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_schemawhere action is one of:
ADD [ COLUMN ] columndata_type [ COLLATE collation ] [ column_constraint [ ... ] ]
DROP [ COLUMN ] [ IF EXISTS ] column [ RESTRICT | CASCADE ]
ALTER [ COLUMN ] column [ SET DATA ] TYPE data_type [ COLLATE collation ] [ 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 ( attribute_option = value [, ... ] )
ALTER [ COLUMN ] column RESET ( attribute_option [, ... ] )
ALTER [ COLUMN ] column SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
ADD table_constraint [ NOT VALID ]
ADD table_constraint_using_index
VALIDATE CONSTRAINT constraint_name
DROP CONSTRAINT [ IF EXISTS ] constraint_name [ RESTRICT | CASCADE ]
DISABLE TRIGGER [ trigger_name | ALL | USER ]
ENABLE TRIGGER [ trigger_name | ALL | USER ]
ENABLE REPLICA TRIGGER trigger_name
ENABLE ALWAYS TRIGGER trigger_name
DISABLE RULE rewrite_rule_name
ENABLE RULE rewrite_rule_name
ENABLE REPLICA RULE rewrite_rule_name
ENABLE ALWAYS RULE rewrite_rule_name
CLUSTER ON index_name
SET WITHOUT CLUSTER
SET WITH OIDS
SET WITHOUT OIDS
SET ( storage_parameter = value [, ... ] )
RESET ( storage_parameter [, ... ] )
INHERIT parent_table
NO INHERIT parent_table
OF type_name
NOT OF
OWNER TO new_owner
SET TABLESPACE new_tablespaceand table_constraint_using_index is:
[ CONSTRAINT constraint_name ]
{ UNIQUE | PRIMARY KEY } USING INDEX index_name
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
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.
DROP COLUMN [ IF EXISTS ]
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.
If IF EXISTS is specified and the column
does not exist, no error is thrown. In this case a notice
is issued instead.
SET DATA 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 COLLATE clause specifies a collation
for the new column; if omitted, the collation is the default for the
new column type.
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 can 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 10000; 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 14.2.
SET ( attribute_option = value [, ... ] ) RESET ( attribute_option [, ... ] )
This form sets or resets per-attribute options. Currently, the only
defined per-attribute options are n_distinct and
n_distinct_inherited, which override the
number-of-distinct-values estimates made by subsequent
ANALYZE
operations. n_distinct affects the statistics for the table
itself, while n_distinct_inherited affects the statistics
gathered for the table plus its inheritance children. When set to a
positive value, ANALYZE will assume that the column contains
exactly the specified number of distinct nonnull values. When set to a
negative value, which must be greater
than or equal to -1, ANALYZE will assume that the number of
distinct nonnull values in the column is linear in the size of the
table; the exact count is to be computed by multiplying the estimated
table size by the absolute value of the given number. For example,
a value of -1 implies that all values in the column are distinct, while
a value of -0.5 implies that each value appears twice on the average.
This can be useful when the size of the table changes over time, since
the multiplication by the number of rows in the table is not performed
until query planning time. Specify a value of 0 to revert to estimating
the number of distinct values normally. For more information on the use
of statistics by the PostgreSQL query
planner, refer to Section 14.2.
SET STORAGE
This form sets the storage mode for a column. This controls whether this
column is held inline or in a secondary TOAST 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
very large text and bytea values run 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 55.2 for more information.
ADD table_constraint [ NOT VALID ]
This form adds a new constraint to a table using the same syntax as
CREATE TABLE, plus the option NOT
VALID, which is currently only allowed for foreign key
constraints.
If the constraint is marked NOT VALID, the
potentially-lengthy initial check to verify that all rows in the table
satisfy the constraint is skipped. The constraint will still be
enforced against subsequent inserts or updates (that is, they'll fail
unless there is a matching row in the referenced table). But the
database will not assume that the constraint holds for all rows in
the table, until it is validated by using the VALIDATE
CONSTRAINT option.
ADD table_constraint_using_index
This form adds a new PRIMARY KEY or UNIQUE
constraint to a table based on an existing unique index. All the
columns of the index will be included in the constraint.
The index cannot have expression columns nor be a partial index.
Also, it must be a b-tree index with default sort ordering. These
restrictions ensure that the index is equivalent to one that would be
built by a regular ADD PRIMARY KEY or ADD UNIQUE
command.
If PRIMARY KEY is specified, and the index's columns are not
already marked NOT NULL, then this command will attempt to
do ALTER COLUMN SET NOT NULL against each such column.
That requires a full table scan to verify the column(s) contain no
nulls. In all other cases, this is a fast operation.
If a constraint name is provided then the index will be renamed to match
the constraint name. Otherwise the constraint will be named the same as
the index.
After this command is executed, the index is "owned" by the
constraint, in the same way as if the index had been built by
a regular ADD PRIMARY KEY or ADD UNIQUE
command. In particular, dropping the constraint will make the index
disappear too.
Note: Adding a constraint using an existing index can be helpful in
situations where a new constraint needs to be added without blocking
table updates for a long time. To do that, create the index using
CREATE INDEX CONCURRENTLY, and then install it as an
official constraint using this syntax. See the example below.
VALIDATE CONSTRAINT
This form validates a foreign key constraint that was previously created
as NOT VALID, by scanning the table to ensure there
are no unmatched rows. Nothing happens if the constraint is
already marked valid.
