I understand you want to be in "control" of how exactly a query is beeing executed (what indices it uses). There is a lot to be said about query optimization but it is not "random". My guess is that most queries can be optimized automatically and it shows that sometimes indices are not even needed (see https://robots.thoughtbot.com/why-postgres-wont-always-use-a...) whereas the "manual" approach would always use an index.

In PostgreSQL if you want to know what indices are used by a query, just ask the system using an EXPLAIN ANALYZE query and if it does not use any indices, create them (or live with the performance).

Also there are "Index-only scans" which I do not know much about but may well fit your approach: https://wiki.postgresql.org/wiki/Index-only_scans

So to summarize: automatic index selection is not random and slow queries can be identified pretty easy.