To optimize application speed, everyone always advises to minimize the number of queries an application makes to the database, consolidating them into fewer queries that retrieve more wherever possible.

However, this also always comes with the caution that data transferred is still data transferred, and just because you are making fewer queries doesn't make the data transferred free.

I'm in a situation where I can over-include on the query in order to cut down the number of queries, and simply remove the unwanted data in the application code.

Is there any type of a rule of thumb on how much of a cost there is to each query, to know when to optimize number of queries versus size of queries? I've tried to Google for objective performance analysis data, but surprisingly haven't been able to find anything like that.

Clearly this relationship will change for factors such as when the database grows in size, making this somewhat individualized, but surely this is not so individualized that a broad sense of the landscape can't be drawn out?

I'm looking for general answers, but for what it's worth, I'm running an application on Heroku.com, which means Ruby on Rails with a Postgres database.