On 01/31/2016 09:02 AM, Tom Lane wrote:
Harald Fuchs <hari.fuchs@xxxxxxxxx> writes:
Ben Leslie <benno@xxxxxxxxxxx> 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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