fn index_parent_of_id(b: &RecordBatch) -> Vec<u32>Expand description
The parent_id of each child row, indexed by that row’s own id.
The inverse of crate::series::index_by_parent and the same trick: ids
are dense from zero per block, so the id is the slot. A table missing either
column indexes as empty, and every lookup then misses — which is the same
answer a scan of it would give.