YoLinux.com: alter_table manpage

ALTER TABLE() SQL Commands ALTER TABLE()

NAME
ALTER TABLE - change the definition of a table

SYNOPSIS
ALTER TABLE [ ONLY ] name [ * ]
action [, ... ]
ALTER TABLE [ ONLY ] name [ * ]
RENAME [ COLUMN ] column TO new_column
ALTER TABLE name
RENAME TO new_name
ALTER TABLE name
SET SCHEMA new_schema

where action is one of:

ADD [ COLUMN ] column type [ column_constraint [ ... ] ]
DROP [ COLUMN ] column [ RESTRICT | CASCADE ]
ALTER [ COLUMN ] column TYPE type [ USING expression ]
ALTER [ COLUMN ] column SET DEFAULT expression
ALTER [ COLUMN ] column DROP DEFAULT
ALTER [ COLUMN ] column { SET | DROP } NOT NULL
ALTER [ COLUMN ] column SET STATISTICS integer
ALTER [ COLUMN ] column SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
ADD table_constraint
DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ]
DISABLE TRIGGER [ trigger_name | ALL | USER ]
ENABLE TRIGGER [ trigger_name | ALL | USER ]
CLUSTER ON index_name
SET WITHOUT CLUSTER
SET WITHOUT OIDS
OWNER TO new_owner
SET TABLESPACE new_tablespace

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 table
depends on the column, for example, foreign key references or
views.

ALTER COLUMN TYPE
This form changes the type of a column of a table. Indexes and
simple table constraints involving the column will be automati-
cally converted to use the new column type by reparsing the
originally supplied expression. The optional USING clause speci-
fies how to compute the new column value from the old; if omit-
ted, the default conversion is the same as an assignment cast
from old data type to new. A USING clause must be provided if
there is no implicit or assignment cast from old to new type.

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 val-
ues 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
(default_statistics_target). For more information on the use of
statistics by the PostgreSQL query planner, refer to the docu-
mentation.

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 external,
compressed data. EXTENDED is the default for most data types
that support non-PLAIN storage. Use of EXTERNAL will make sub-
string operations on text and bytea columns faster, at the
penalty of increased storage space. Note that SET STORAGE
doesn't itself change anything in the table, it just sets the
strategy to be pursued during future table updates. See the
documentation for more information.

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

DROP CONSTRAINT
This form drops the specified constraint on a table.

DISABLE/ENABLE TRIGGER
These forms disable or enable trigger(s) belonging to the table.
A disabled trigger is still known to the system, but is not exe-
cuted when its triggering event occurs. For a deferred trigger,
the enable status is checked when the event occurs, not when the
trigger function is actually executed. One may disable or enable
a single trigger specified by name, or all triggers on the ta-
ble, or only user triggers (this option excludes triggers that
are used to implement foreign key constraints). Disabling or
enabling constraint triggers requires superuser privileges; it
should be done with caution since of course the integrity of the
constraint cannot be guaranteed if the triggers are not exe-
cuted.

CLUSTER
This form selects the default index for future CLUSTER [clus-
ter(7)] operations. It does not actually re-cluster the table.

SET WITHOUT CLUSTER
This form removes the most recently used CLUSTER [cluster(7)]
index specification from the table. This affects future cluster
operations that don't specify an index.

SET WITHOUT OIDS
This form removes the oid system column from the table. This is
exactly equivalent to DROP COLUMN oid RESTRICT, except that it
will not complain if there is already no oid column.

Note that there is no variant of ALTER TABLE that allows OIDs to
be restored to a table once they have been removed.

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

SET TABLESPACE
This form changes the table's tablespace to the specified
tablespace and moves the data file(s) associated with the table
to the new tablespace. Indexes on the table, if any, are not
moved; but they can be moved separately with additional SET
TABLESPACE commands. See also CREATE TABLESPACE [cre-
ate_tablespace(7)].

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.

SET SCHEMA
This form moves the table into another schema. Associated
indexes, constraints, and SERIAL-column sequences are moved as
well.

All the actions except RENAME and SET SCHEMA can be combined into a
list of multiple alterations to apply in parallel. For example, it is
possible to add several columns and/or alter the type of several col-
umns in a single command. This is particularly useful with large
tables, since only one pass over the table need be made.

