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Improve documentation about PRIMARY KEY constraints.

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Get rid of the false implication that PRIMARY KEY is exactly equivalent to
UNIQUE + NOT NULL.  That was more-or-less true at one time in our
implementation, but the standard doesn't say that, and we've grown various
features (many of them required by spec) that treat a pkey differently from
less-formal constraints.  Per recent discussion on pgsql-general.

I failed to resist the temptation to do some other wordsmithing in the
same area.
This commit is contained in:
Tom Lane
2016-02-07 16:02:44 -05:00
parent cc2ca9319a
commit c477e84fe2
2 changed files with 43 additions and 41 deletions
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28
doc/src/sgml/ref/create_table.sgml
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@@ -506,25 +506,25 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXI
<term><literal>PRIMARY KEY ( <replaceable class="PARAMETER">column_name</replaceable> [, ... ] )</> (table constraint)</term>
<listitem>
<para>
The primary key constraint specifies that a column or columns of a table
can contain only unique (non-duplicate), nonnull values.
Technically, <literal>PRIMARY KEY</literal> is merely a
combination of <literal>UNIQUE</> and <literal>NOT NULL</>, but
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.
</para>
<para>
Only one primary key can be specified for a table, whether as a
The <literal>PRIMARY KEY</> constraint specifies that a column or
columns of a table can contain only unique (non-duplicate), nonnull
values. Only one primary key can be specified for a table, whether as a
column constraint or a table constraint.
</para>
<para>
The primary key constraint should name a set of columns that is
different from other sets of columns named by any unique
constraint defined for the same table.
different from the set of columns named by any unique
constraint defined for the same table. (Otherwise, the unique
constraint is redundant and will be discarded.)
</para>
<para>
<literal>PRIMARY KEY</literal> enforces the same data constraints as
a combination of <literal>UNIQUE</> and <literal>NOT NULL</>, but
identifying a set of columns as the primary key also provides metadata
about the design of the schema, since a primary key implies that other
tables can rely on this set of columns as a unique identifier for rows.
</para>
</listitem>
</varlistentry>