*/) const override
47 {
48 return std::make_shared<DataTypeUInt8>();
49 }
50
51 void executeImpl(Block & block, const ColumnNumbers &, size_t result, size_t input_rows_count) override
52 {
53 block.getByPosition(result).column = DataTypeUInt8().createColumnConst(input_rows_count, 1u);
54 }
55};
56
57
58void registerFunctionIndexHint(FunctionFactory & factory)
59{
60 factory.registerFunction<FunctionIndexHint>();
61}
62
63}
64