On Tue, Oct 15, 2013 at 1:19 PM, Peter Geoghegan <p...@heroku.com> wrote:
>> There could be
>> other ways of avoiding that problem, though.  Here's an example:
>>
>> UPSERT table (keycol1, ..., keycoln) = (keyval1, ..., keyvaln) SET
>> (nonkeycol1, ..., nonkeycoln) = (nonkeyval1, ..., nonkeyvaln)
>>
>> That's pretty ugly on multiple levels, and I'm definitely not
>> proposing that exact thing, but the idea is: look for a record that
>> matches on the key columns/values; if found, update the non-key
>> columns with the corresponding values; if not found, construct a new
>> row with both the key and nonkey column sets and insert it.  If no
>> matching unique index exists we'll have to fail, but we stop short of
>> having to mention the name of that index.
>
> What if you want to update the key columns - either the potential
> conflict-causing one, or another?

I'm not sure what that means in the context of an UPSERT operation.
If the update case is, when a = 1 then make a = 2, then which value
goes in column a when we insert, 1 or 2?  But I suppose if you can
work that out it's just a matter of mentioning the column as both a
key column and a non-key column.

> What about composite unique
> constraints? MySQL certainly supports all that, for example.

That's why it allows you to specify N key columns rather than
restricting you to just one.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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