1 | #include <Functions/IFunctionImpl.h> |
2 | #include <Functions/FunctionFactory.h> |
3 | #include <DataTypes/DataTypesNumber.h> |
4 | |
5 | |
6 | namespace 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 | */ |
22 | class FunctionIndexHint : public IFunction |
23 | { |
24 | public: |
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 | |
58 | void registerFunctionIndexHint(FunctionFactory & factory) |
59 | { |
60 | factory.registerFunction<FunctionIndexHint>(); |
61 | } |
62 | |
63 | } |
64 | |