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71 lines
2.4 KiB
C++
71 lines
2.4 KiB
C++
#pragma once
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#include <memory>
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#include <DataTypes/IDataType.h>
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#include <AggregateFunctions/IAggregateFunction.h>
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namespace DB
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{
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/** Aggregate function combinator allows to take one aggregate function
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* and transform it to another aggregate function.
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*
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* In SQL language they are used as suffixes for existing aggregate functions.
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*
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* Example: -If combinator takes an aggregate function and transforms it
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* to aggregate function with additional argument at end (condition),
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* that will pass values to original aggregate function when the condition is true.
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*
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* More examples:
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*
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* sum(x) - calculate sum of x
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* sumIf(x, cond) - calculate sum of x for rows where condition is true.
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* sumArray(arr) - calculate sum of all elements of arrays.
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*
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* PS. Please don't mess it with so called "combiner" - totally unrelated notion from Hadoop world.
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* "combining" - merging the states of aggregate functions - is supported naturally in ClickHouse.
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*/
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class IAggregateFunctionCombinator
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{
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public:
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virtual String getName() const = 0;
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virtual bool isForInternalUsageOnly() const { return false; }
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/** From the arguments for combined function (ex: UInt64, UInt8 for sumIf),
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* get the arguments for nested function (ex: UInt64 for sum).
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* If arguments are not suitable for combined function, throw an exception.
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*/
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virtual DataTypes transformArguments(const DataTypes & arguments) const
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{
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return arguments;
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}
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/** From the parameters for combined function,
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* get the parameters for nested function.
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* If arguments are not suitable for combined function, throw an exception.
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*/
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virtual Array transformParameters(const Array & parameters) const
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{
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return parameters;
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}
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/** Create combined aggregate function (ex: sumIf)
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* from nested function (ex: sum)
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* and arguments for combined agggregate function (ex: UInt64, UInt8 for sumIf).
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* It's assumed that function transformArguments was called before this function and 'arguments' are validated.
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*/
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virtual AggregateFunctionPtr transformAggregateFunction(
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const AggregateFunctionPtr & nested_function,
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const DataTypes & arguments,
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const Array & params) const = 0;
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virtual ~IAggregateFunctionCombinator() {}
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};
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using AggregateFunctionCombinatorPtr = std::shared_ptr<const IAggregateFunctionCombinator>;
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}
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