Re: Add A Glossary

Pushed now.  Many thanks to Corey who put the main thrust, and to Jürgen
and Roger for the great help, and to Justin for the extensive review and
Fabien for the initial discussion.

This is just a starting point.  Let's keep improving it.  And how that
we have it, we can start thinking of patching the main part of the docs
to make reference to it by using <glossterm> in key spots.  Right now
the glossary links to itself, but it makes lots of sense to have other
places point to it.

On 2020-Apr-02, Justin Pryzby wrote:

> We already have Session:
>     A Connection to the Database. 

Yes, but I didn't like that much, so I rewrote it -- I was asking for
suggestions on how to improve it further.  While I think we use those
terms (connection and session) interchangeably sometimes, they're not
exactly the same and the glossary should be more precise or at least
less vague about the distinction.

> I propose: Client:
> 	A host (or a process on a host) which connects to a server to make
> queries or other requests.
> 
> But note, "host" is still defined as "server", which I didn't like.
> 
> Maybe it should be:
> 	A computer which may act as a >client< or a >server<.

I changed all these terms, and a few others, added a couple more and
commented out some that I was not happy with, and pushed.

I think this still needs more work:

* We had "serializable", but none of the other isolation levels were
  defined.  If we think we should define them, let's define them all.
  But also the definition we had for serializable was not correct;
  it seemed more suited to define "repeatable read".

* I commented out the definition of "sequence", which seemed to go into
  excessive detail.  Let's have a more concise definition?

* We're missing exclusion constraints, and NOT NULL which is also a
  weird type of constraint.

Patches for these omissions, and other contributions, welcome.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services