You must own the table to use ALTER TABLE. To change the schema of a
table, you must also have CREATE privilege on the new schema. To alter
the owner, you must also be a direct or indirect member of the new own-
ing role, and that role must have CREATE privilege on the table's
schema. (These restrictions enforce that altering the owner doesn't do
anything you couldn't do by dropping and recreating the table. How-
ever, a superuser can alter ownership of any table anyway.)

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 indi-
cate that descendant tables are to be altered, but in the cur-
rent 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_inheritance.)

column Name of a new or existing column.

new_column
New name for an existing column.

new_name
New name for the table.

type Data type of the new column, or new data type for an existing
column.

table_constraint
New table constraint for the table.

constraint_name
Name of an existing constraint to drop.

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.

trigger_name
Name of a single trigger to disable or enable.

ALL Disable or enable all triggers belonging to the table. (This
requires superuser privilege if any of the triggers are for for-
eign key constraints.)

USER Disable or enable all triggers belonging to the table except for
foreign key constraint triggers.

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

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

new_tablespace
The name of the tablespace to which the table will be moved.

new_schema
The name of the schema to which the table will be moved.

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

When a column is added with ADD COLUMN, all existing rows in the table
are initialized with the column's default value (NULL if no DEFAULT
clause is specified).

Adding a column with a non-null default or changing the type of an
existing column will require the entire table to be rewritten. This may
take a significant amount of time for a large table; and it will tempo-
rarily require double the disk space.

Adding a CHECK or NOT NULL constraint requires scanning the table to
verify that existing rows meet the constraint.

The main reason for providing the option to specify multiple changes in
a single ALTER TABLE is that multiple table scans or rewrites can
thereby be combined into a single pass over the table.

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.

The fact that ALTER TYPE requires rewriting the whole table is some-
times an advantage, because the rewriting process eliminates any dead
space in the table. For example, to reclaim the space occupied by a
dropped column immediately, the fastest way is

ALTER TABLE table ALTER COLUMN anycol TYPE anytype;

where anycol is any remaining table column and anytype is the same type
that column already has. This results in no semantically-visible
change in the table, but the command forces rewriting, which gets rid
of no-longer-useful data.

The USING option of ALTER TYPE can actually specify any expression
involving the old values of the row; that is, it can refer to other
columns as well as the one being converted. This allows very general
conversions to be done with the ALTER TYPE syntax. Because of this
flexibility, the USING expression is not applied to the column's
default value (if any); the result might not be a constant expression
as required for a default. This means that when there is no implicit
or assignment cast from old to new type, ALTER TYPE may fail to convert
the default even though a USING clause is supplied. In such cases, drop
the default with DROP DEFAULT, perform the ALTER TYPE, and then use SET
DEFAULT to add a suitable new default. Similar considerations apply to
indexes and constraints involving the column.

If a table has any descendant tables, it is not permitted to add,
rename, or change the type of 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 match-
ing the parent.

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

The TRIGGER, CLUSTER, OWNER, and TABLESPACE actions never recurse to
descendant tables; that is, they always act as though ONLY were speci-
fied. Adding a constraint can recurse only for CHECK constraints.

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

Refer to CREATE TABLE [create_table(7)] for a further description of
valid parameters. the documentation has further information on inheri-
tance.

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 change the types of two existing columns in one operation:

ALTER TABLE distributors
ALTER COLUMN address TYPE varchar(80),
ALTER COLUMN name TYPE varchar(100);

To change an integer column containing UNIX timestamps to timestamp
with time zone via a USING clause:

ALTER TABLE foo
ALTER COLUMN foo_timestamp TYPE timestamp with time zone
USING
timestamp with time zone 'epoch' + foo_timestamp * interval '1 second';

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, noting
that a table can only ever have one primary key:

ALTER TABLE distributors ADD PRIMARY KEY (dist_id);

To move a table to a different tablespace:

ALTER TABLE distributors SET TABLESPACE fasttablespace;

To move a table to a different schema:

ALTER TABLE myschema.distributors SET SCHEMA yourschema;

COMPATIBILITY
The ADD, DROP, and SET DEFAULT forms conform with the SQL standard. The
other forms are PostgreSQL extensions of the SQL standard. Also, the
ability to specify more than one manipulation in a single ALTER TABLE
command is an extension.

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

SQL - Language Statements 2010-12-14 ALTER TABLE()

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