alter_table

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ALTER TABLE() SQL Commands ALTER TABLE()

NAME
ALTER TABLE - change the definition of a table

SYNOPSIS
ALTER TABLE [ ONLY ] name [ * ]
ADD [ COLUMN ] column type [ column_constraint [ ... ] ]
ALTER TABLE [ ONLY ] name [ * ]
DROP [ COLUMN ] column [ RESTRICT | CASCADE ]
ALTER TABLE [ ONLY ] name [ * ]
ALTER [ COLUMN ] column { SET DEFAULT expression | DROP DEFAULT }
ALTER TABLE [ ONLY ] name [ * ]
ALTER [ COLUMN ] column { SET | DROP } NOT NULL
ALTER TABLE [ ONLY ] name [ * ]
ALTER [ COLUMN ] column SET STATISTICS integer
ALTER TABLE [ ONLY ] name [ * ]
ALTER [ COLUMN ] column SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
ALTER TABLE [ ONLY ] name [ * ]
SET WITHOUT OIDS
ALTER TABLE [ ONLY ] name [ * ]
RENAME [ COLUMN ] column TO new_column
ALTER TABLE name
RENAME TO new_name
ALTER TABLE [ ONLY ] name [ * ]
ADD table_constraint
ALTER TABLE [ ONLY ] name [ * ]
DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ]
ALTER TABLE name
OWNER TO new_owner
ALTER TABLE name
CLUSTER ON index_name

DESCRIPTION
ALTER TABLE changes the definition of an existing table. There are
several subforms:

ADD COLUMN
This form adds a new column to the table using the same syntax
as CREATE TABLE [create_table(7)].

DROP COLUMN
This form drops a column from a table. Indexes and table con-
straints involving the column will be automatically dropped as
well. You will need to say CASCADE if anything outside the ta-
ble depends on the column, for example, foreign key references
or views.

SET/DROP DEFAULT
These forms set or remove the default value for a column. The
default values only apply to subsequent INSERT commands; they
do not cause rows already in the table to change. Defaults may
also be created for views, in which case they are inserted into
INSERT statements on the view before the view’s ON INSERT rule
is applied.

SET/DROP NOT NULL
These forms change whether a column is marked to allow null
values or to reject null values. You can only use SET NOT NULL
when the column contains no null values.

SET STATISTICS
This form sets the per-column statistics-gathering target for
subsequent ANALYZE [analyze(7)] operations. The target can be
set in the range 0 to 1000; alternatively, set it to -1 to
revert to using the system default statistics target.

SET STORAGE
This form sets the storage mode for a column. This controls
whether this column is held inline or in a supplementary table,
and whether the data should be compressed or not. PLAIN must be
used for fixed-length values such as integer and is inline,
uncompressed. MAIN is for inline, compressible data. EXTERNAL
is for external, uncompressed data, and EXTENDED is for exter-
nal, compressed data. EXTENDED is the default for all data
types that support it. The use of EXTERNAL will, for example,
make substring operations on a text column faster, at the
penalty of increased storage space.

SET WITHOUT OIDS
This form removes the oid column from the table. Removing OIDs
from a table does not occur immediately. The space that the
OID uses will be reclaimed when the row is updated. Without
updating the row, both the space and the value of the OID are
kept indefinitely. This is semantically similar to the DROP
COLUMN process.

RENAME The RENAME forms change the name of a table (or an index,
sequence, or view) or the name of an individual column in a ta-
ble. There is no effect on the stored data.

ADD table_constraint
This form adds a new constraint to a table using the same syn-
tax as CREATE TABLE [create_table(7)].

DROP CONSTRAINT
This form drops constraints on a table. Currently, constraints
on tables are not required to have unique names, so there may
be more than one constraint matching the specified name. All
such constraints will be dropped.

OWNER This form changes the owner of the table, index, sequence, or
view to the specified user.

CLUSTER
This form marks a table for future CLUSTER [cluster(7)] opera-
tions.

You must own the table to use ALTER TABLE; except for ALTER TABLE
OWNER, which may only be executed by a superuser.

