Optimizer and Indexes with Joins

I have a question regarding the optimizer and indexes. Having a boss that is fluent in data flex but trying to understand postgresql, I am trying to find out if postgresql has a way to basically tell the optimizer which index to use. The boss states that in data flex you "tell" it to use index 7 and it uses index 7.

We utilize vacuum analyze and we also structure our where clauses to fit the specific indexes. Further, we set the "enable sequence scan" to off and it did not improve any performace issues. Most of our selects are "joins" though and that may be causing most of our performance issues. Any advice out there?

If I need to provide any further information, please advise....

TIA,

LA Hargis
(Trying to get better at postgres)