Validation can be a long process on larger tables and currently requires
an ACCESS EXCLUSIVE lock. The value of separating
validation from initial creation is that you can defer validation to less
busy times, or can be used to give additional time to correct pre-existing
errors while preventing new errors.
DROP CONSTRAINT [ IF EXISTS ]
This form drops the specified constraint on a table.
If IF EXISTS is specified and the constraint
does not exist, no error is thrown. In this case a notice is issued instead.
DISABLE/ENABLE [ REPLICA | ALWAYS ] TRIGGER
These forms configure the firing of 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 can disable or enable a single
trigger specified by name, or all triggers on the table, or only
user triggers (this option excludes internally generated constraint
triggers such as those that are used to implement foreign key
constraints or deferrable uniqueness and exclusion constraints).
Disabling or enabling internally generated 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.
The trigger firing mechanism is also affected by the configuration
variable session_replication_role. Simply enabled
triggers will fire when the replication role is "origin"
(the default) or "local". Triggers configured as ENABLE
REPLICA will only fire if the session is in "replica"
mode, and triggers configured as ENABLE ALWAYS will
fire regardless of the current replication mode.
DISABLE/ENABLE [ REPLICA | ALWAYS ] RULE
These forms configure the firing of rewrite rules belonging to the table.
A disabled rule is still known to the system, but is not applied
during query rewriting. The semantics are as for disabled/enabled
triggers. This configuration is ignored for ON SELECT rules, which
are always applied in order to keep views working even if the current
session is in a non-default replication role.
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 WITH OIDS
This form adds an oid system column to the
table (see Section 5.4).
It does nothing if the table already has OIDs.
Note that this is not equivalent to ADD COLUMN oid oid;
that would add a normal column that happened to be named
oid, not a system column.
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.
SET ( storage_parameter = value [, ... ] )
This form changes one or more storage parameters for the table. See
Storage Parameters
for details on the available parameters. Note that the table contents
will not be modified immediately by this command; depending on the
parameter you might need to rewrite the table to get the desired effects.
That can be done with VACUUM
FULL, CLUSTER or one of the forms
of ALTER TABLE that forces a table rewrite.
Note: While CREATE TABLE allows OIDS to be specified
in the WITH (storage_parameter) syntax,
ALTER TABLE does not treat OIDS as a
storage parameter. Instead use the SET WITH OIDS
and SET WITHOUT OIDS forms to change OID status.
RESET ( storage_parameter [, ... ] )
This form resets one or more storage parameters to their
defaults. As with SET, a table rewrite might 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 might 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.
OF type_name
This form links the table to a composite type as though CREATE
TABLE OF had formed it. The table's list of column names and types
must precisely match that of the composite type; the presence of
an oid system column is permitted to differ. The table must
not inherit from any other table. These restrictions ensure
that CREATE TABLE OF would permit an equivalent table
definition.
NOT OF
This form dissociates a typed table from its type.
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.)
Parameters
name
The name (optionally schema-qualified) of an existing table to
alter. If ONLY is specified before the table name, only
that table is altered. If ONLY is not specified, the table
and all its descendant tables (if any) are altered. Optionally,
* can be specified after the table name to explicitly
indicate that descendant tables are included.
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
internally generated constraint triggers such as those that are used
to implement foreign key constraints or deferrable uniqueness and
exclusion constraints.)
USER
Disable or enable all triggers belonging to the table except for
internally generated constraint triggers such as those that are used
to implement foreign key constraints or deferrable uniqueness and
exclusion constraints.
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.
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 and indexes to be rewritten.
As an exception, if the USING clause does not change the column
contents and the old type is either binary coercible to the new type or
an unconstrained domain over the new type, a table rewrite is not needed,
but any indexes on the affected columns must still be rebuilt. Adding or
removing a system oid column also requires rewriting the entire
table. Table and/or index rebuilds may take a significant amount of time
for a large table; and will temporarily require as much as double the disk
space.
Adding a CHECK or NOT NULL constraint requires
scanning the table to verify that existing rows meet the constraint,
but does not require a table rewrite.
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. (These statements do
not apply when dropping the system oid column; that is done
with an immediate rewrite.)
To force immediate reclamation of space occupied by a dropped column,
you can execute one of the forms of ALTER TABLE that
performs a rewrite of the whole table. This results in reconstructing
each row with the dropped column replaced by a null value.
The rewriting forms of ALTER TABLE are not MVCC-safe.
After a table rewrite, the table will appear empty to concurrent
transactions, if they are using a snapshot taken before the rewrite
occurred. See Section 13.5 for more details.
The USING option of SET DATA 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 SET DATA 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, SET DATA TYPE might 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,
and is required to do so for such 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 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 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 SET DATA 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 and all its children:
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 remove a check constraint from one table only:
ALTER TABLE ONLY distributors DROP CONSTRAINT zipchk;
(The check constraint remains in place for any child tables.)
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;
To recreate a primary key constraint, without blocking updates while the
index is rebuilt:
CREATE UNIQUE INDEX CONCURRENTLY dist_id_temp_idx ON distributors (dist_id);
ALTER TABLE distributors DROP CONSTRAINT distributors_pkey,
ADD CONSTRAINT distributors_pkey PRIMARY KEY USING INDEX dist_id_temp_idx;
Compatibility
The forms ADD (without USING INDEX),
DROP, SET DEFAULT,
and SET DATA TYPE (without USING)
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.