#####################
## Database schema ##
#####################

* After some time I've noticed entries like this one in my from_awl;
-undef- | -undef- | aaa.bbb.ccc.ddd | xxxx-xx-xx xx:xx:xx.xxxxxx

This is expected. Mail servers send you e-mail notices when they can't
deliver your messages. When doing so, the Return-Path is empty.
As at least MySQL doesn't support NULL in PRIMARY KEYs, SQLgrey creates
these '-undef- | -undef-' entries to recognise these cases.

* Why isn't there any PRIMARY KEY on the connect table ?
* Why isn't there an INDEX on the column <xxx> ?

I usually add a PRIMARY KEY to each table where there's a (KEY, VALUE)
structure. It's rarely bad for performance and help detect bugs in
the way the application layer handles the data.
Unfortunately MySQL doesn't support PRIMARY KEYs on columns
whose aggregate size is more than 500 bytes :
ERROR 1071: Specified key was too long. Max key length is 500

When adding support to PostgreSQL I could have added a PostgreSQL
specific PRIMARY KEY. I chose not to because I was not convinced it
was a good idea to introduce different database schema depending on
the actual database system.

There are two indexes on the connect table to speed up the queries:
- one on first_seen
- one on (src, sender_domain, sender_name)

* Postfix starts refusing messages some time after installing SQLgrey,
  what's going on?

That was a common problem with 1.4.x and large installs. The connect table
didn't have any index and needed full table scans for most connections.
The database was overloaded and SQLgrey couldn't answer in time. If it
happens with 1.5.x, you have an unordinary setup (insane trafic and a 386
for the database for example). Look for heavy disk usage, try to tune
your database server to allow it to load indexes in RAM and the table itself
if needed.
In any case, report your problem to sqlgrey-users@lists.sourceforge.net

##########
## Misc ##
##########

* Is /var/sqlgrey really necessary ? Wouldn't it be enough to start
  sqlgrey in /tmp ?

/var/sqlgrey is needed for SQLite users. MySQL and PostgreSQL don't need it.
Unfortunately there's no way to guess which database will be used.