Another trap people fall into is doing joins in their application instead of in the database. I see this all the time where people will query some table with a

Another trap people fall into is doing joins in their application instead of in the database. I see this all the time where people will query some table with a condition, map the ids from the result and then query another table (which also joins the first table) and then filters by the id of the first query. Or just doing massive aggregates in ruby instead of having the database do it and just spitting out the result. A little bit of knowledge about SQL and how a relational database works goes a long way.