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ALTER TABLE(7) PostgreSQL 14.23 Documentation ALTER TABLE(7)

NAME
ALTER_TABLE - change the definition of a table

SYNOPSIS
ALTER TABLE [ IF EXISTS ] [ ONLY ] name [ * ]
action [, ... ]
ALTER TABLE [ IF EXISTS ] [ ONLY ] name [ * ]
RENAME [ COLUMN ] column_name TO new_column_name
ALTER TABLE [ IF EXISTS ] [ ONLY ] name [ * ]
RENAME CONSTRAINT constraint_name TO new_constraint_name
ALTER TABLE [ IF EXISTS ] name
RENAME TO new_name
ALTER TABLE [ IF EXISTS ] name
SET SCHEMA new_schema
ALTER TABLE ALL IN TABLESPACE name [ OWNED BY role_name [, ... ] ]
SET TABLESPACE new_tablespace [ NOWAIT ]
ALTER TABLE [ IF EXISTS ] name
ATTACH PARTITION partition_name { FOR VALUES partition_bound_spec | DEFAULT }
ALTER TABLE [ IF EXISTS ] name
DETACH PARTITION partition_name [ CONCURRENTLY | FINALIZE ]

where action is one of:

ADD [ COLUMN ] [ IF NOT EXISTS ] column_name data_type [ COLLATE collation ] [ column_constraint [ ... ] ]
DROP [ COLUMN ] [ IF EXISTS ] column_name [ RESTRICT | CASCADE ]
ALTER [ COLUMN ] column_name [ SET DATA ] TYPE data_type [ COLLATE collation ] [ USING expression ]
ALTER [ COLUMN ] column_name SET DEFAULT expression
ALTER [ COLUMN ] column_name DROP DEFAULT
ALTER [ COLUMN ] column_name { SET | DROP } NOT NULL
ALTER [ COLUMN ] column_name DROP EXPRESSION [ IF EXISTS ]
ALTER [ COLUMN ] column_name ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( sequence_options ) ]
ALTER [ COLUMN ] column_name { SET GENERATED { ALWAYS | BY DEFAULT } | SET sequence_option | RESTART [ [ WITH ] restart ] } [...]
ALTER [ COLUMN ] column_name DROP IDENTITY [ IF EXISTS ]
ALTER [ COLUMN ] column_name SET STATISTICS integer
ALTER [ COLUMN ] column_name SET ( attribute_option = value [, ... ] )
ALTER [ COLUMN ] column_name RESET ( attribute_option [, ... ] )
ALTER [ COLUMN ] column_name SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
ALTER [ COLUMN ] column_name SET COMPRESSION compression_method
ADD table_constraint [ NOT VALID ]
ADD table_constraint_using_index
ALTER CONSTRAINT constraint_name [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
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
DISABLE ROW LEVEL SECURITY
ENABLE ROW LEVEL SECURITY
FORCE ROW LEVEL SECURITY
NO FORCE ROW LEVEL SECURITY
CLUSTER ON index_name
SET WITHOUT CLUSTER
SET WITHOUT OIDS
SET TABLESPACE new_tablespace
SET { LOGGED | UNLOGGED }
SET ( storage_parameter [= value] [, ... ] )
RESET ( storage_parameter [, ... ] )
INHERIT parent_table
NO INHERIT parent_table
OF type_name
NOT OF
OWNER TO { new_owner | CURRENT_ROLE | CURRENT_USER | SESSION_USER }
REPLICA IDENTITY { DEFAULT | USING INDEX index_name | FULL | NOTHING }

and partition_bound_spec is:

IN ( partition_bound_expr [, ...] ) |
FROM ( { partition_bound_expr | MINVALUE | MAXVALUE } [, ...] )
TO ( { partition_bound_expr | MINVALUE | MAXVALUE } [, ...] ) |
WITH ( MODULUS numeric_literal, REMAINDER numeric_literal )

and column_constraint is:

[ CONSTRAINT constraint_name ]
{ NOT NULL |
NULL |
CHECK ( expression ) [ NO INHERIT ] |
DEFAULT default_expr |
GENERATED ALWAYS AS ( generation_expr ) STORED |
GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( sequence_options ) ] |
UNIQUE index_parameters |
PRIMARY KEY index_parameters |
REFERENCES reftable [ ( refcolumn ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
[ ON DELETE referential_action ] [ ON UPDATE referential_action ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

and table_constraint is:

