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It sounds simple, but you'd be surprised how many programmers don't grok the fact that types/data have sizes (especially numeric types). For many tasks, this doesn't matter, but when it does matter, you need people who understand.

As an example, an IPv4 address is 32 bits. Don't convert it to a string and put it in a varchar(64) in your database when you are optimizing for space (I actually saw this once). And yes, the DB had an inet type, but no one knew how to use it, what it was or why it mattered.