Enum Cardinality 

pub enum Cardinality {
    ExactlyOne,
    AtMostOne,
    Unverified,
}

How many parent rows a child row can match.

The three cases are not decoration. Each licenses a different rewrite in section 5, and the difference between the first two is the difference between a LEFT JOIN that can be turned into an inner one and one that cannot.

Variants

ExactlyOne

Every child row matches exactly one parent row: the key is not null anywhere and every value is present in the parent. This licenses pushing an aggregate through the relationship and turns a LEFT JOIN into an inner join.

AtMostOne

Every child row matches at most one parent row, which is the ordinary case: a null key or a key with no parent row matches nothing.

Unverified

The parent side was not unique, or verification did not run. No link is built. The planner is told so that it plans an ordinary hash join rather than waiting for a structure that is never going to arrive.

Implementations

impl Cardinality

pub fn links(self) -> bool

Whether a link may be built for a relationship of this cardinality.

pub fn total(self) -> bool

Whether every child row is guaranteed a parent row, which is what licenses the aggregate push-through and the LEFT JOIN rewrite.

pub fn tag(self) -> u8

The tag this cardinality takes in a section header.

pub fn label(self) -> &'static str

What this cardinality is called where a person reads it, which is rudb_links().

pub fn from_tag(tag: u8) -> Result<Self>

The cardinality a header tag names.

Errors

If the tag is not one of the three, which means a file from a later build. The caller drops the section, per the rule in section 3.2 that a reader ignores what it does not know.

Trait Implementations

impl Clone for Cardinality

fn clone(&self) -> Self

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Copy for Cardinality

impl Debug for Cardinality

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

Formats the value using the given formatter.

impl Display for Cardinality

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

Formats the value using the given formatter.

impl Eq for Cardinality

impl PartialEq for Cardinality

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

Equality operator ==.

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

Inequality operator !=.

impl StructuralPartialEq for Cardinality