diff options
| -rw-r--r-- | doc/src/sgml/ref/create_table.sgml | 22 |
1 files changed, 15 insertions, 7 deletions
diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml index 4964c46213a..f168b13390a 100644 --- a/doc/src/sgml/ref/create_table.sgml +++ b/doc/src/sgml/ref/create_table.sgml @@ -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> |