[ CONSTRAINT constraint_name ]
{ CHECK ( expression ) [ NO INHERIT ] |
UNIQUE ( column_name [, ... ] ) index_parameters |
PRIMARY KEY ( column_name [, ... ] ) index_parameters |
EXCLUDE [ USING index_method ] ( exclude_element WITH operator [, ... ] ) index_parameters [ WHERE ( predicate ) ] |
FOREIGN KEY ( column_name [, ... ] ) REFERENCES reftable [ ( refcolumn [, ... ] ) ]
[ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE referential_action ] [ ON UPDATE referential_action ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

and table_constraint_using_index is:

[ CONSTRAINT constraint_name ]
{ UNIQUE | PRIMARY KEY } USING INDEX index_name
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

index_parameters in UNIQUE, PRIMARY KEY, and EXCLUDE constraints are:

[ INCLUDE ( column_name [, ... ] ) ]
[ WITH ( storage_parameter [= value] [, ... ] ) ]
[ USING INDEX TABLESPACE tablespace_name ]

exclude_element in an EXCLUDE constraint is:

{ column_name | ( expression ) } [ COLLATE collation ] [ opclass [ ( opclass_parameter = value [, ... ] ) ] ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ]

DESCRIPTION
ALTER TABLE changes the definition of an existing table. There are several subforms described
below. Note that the lock level required may differ for each subform. An ACCESS EXCLUSIVE
lock is acquired unless explicitly noted. When multiple subcommands are given, the lock
acquired will be the strictest one required by any subcommand.

ADD [ COLUMN ] [ IF NOT EXISTS ]
This form adds a new column to the table, using the same syntax as CREATE TABLE. If IF
NOT EXISTS is specified and a column already exists with this name, no error is thrown.

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. Multivariate statistics referencing the dropped
column will also be removed if the removal of the column would cause the statistics to
contain data for only a single column. 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 (where removal is equivalent to
setting the default value to NULL). The new default value will only apply in subsequent
INSERT or UPDATE commands; it does not cause rows already in the table to change.

SET/DROP NOT NULL
These forms change whether a column is marked to allow null values or to reject null
values.

SET NOT NULL may only be applied to a column provided none of the records in the table
contain a NULL value for the column. Ordinarily this is checked during the ALTER TABLE by
scanning the entire table; however, if a valid CHECK constraint exists (and is not
dropped in the same command) which proves no NULL can exist, then the table scan is
skipped.

If this table is a partition, one cannot perform DROP NOT NULL on a column if it is
marked NOT NULL in the parent table. To drop the NOT NULL constraint from all the
partitions, perform DROP NOT NULL on the parent table. Even if there is no NOT NULL
constraint on the parent, such a constraint can still be added to individual partitions,
if desired; that is, the children can disallow nulls even if the parent allows them, but
not the other way around.

DROP EXPRESSION [ IF EXISTS ]
This form turns a stored generated column into a normal base column. Existing data in the
columns is retained, but future changes will no longer apply the generation expression.

If DROP EXPRESSION IF EXISTS is specified and the column is not a stored generated
column, no error is thrown. In this case a notice is issued instead.

ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY
SET GENERATED { ALWAYS | BY DEFAULT }
DROP IDENTITY [ IF EXISTS ]
These forms change whether a column is an identity column or change the generation
attribute of an existing identity column. See CREATE TABLE for details. Like SET DEFAULT,
these forms only affect the behavior of subsequent INSERT and UPDATE commands; they do
not cause rows already in the table to change.

If DROP IDENTITY IF EXISTS is specified and the column is not an identity column, no
error is thrown. In this case a notice is issued instead.

SET sequence_option
RESTART
These forms alter the sequence that underlies an existing identity column.
sequence_option is an option supported by ALTER SEQUENCE such as INCREMENT BY.

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 STATISTICS acquires a SHARE UPDATE EXCLUSIVE lock.

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, and for the statistics
gathered for partitioned tables. When the value specified is a positive value, the query
planner will assume that the column contains exactly the specified number of distinct
nonnull values. Fractional values may also be specified by using values below 0 and above
or equal to -1. This instructs the query planner to estimate the number of distinct
values by multiplying the absolute value of the specified number by the estimated number
of rows in the table. 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 average.
This can be useful when the size of the table changes over time. For more information on
the use of statistics by the PostgreSQL query planner, refer to Section 14.2.

