Managing users

Dear All,

Forgive me if this is a silly question, but I need some advice.

I'm setting up a db where users will be able to insert, update, select. I
will have three types of users. Lowest can do very little, highest to do
everything.

I'm thinking of creating three groups (Level1 to Level3) for the 3 types of
users. Is this the best way of doing this?

If a user logs on, say through a web  interface, would I have a table with
username/passwd to check the login to the database? Or should I somehow use
the pg_user table?

Also, is there a way of monitoring how much memory a user takes up. For
example, I have a table username/Data. How much memory does each username
take up on that table?

Thanks for your time and help

Colin

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