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Comment by esaym | Hacker News Reader

These are good, I've forgotten about some of them as well. As to:

  EXCLUDE USING gist (
   id WITH =,
   period WITH &&
  );"

What the heck?

It's so you can't define two billings for the same period (say, the same month). To prevent you from double-billing someone.

&& is the range operator that checks for overlap.[1]

You have to use gist indexes to do this. And in fact this page doesn't mention it but you have to enable the btree_gist extension with:

    CREATE EXTENSION btree_gist;

before the snippet above will work.

They're very nifty, as you can use this to easily prevent all kinds of date range overlaps.