Changing per-attribute options acquires a SHARE UPDATE EXCLUSIVE lock.

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 70.2 for more information.

SET COMPRESSION compression_method
This form sets the compression method for a column, determining how values inserted in
future will be compressed (if the storage mode permits compression at all). This does not
cause the table to be rewritten, so existing data may still be compressed with other
compression methods. If the table is restored with pg_restore, then all values are
rewritten with the configured compression method. However, when data is inserted from
another relation (for example, by INSERT ... SELECT), values from the source table are
not necessarily detoasted, so any previously compressed data may retain its existing
compression method, rather than being recompressed with the compression method of the
target column. The supported compression methods are pglz and lz4. (lz4 is available only
if --with-lz4 was used when building PostgreSQL.) In addition, compression_method can be
default, which selects the default behavior of consulting the default_toast_compression
setting at the time of data insertion to determine the method to use.

ADD table_constraint [ NOT VALID ]
This form adds a new constraint to a table using the same constraint syntax as CREATE
TABLE, plus the option NOT VALID, which is currently only allowed for foreign key and
CHECK constraints.

Normally, this form will cause a scan of the table to verify that all existing rows in
the table satisfy the new constraint. But if the NOT VALID option is used, this
potentially-lengthy scan 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, in the case of foreign keys, or they'll fail unless the new row
matches the specified check condition). 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. See Notes below for more information about using the NOT VALID option.

Although most forms of ADD table_constraint require an ACCESS EXCLUSIVE lock, ADD FOREIGN
KEY requires only a SHARE ROW EXCLUSIVE lock. Note that ADD FOREIGN KEY also acquires a
SHARE ROW EXCLUSIVE lock on the referenced table, in addition to the lock on the table on
which the constraint is declared.

Additional restrictions apply when unique or primary key constraints are added to
partitioned tables; see CREATE TABLE. Also, foreign key constraints on partitioned tables
may not be declared NOT VALID at present.

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.

This form is not currently supported on partitioned tables.

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.

ALTER CONSTRAINT
This form alters the attributes of a constraint that was previously created. Currently
only foreign key constraints may be altered.

VALIDATE CONSTRAINT
This form validates a foreign key or check constraint that was previously created as NOT
VALID, by scanning the table to ensure there are no rows for which the constraint is not
satisfied. Nothing happens if the constraint is already marked valid. (See Notes below
for an explanation of the usefulness of this command.)

This command acquires a SHARE UPDATE EXCLUSIVE lock.

DROP CONSTRAINT [ IF EXISTS ]
This form drops the specified constraint on a table, along with any index underlying the
constraint. 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 (the default) 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 role.

The effect of this mechanism is that in the default configuration, triggers do not fire
on replicas. This is useful because if a trigger is used on the origin to propagate data
between tables, then the replication system will also replicate the propagated data, and
the trigger should not fire a second time on the replica, because that would lead to
duplication. However, if a trigger is used for another purpose such as creating external
alerts, then it might be appropriate to set it to ENABLE ALWAYS so that it is also fired
on replicas.

This command acquires a SHARE ROW EXCLUSIVE lock.

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.

The rule firing mechanism is also affected by the configuration variable
session_replication_role, analogous to triggers as described above.

DISABLE/ENABLE ROW LEVEL SECURITY
These forms control the application of row security policies belonging to the table. If
enabled and no policies exist for the table, then a default-deny policy is applied. Note
that policies can exist for a table even if row-level security is disabled. In this case,
the policies will not be applied and the policies will be ignored. See also CREATE
POLICY.

NO FORCE/FORCE ROW LEVEL SECURITY
These forms control the application of row security policies belonging to the table when
the user is the table owner. If enabled, row-level security policies will be applied when
the user is the table owner. If disabled (the default) then row-level security will not
be applied when the user is the table owner. See also CREATE POLICY.

CLUSTER ON
This form selects the default index for future CLUSTER operations. It does not actually
re-cluster the table.

Changing cluster options acquires a SHARE UPDATE EXCLUSIVE lock.

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.

Changing cluster options acquires a SHARE UPDATE EXCLUSIVE lock.

SET WITHOUT OIDS
Backward-compatible syntax for removing the oid system column. As oid system columns
cannot be added anymore, this never has an effect.

