Struct ScalarProjection 

pub struct ScalarProjection {
    pub col: usize,
    pub identity: OwnedValue,
}

A scalar projection annotation on a relationship slot (REDUCE-DESIGN.md §9, Tier 1). A relationship aggregate — issue { commentCount: count(comments) } — lives in the engine tree as an ordinary singular one-row relationship whose child is the synthetic aggregate row [group_key…, agg]. This annotation tells the presentation boundary (the View marshaller and every wire receiver) to unwrap that one-row child into a plain scalar field named after the relationship: instead of commentCount: [{ issueID: 1, count: 5 }] the consumer sees commentCount: 5. It is purely a result-shape concern — the dataflow and the reconstructed tree stay a plural relationship of one row.

Fields

col: usize

Which child column to surface as the scalar value (the aggregate column — count in [issueID, count]). Indexed into the child schema’s columns.

identity: OwnedValue

The value to substitute when the relationship is empty — a childless parent has no group in the reduce, so the projection emits the aggregate’s identity (0 for count, NULL for sum/avg), the boundary analogue of SQL’s LEFT JOIN … count → 0 (REDUCE-DESIGN.md §9). Without this an absent group would render as null, which is wrong for count.

Trait Implementations

impl Clone for ScalarProjection

fn clone(&self) -> ScalarProjection

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for ScalarProjection

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

Formats the value using the given formatter.