Struct Child 

struct Child {
    label: &'static str,
    rows: RecordBatch,
    attrs: Option<Attrs>,
    by_parent: Vec<Vec<u32>>,
    parent_of_id: Vec<u32>,
}

A child table and the attribute table keyed by its id.

Both indexes are built once when the block is opened, because both are read once per emitted span and once per attribute term: rebuilding either per span made emission quadratic in the number of spans in the block, which is the one place a limit does not bound the work.

Fields

label: &'static str

What the array is called in the emitted row.

rows: RecordBatch

attrs: Option<Attrs>

by_parent: Vec<Vec<u32>>

Child rows grouped by the root row they hang off, indexed by it.

parent_of_id: Vec<u32>

The root row each child id belongs to, indexed by the id. Child ids are dense from zero per block, so this is an array store like every other join here — but it is not the identity, because id numbers a child and the row number is where it happens to sit.