Enum Class 

pub enum Class {
    Exact,
    Certified {
        bound: f64,
    },
    Estimated {
        source: Source,
    },
}

How much of an answer is knowledge.

Three cases, ordered by how much they permit. Exact permits anything, including changing an answer by folding a predicate away. Certified permits a decision whose fallback survives being wrong, which is the case a safe bound is for. Estimated permits choosing between two plans that produce the same rows, and nothing else.

Variants

Exact

The value is the value. A count that was counted, a null count that was maintained, a minimum that was compared.

Certified

The value is wrong by no more than bound, as a fraction of itself, and the structure that produced it can prove that.

A quantile summary with an epsilon is the usual source. The number is what makes a threshold safe to seed from, so a producer that cannot state one should say Estimated instead of picking a bound that sounds about right.

Fields

bound: f64

The relative error bound, where 0.01 is one percent.

Estimated

The value is a guess, and this is where it came from.

Fields

source: Source

What produced the guess, because EXPLAIN prints it.

Implementations

impl Class

pub const fn is_exact(self) -> bool

Whether this is knowledge rather than a guess, which is the test the folding rule applies.

pub fn combine(self, other: Self) -> Self

The class of an answer computed from two others.

Exact combined with anything else is the anything else, which is the honest direction and the easy one to get backwards. Two certified bounds add, because a combination of two bounded errors is bounded by their sum, and the sum saturates at one because a bound of more than a hundred percent says nothing that Estimated does not say. Anything involving an estimate is an estimate, and the source of a combination is Source::Propagation unless one of the two was a bare constant, in which case the answer is as weak as the constant was.

Trait Implementations

impl Clone for Class

fn clone(&self) -> Class

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for Class

impl Debug for Class

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for Class

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl PartialEq for Class

fn eq(&self, other: &Class) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for Class