Documentation suggestions

[kengruven]

This looks like a pretty neat extension, but there were several aspects which I couldn't figure out from just the documentation.

• @ looks like a conversion operator, but I can't guess what some of the other functions/symbols do. I think a lot are standard (if uncommon) Postgres operators which are simply overloaded, but ASCII art is hard to search for in a manual. (I eventually found a couple in the postgresql-unit changelog, and a couple in a third-party Postgres blog article.) The list of Operators and Functions look like they fall into distinct categories: shortcuts for applying a unit (ampere), new unit functionality (unit_at), overridden Postgres functionality (<=), and so on. It might be helpful to group them this way, since you don't need to document each unit application function separately, for example.
• This extension added tables unit_prefixes and unit_units in my public schema; is it possible to install them in a different place, or move them? (I don't know if this is common practice with Postgres extensions or not. I don't even know if it uses the "public" schema, or the current schema when you run the CREATE EXTENSION command.)
• It's not clear where the "config" should go. Is this server config, or client config? Is it per-session or per-database or do I need to set it the same globally? I'm also not sure how to query this, or if I might need to make my client program handle these output formats.
• It might be helpful to suggest using a CHECK constraint (like CHECK dimension(length) = '1 m') on unit-typed columns, since there doesn't appear to be any other way to force a column to have a particular dimension. If someone inserted incompatible data, then querying with < would return nonsensical results, and using << would return an error. Presumably, having consistent data (and simpler queries) would also allow an index to be used more efficiently, too, but I haven't tested this yet.
• When reading a result set with psycopg, I seem to get data out as a string ("1 kg"), which is fine. Do I need to look up this OID in the schema? I think some Postgres extensions hardcode their OIDs, but I don't see this number in the postgresql-unit source.

Thanks for the nice extension!