Now that all of the ways a query executes have been covered, some of the more obscure query planning parameters can be explained usefully.
Defaulting to 128 MB,
effective_cache_size is used to represent approximately how much total disk space is available for caching the database. This is normally set to the total of
shared_buffers plus the size of the operating system disk buffer cache after the database is started. This turns out to be greater than half of the total system memory on a typical dedicated database server. This setting does not allocate any memory itself, it simply serves as an advisory value for the planner about what should likely be available.
The only thing this is used for is ...