ALTER TABLE — change the definition of a table
ALTER TABLE [ ONLY ]name
[ * ]action
[, ... ] ALTER TABLE [ ONLY ]name
[ * ] RENAME [ COLUMN ]column
TOnew_column
ALTER TABLEname
RENAME TOnew_name
ALTER TABLEname
SET SCHEMAnew_schema
whereaction
is one of: ADD [ COLUMN ]column
type
[column_constraint
[ ... ] ] DROP [ COLUMN ]column
[ RESTRICT | CASCADE ] ALTER [ COLUMN ]column
TYPEtype
[ USINGexpression
] ALTER [ COLUMN ]column
SET DEFAULTexpression
ALTER [ COLUMN ]column
DROP DEFAULT ALTER [ COLUMN ]column
{ SET | DROP } NOT NULL ALTER [ COLUMN ]column
SET STATISTICSinteger
ALTER [ COLUMN ]column
SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN } ADDtable_constraint
DROP CONSTRAINTconstraint_name
[ RESTRICT | CASCADE ] DISABLE TRIGGER [trigger_name
| ALL | USER ] ENABLE TRIGGER [trigger_name
| ALL | USER ] CLUSTER ONindex_name
SET WITHOUT CLUSTER SET WITHOUT OIDS SET (storage_parameter
=value
[, ... ] ) RESET (storage_parameter
[, ... ] ) INHERITparent_table
NO INHERITparent_table
OWNER TOnew_owner
SET TABLESPACEnew_tablespace
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.
DROP COLUMN
This form drops a column from a table. Indexes and
table constraints 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 automatically
converted to use the new column type by reparsing the originally
supplied expression. The optional USING
clause specifies how to compute the new column value from the old;
if omitted, 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
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 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 Section 13.2, “Statistics Used by the Planner”.
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 substring 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 Section 52.2, “TOAST” for more information.
ADD table_constraint
This form adds a new constraint to a table using the same syntax as CREATE TABLE.
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 executed 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 table, 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 executed.
CLUSTER
This form selects the default index for future CLUSTER operations. It does not actually re-cluster the table.
SET WITHOUT CLUSTER
This form removes the most recently used CLUSTER 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.
SET ( storage_parameter
= value
[, ... ] )
This form changes one or more storage parameters for the table. See
CREATE TABLE
for details on the available parameters. Note that the table contents
will not be modified immediately by this command; depending on the
parameter you may need to rewrite the table to get the desired effects.
That can be done with CLUSTER or one of the forms of ALTER
TABLE
that forces a table rewrite.
While CREATE TABLE
allows OIDS
to be specified
in the WITH (
syntax,
storage_parameter
)ALTER TABLE
does not treat OIDS
as a
storage parameter.
RESET ( storage_parameter
[, ... ] )
This form resets one or more storage parameters to their
defaults. As with SET
, a table rewrite may be
needed to update the table entirely.
INHERIT parent_table
This form adds the target table as a new child of the specified parent
table. Subsequently, queries against the parent will include records
of the target table. To be added as a child, the target table must
already contain all the same columns as the parent (it could have
additional columns, too). The columns must have matching data types,
and if they have NOT NULL
constraints in the parent
then they must also have NOT NULL
constraints in the
child.
There must also be matching child-table constraints for all
CHECK
constraints of the parent. Currently
UNIQUE
, PRIMARY KEY
, and
FOREIGN KEY
constraints are not considered, but
this may change in the future.
NO INHERIT parent_table
This form removes the target table from the list of children of the specified parent table. Queries against the parent table will no longer include records drawn from the target table.
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.
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 table. There is no effect on the stored data.
SET SCHEMA
This form moves the table into another schema. Associated indexes, constraints, and sequences owned by table columns 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
columns 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 add the table as a new child of a parent table, you must own the
parent table as well.
To alter the owner, you must also be a direct or indirect member of the new
owning 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.
However, a superuser can alter ownership of any table anyway.)
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_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 dependent 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 foreign 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 clustering.
storage_parameter
The name of a table storage parameter.
value
The new value for a table storage parameter. This might be a number or a word depending on the parameter.
parent_table
A parent table to associate or de-associate with this table.
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.
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 temporarily 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 sometimes 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 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 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 specified.
Adding a constraint can recurse only for CHECK
constraints.
Changing any part of a system catalog table is not permitted.
Refer to CREATE TABLE for a further description of valid parameters. Chapter 5, Data Definition has further information on inheritance.
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';
The same, when the column has a default expression that won't automatically cast to the new data type:
ALTER TABLE foo ALTER COLUMN foo_timestamp DROP DEFAULT, ALTER COLUMN foo_timestamp TYPE timestamp with time zone USING timestamp with time zone 'epoch' + foo_timestamp * interval '1 second', ALTER COLUMN foo_timestamp SET DEFAULT now();
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;
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 disallows zero-column tables.