Lukas Eder lays out one of the biggest performance drains today:
We’re using 8x as much memory in the database when doing
SELECT *rather thanSELECT film, rating. That’s not really surprising though, is it? We knew that. Yet we accepted it in many many of our queries where we simply didn’t need all that data. We generated needless, mandatory work for the database, and it does sum up. We’re using 8x too much memory (the number will differ, of course).Now, all the other steps (disk I/O, wire transfer, client memory consumption) are also affected in the same way, but I’m skipping those.
This article is absolutely worth reading and sharing with developers.