The problem is that in SQL "primary" also implies "unique".

OK, I'll amend my statement to be "I can't imaging having a table
without a UNIQUE primary key" :)

And even that wouldn't be so bad if it didn't add a bunch of overhead.

I've not done the research, but given that you most likely have an index
on this non-unique key anyway, wouldn't the added overhead for unique be
trivial. You're already maintaining the index, all you're adding is a
statement that says "if there's already a row in this index with this
key, throw". You had to find the right point in the index to insert the
non-unique key, so the page is already in memory.

-Walden


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