On 01/31/2016 09:02 AM, Tom Lane wrote:
> Harald Fuchs <hari.fuchs@gmail.com> writes:
>> Ben Leslie <benno@benno.id.au> writes:
>>> "Technically, PRIMARY KEY is merely a combination of UNIQUE and NOT NULL"
>>>
>>> I wanted to clarify if that was, technically, true.
>
>> Yes, but see below.
>
>>> "identifying a set of columns as primary key also provides metadata
>>> about the design of the schema, as a primary key implies that other
>>> tables can rely on this set of columns as a unique identifier for
>>> rows."
>
> Yeah. The extra metadata has several other effects. Perhaps it would be
> better to reword this sentence to make it clear that PRIMARY KEY is
> equivalent to UNIQUE+NOTNULL in terms of the data constraint that it
> enforces, without implying that there is no other difference. I'm not
> sure about a short and clear expression of that though ...
The practical implementation of a PRIMARY KEY is the equivalent to
UNIQUE+NOTNULL. However, ...
>
> regards, tom lane
>
>
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