PARAMETERS
name The name (possibly schema-qualified) of an existing table to
alter. If ONLY is specified, only that table is altered. If
ONLY is not specified, the table and all its descendant tables
(if any) are updated. * can be appended to the table name to
indicate that descendant tables are to be altered, but in the
current version, this is the default behavior. (In releases
before 7.1, ONLY was the default behavior. The default can be
altered by changing the configuration parameter sql_inheri-
tance.)

column Name of a new or existing column.

type Data type of the new column.

new_column
New name for an existing column.

new_name
New name for the table.

table_constraint
New table constraint for the table.

constraint_name
Name of an existing constraint to drop.

new_owner
The user name of the new owner of the table.

index_name
The index name on which the table should be marked for cluster-
ing.

CASCADE
Automatically drop objects that depend on the dropped column or
constraint (for example, views referencing the column).

RESTRICT
Refuse to drop the column or constraint if there are any depen-
dent objects. This is the default behavior.

NOTES
The key word COLUMN is noise and can be omitted.

In the current implementation of ADD COLUMN, default and NOT NULL
clauses for the new column are not supported. The new column always
comes into being with all values null. You can use the SET DEFAULT
form of ALTER TABLE to set the default afterward. (You may also want
to update the already existing rows to the new default value, using
UPDATE [update(7)].) If you want to mark the column non-null, use the
SET NOT NULL form after you’ve entered non-null values for the column
in all rows.

The DROP COLUMN form does not physically remove the column, but simply
makes it invisible to SQL operations. Subsequent insert and update
operations in the table will store a null value for the column. Thus,
dropping a column is quick but it will not immediately reduce the on-
disk size of your table, as the space occupied by the dropped column
is not reclaimed. The space will be reclaimed over time as existing
rows are updated. To reclaim the space at once, do a dummy UPDATE of
all rows and then vacuum, as in:

UPDATE table SET col = col;
VACUUM FULL table;

If a table has any descendant tables, it is not permitted to add or
rename a column in the parent table without doing the same to the
descendants. That is, ALTER TABLE ONLY will be rejected. This ensures
that the descendants always have columns matching the parent.

A recursive DROP COLUMN operation will remove a descendant table’s
column only if the descendant does not inherit that column from any
other parents and never had an independent definition of the column. A
nonrecursive DROP COLUMN (i.e., ALTER TABLE ONLY ... DROP COLUMN)
never removes any descendant columns, but instead marks them as inde-
pendently defined rather than inherited.

Changing any part of a system catalog table is not permitted.

Refer to CREATE TABLE for a further description of valid parameters.
[XRef to DDL] has further information on inheritance.

EXAMPLES
To add a column of type varchar to a table:

ALTER TABLE distributors ADD COLUMN address varchar(30);

To drop a column from a table:

ALTER TABLE distributors DROP COLUMN address RESTRICT;

To rename an existing column:

ALTER TABLE distributors RENAME COLUMN address TO city;

To rename an existing table:

ALTER TABLE distributors RENAME TO suppliers;

To add a not-null constraint to a column:

ALTER TABLE distributors ALTER COLUMN street SET NOT NULL;

To remove a not-null constraint from a column:

ALTER TABLE distributors ALTER COLUMN street DROP NOT NULL;

To add a check constraint to a table:

ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);

To remove a check constraint from a table and all its children:

ALTER TABLE distributors DROP CONSTRAINT zipchk;

To add a foreign key constraint to a table:

ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) MATCH FULL;

To add a (multicolumn) unique constraint to a table:

ALTER TABLE distributors ADD CONSTRAINT dist_id_zipcode_key UNIQUE (dist_id, zipcode);

To add an automatically named primary key constraint to a table, not-
ing that a table can only ever have one primary key:

ALTER TABLE distributors ADD PRIMARY KEY (dist_id);

COMPATIBILITY
The ADD COLUMN form conforms with the SQL standard, with the exception
that it does not support defaults and not-null constraints, as
explained above. The ALTER COLUMN form is in full conformance.

The clauses to rename tables, columns, indexes, views, and sequences
are PostgreSQL extensions of the SQL standard.

ALTER TABLE DROP COLUMN can be used to drop the only column of a ta-
ble, leaving a zero-column table. This is an extension of SQL, which
disallows zero-column tables.

SQL - Language Statements 2008-01-03 ALTER TABLE()