On Sun, 31 Jan 2016 18:02:38 +0100 Tom Lane <tgl@xxxxxxxxxxxxx> 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 ... How about: "PRIMARY KEY is merly a combination of UNIQUE and NOT NULL with regard to data consistency behavior." "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. This metadata may be used by external programs, but is also utilized interally by the server in some cases." -- Bill Moran -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
