Composite Primary keys and auto increment? i'm developing SaaS app with multi-tenancy, and i've decide to use single DB (MySQL Innodb for now) for client's data. I chose to use composite primary keys like  PK(client_id, id). I have 2 ways here, 

1: increment "id" by myself (by trigger or from code) 

or 

2: Make "id" as auto-increment by mysql.

In first case i will have unique id's for each client, so each client will have id 1, 2, 3 etc..
In second case id's will grow for all clients.
What is the best practice here? My priorities are: performace, security &amp; scaling. Thanks!
Same question at stackoverflow with more answers
The second option should be the fastest. It was made for this. Also it should have no bugs since it is used a lot and already for a long time. If you go for this then you do not even need to use a composite primary key.

In my opinion the only reason to use the first option is if you need a numbering starting from 1 per client.