I would like to improve this query. First: the link to :tracking/invoice has either no or one reference. Is it possible to express this in the pull syntax? Currently I get a vector of :tracking/invocie. And then I want to filter by :invoice/status. I put the contains? clause, but I think there must be a way how I give a vector of possible matches. I guess with a vector the query will be run faster?

(def xero-orders-tracking
'[:find [(pull ?id [* {:tracking/_invoice [:tracking/tag :tracking/number]}]) ...]
    :in $ 
    :where 
    [?id :invoice/id _]
    [?id :invoice/type "ACCREC"]
    [?id :invoice/status ?status]
    [(contains? #{"AUTHORISED" "PAID"} ?status)]
   ])

back-references like :tracking/_invoice can't help but be "many", even if your data happens to have only one, since there's no way to control how many incoming refs there might be.

you'll have to post-process that with (update _ :tracking/invoice first) or similar.

you can probably skip the [?id :invoice/id _] at the start, unless there are entities with :invoice/type and :invoice/status that don't have :invoice/id.

you can express the status constraint as (or [?id :invoice/status "AUTHORIZED"] [?id :invoice/status "PAID"])