Index selection is based on rule based (Deep Drive into couchbase n1ql query optimization https://blog.couchbase.com/wp-content/uploads/2017/10/N1QL-A-Practical-Guide-2nd-Edition.pdf).
You can explore Cost Based Optimizer 6.50 EE Developer Preview feature https://blog.couchbase.com/cost-based-optimizer-for-couchbase-n1ql-sql-for-json/