How to do SQL lookup in before_send hook?

[MRob]

I found a very helpful example on how to write a hook:

https://marc.info/?l=postfix-users&m=128438735413057&w=2

But I need to do a SQL lookup in the before_send hook for outbound mail. 
Two questions:

What's the most reliable way to retrieve our sending user? I found that 
$msginfo->sender seems to work in my first test but I was not sure if 
this was recommended or reliable.

I could write my own DB code to create a SQL connection and do the 
needed lookup, but that might not be recommended (does amavis pool 
database connections for instance?). What's the amavis way to do that?