From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | "Craig Boucher" <craig(at)wesvic(dot)com> |
Cc: | "'David G(dot) Johnston'" <david(dot)g(dot)johnston(at)gmail(dot)com>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: Column order in multi column primary key |
Date: | 2016-08-08 20:46:38 |
Message-ID: | 22408.1470689198@sss.pgh.pa.us |
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Lists: | pgsql-general |
"Craig Boucher" <craig(at)wesvic(dot)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 would be 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
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