Trait rusqlite::functions::Aggregate

pub trait Aggregate<A, T> where
    A: RefUnwindSafe + UnwindSafe,
    T: ToSql, {
    fn init(&self) -> A;
    fn step(&self, _: &mut Context, _: &mut A) -> Result<()>;
    fn finalize(&self, _: Option<A>) -> Result<T>;
}

Aggregate is the callback interface for user-defined aggregate function.

A is the type of the aggregation context and T is the type of the final result. Implementations should be stateless.

Required methods

fn init(&self) -> A

Initializes the aggregation context. Will be called prior to the first call to step() to set up the context for an invocation of the function. (Note: init() will not be called if there are no rows.)

fn step(&self, _: &mut Context, _: &mut A) -> Result<()>

"step" function called once for each row in an aggregate group. May be called 0 times if there are no rows.

fn finalize(&self, _: Option<A>) -> Result<T>

Computes and returns the final result. Will be called exactly once for each invocation of the function. If step() was called at least once, will be given Some(A) (the same A as was created by init and given to step); if step() was not called (because the function is running against 0 rows), will be given None.

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Implementors

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