diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index 281f649145..9300b71e75 100644
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -877,15 +877,17 @@ WITH ( MODULUS numeric_literal, REM
UNIQUE (column constraint)
UNIQUE ( column_name [, ... ] )
- INCLUDE ( column_name [, ...]) (table constraint)
+ INCLUDE ( column_name [, ...]) (table constraint)
The UNIQUE 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.
@@ -894,10 +896,10 @@ WITH ( MODULUS numeric_literal, REM
- 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.)
@@ -910,10 +912,15 @@ WITH ( MODULUS numeric_literal, REM
Adding a unique constraint will automatically create a unique btree
index on the column or group of columns used in the constraint.
- The optional clause INCLUDE adds to that index
- one or more columns on which the uniqueness is not enforced.
- Note that although the constraint is not enforced on the included columns,
- it still depends on them. Consequently, some operations on these columns
+
+
+
+ The optional INCLUDE clause adds to that index
+ one or more columns that are simply payload
: uniqueness
+ is not enforced on them, and the index cannot be searched on the basis
+ of those columns. However they can be retrieved by an index-only scan.
+ Note that although the constraint is not enforced on included columns,
+ it still depends on them. Consequently, some operations on such columns
(e.g., DROP COLUMN) can cause cascaded constraint and
index deletion.
@@ -923,7 +930,7 @@ WITH ( MODULUS numeric_literal, REM
PRIMARY KEY (column constraint)
PRIMARY KEY ( column_name [, ... ] )
- INCLUDE ( column_name [, ...]) (table constraint)
+ INCLUDE ( column_name [, ...]) (table constraint)
The PRIMARY KEY constraint specifies that a column or
@@ -941,27 +948,34 @@ WITH ( MODULUS numeric_literal, REM
PRIMARY KEY enforces the same data constraints as
- a combination of UNIQUE and NOT NULL, but
+ a combination of UNIQUE and 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.
- PRIMARY KEY constraints share the restrictions that
- UNIQUE constraints have when placed on partitioned
- tables.
+ When placed on a partitioned table, PRIMARY KEY
+ constraints share the restrictions previously decribed
+ for UNIQUE constraints.
Adding a PRIMARY KEY constraint will automatically
create a unique btree index on the column or group of columns used in the
- constraint. The optional INCLUDE clause allows a list
- of columns to be specified which will be included in the non-key portion
- of the index. Although uniqueness is not enforced on the included columns,
- the constraint still depends on them. Consequently, some operations on the
- included columns (e.g., DROP COLUMN) can cause cascaded
- constraint and index deletion.
+ constraint.
+
+
+
+ The optional INCLUDE clause adds to that index
+ one or more columns that are simply payload
: uniqueness
+ is not enforced on them, and the index cannot be searched on the basis
+ of those columns. However they can be retrieved by an index-only scan.
+ Note that although the constraint is not enforced on included columns,
+ it still depends on them. Consequently, some operations on such columns
+ (e.g., DROP COLUMN) can cause cascaded constraint and
+ index deletion.