Both of the below visualizers can help, but require some knowledge about comparing estimated vs actual row estimates, disk sorts vs in memory, etc. Drawing implications about whether your schema needs to change or just the query will take time to master as well.
Often you need to provide much more information to get good advice. What version of Postgres are you on? What are stats like for the involved tables? What other indexes are available or might you add? What is the use of this system-- single user or thousands? Heavy writes continually, or load & analysis? Can you share the full query text and output of EXPLAIN ANALYZE on it?
