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Currently, aggregate functions are implemented in a statefull manner. The accumulator is stored internally in an aggregate_function::aggregate, requiring each query to instantiate new instances (see aggregate_function_selector's constructor, and note how it's called from selector::new_instance()). This makes aggregates hard to use in expressions, since expressions are stateless (with state only provided to evaluate()). To facilitate migration towards stateless expressions, we define a stateless_aggregate_function (modelled after user-defined aggregates, which are already stateless). This new struct defines the aggregate in terms of three scalar functions: one to aggregate a new input into an accumulator (provided in the first parameter), one to finalize an accumulator into a result, and one to reduce two accumulators for parallelized aggregation. An adapter of the new struct to the aggregate_function interface is also provided, to allow for incremental migration in the following patches.