Function explain_index 

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

Name of the first declared index the predicate’s leaves can use, for EXPLAIN’s access path.

Errors

Returns the underlying storage error from an index-catalog read.