Function read 

pub fn read(db: &Db, scan: &Scan) -> Vec<Node>

Read the relation.

The order is NQL’s execution order, and it is load-bearing:

1. candidates — every row, at a sequence or at the tip
2. filters — VALID AS OF, then SEARCH
3. row-set transforms — TRACE, then TRAVERSE

Filters before transforms is the part worth stating. Tracing first and filtering after would apply SEARCH to the CHAIN rather than to the roots the chain was grown from, which is a different question with a plausible-looking answer.