Function explain_walk 

pub fn explain_walk<R: TryRead>(
    snap: &R,
    tenant: TenantId,
    labels: &[String],
    where_: &Expr,
) -> Result<Option<(String, u64, Vec<String>)>, QueryError>

The unindexed walk a query would start with, for EXPLAIN: the property of the most selective eligible leaf when no declared ordered index serves it, and the owners carrying that property (under labels when given), which the planner visits one point read at a time.

Errors

Propagates storage reads from the estimate and support lookups.