mirrors
/
postgres
mirror of https://github.com/postgres/postgres
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

Doc: clarify that CREATE TABLE discards redundant unique constraints. 

The SQL standard says that redundant unique constraints are disallowed,
but we long ago decided that throwing an error would be too
user-unfriendly, so we just drop redundant ones.  The docs weren't very
clear about that though, as this behavior was only explained for PRIMARY
KEY vs UNIQUE, not UNIQUE vs UNIQUE.

While here, I couldn't resist doing some copy-editing and markup-fixing
on the adjacent text about INCLUDE options.

Per bug #16767 from Matthias vd Meent.

Discussion: https://postgr.es/m/16767-1714a2056ca516d0@postgresql.org
This commit is contained in:
Tom Lane 2020-12-08 13:09:48 -05:00
parent 0921b86ca1
commit 14db957922
1 changed files with 15 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

22
doc/src/sgml/ref/create_table.sgml
View File

@ -492,9 +492,11 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXI
<para>
The <literal>UNIQUE</literal> constraint specifies that a
group of one or more columns of a table can contain
only unique values. The behavior of the unique table constraint
is the same as that for column constraints, with the additional
capability to span multiple columns.
only unique values. The behavior of a unique table constraint
is the same as that of a unique column constraint, with the
additional capability to span multiple columns. The constraint
therefore enforces that any two rows must differ in at least one
of these columns.
</para>
<para>
@ -503,10 +505,10 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXI
</para>
<para>
Each unique table constraint must name a set of columns that is
Each unique constraint should name a set of columns that is
different from the set of columns named by any other unique or
primary key constraint defined for the table. (Otherwise it
would just be the same constraint listed twice.)
primary key constraint defined for the table. (Otherwise, redundant
unique constraints will be discarded.)
</para>
</listitem>
</varlistentry>
@ -531,11 +533,17 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXI
<para>
<literal>PRIMARY KEY</literal> enforces the same data constraints as
a combination of <literal>UNIQUE</> and <literal>NOT NULL</>, but
a combination of <literal>UNIQUE</> and <literal>NOT NULL</>. However,
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>
<para>
Adding a <literal>PRIMARY KEY</literal> constraint will automatically
create a unique btree index on the column or group of columns used in the
constraint.
</para>
</listitem>
</varlistentry>