Re: Column order in multi column primary key

"Craig Boucher" <craig@wesvic.com> writes:
> I should have pointed out in my last response that I was wondering if the performance of the pk index on work_session
wouldbe better if my primary key was (customer_id, work_session_id) or if (work_session_id,  customer_id) will be fine.
Customer_id will be repeated quite a bit in the table but work_session_id should be unique across the whole table. 

You almost certainly want the more-unique column first, so far as the
performance of the index itself goes.  See
https://www.postgresql.org/docs/9.5/static/indexes-multicolumn.html

Having said that, I'm pretty skeptical of the notion of redefining what
your PK is on performance grounds.  With this definition, you'd allow
two entries with the same work_session_id, if they chanced to have
different customer_ids.  Is that really OK?

            regards, tom lane