Ah, I've had this same use case: I store many versions of a resource from an external API, but I typically only want the most recent version. I've used the following techniques:

Select the ID with the max sync_token. Easiest for Postgres to optimize, assuming you have a primary key of (id, sync_token).

    SELECT *
    FROM external_api
    WHERE id = :my_id 
      AND sync_token = (SELECT max(sync_token) FROM ext_api WHERE id = :my_id)

Define a view using DISTINCT ON. Convenient for ad-hoc querying. Postgres usually figures out it can use the primary key to avoid a full-table scan.

    SELECT DISTINCT ON (id) *
    FROM external_api
    WHERE id = :my_id 
      AND sync_token = (SELECT max(sync_token) FROM ext_api WHERE id = :my_id)
    ORDER BY id, sync_token DESC

For tricky predicates, I use a trigger to track the most recent resource in a separate table. This is a hacky version of incremental view maintenance. [1]

[1]: https://wiki.postgresql.org/wiki/Incremental_View_Maintenanc...