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.
When applied to a partitioned table, nothing is moved, but any partitions created
afterwards with CREATE TABLE PARTITION OF will use that tablespace, unless overridden by
a TABLESPACE clause.

All tables in the current database in a tablespace can be moved by using the ALL IN
TABLESPACE form, which will lock all tables to be moved first and then move each one.
This form also supports OWNED BY, which will only move tables owned by the roles
specified. If the NOWAIT option is specified then the command will fail if it is unable
to acquire all of the locks required immediately. Note that system catalogs are not moved
by this command; use ALTER DATABASE or explicit ALTER TABLE invocations instead if
desired. The information_schema relations are not considered part of the system catalogs
and will be moved. See also CREATE TABLESPACE.

SET { LOGGED | UNLOGGED }
This form changes the table from unlogged to logged or vice-versa (see UNLOGGED). It
cannot be applied to a temporary table.

SET ( storage_parameter [= value] [, ... ] )
This form changes one or more storage parameters for the table. See Storage Parameters in
the CREATE TABLE documentation 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. For planner related parameters, changes will take effect from the next time the
table is locked so currently executing queries will not be affected.

SHARE UPDATE EXCLUSIVE lock will be taken for fillfactor, toast and autovacuum storage
parameters, as well as the planner parameter parallel_workers.

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, except those marked non-inheritable (that is, created with ALTER TABLE ... ADD
CONSTRAINT ... NO INHERIT) in the parent, which are ignored; all child-table constraints
matched must not be marked non-inheritable. 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 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 TO
This form changes the owner of the table, sequence, view, materialized view, or foreign
table to the specified user.

REPLICA IDENTITY
This form changes the information which is written to the write-ahead log to identify
rows which are updated or deleted. In most cases, the old value of each column is only
logged if it differs from the new value; however, if the old value is stored externally,
it is always logged regardless of whether it changed. This option has no effect except
when logical replication is in use.

DEFAULT
Records the old values of the columns of the primary key, if any. This is the default
for non-system tables.

USING INDEX index_name
Records the old values of the columns covered by the named index, that must be
unique, not partial, not deferrable, and include only columns marked NOT NULL. If
this index is dropped, the behavior is the same as NOTHING.

FULL
Records the old values of all columns in the row.

NOTHING
Records no information about the old row. This is the default for system tables.

RENAME
The RENAME forms change the name of a table (or an index, sequence, view, materialized
view, or foreign table), the name of an individual column in a table, or the name of a
constraint of the table. When renaming a constraint that has an underlying index, the
index is renamed as well. 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.

ATTACH PARTITION partition_name { FOR VALUES partition_bound_spec | DEFAULT }
This form attaches an existing table (which might itself be partitioned) as a partition
of the target table. The table can be attached as a partition for specific values using
FOR VALUES or as a default partition by using DEFAULT. For each index in the target
table, a corresponding one will be created in the attached table; or, if an equivalent
index already exists, it will be attached to the target table's index, as if ALTER INDEX
ATTACH PARTITION had been executed. Note that if the existing table is a foreign table,
it is currently not allowed to attach the table as a partition of the target table if
there are UNIQUE indexes on the target table. (See also CREATE FOREIGN TABLE
(CREATE_FOREIGN_TABLE(7)).) For each user-defined row-level trigger that exists in the
target table, a corresponding one is created in the attached table.

A partition using FOR VALUES uses same syntax for partition_bound_spec as CREATE TABLE.
The partition bound specification must correspond to the partitioning strategy and
partition key of the target table. The table to be attached must have all the same
columns as the target table and no more; moreover, the column types must also match.
Also, it must have all the NOT NULL and CHECK constraints of the target table, not marked
NO INHERIT. Currently FOREIGN KEY constraints are not considered. UNIQUE and PRIMARY KEY
constraints from the parent table will be created in the partition, if they don't already
exist.

If the new partition is a regular table, a full table scan is performed to check that
existing rows in the table do not violate the partition constraint. It is possible to
avoid this scan by adding a valid CHECK constraint to the table that allows only rows
satisfying the desired partition constraint before running this command. The CHECK
constraint will be used to determine that the table need not be scanned to validate the
partition constraint. This does not work, however, if any of the partition keys is an
expression and the partition does not accept NULL values. If attaching a list partition
that will not accept NULL values, also add a NOT NULL constraint to the partition key
column, unless it's an expression.

If the new partition is a foreign table, nothing is done to verify that all the rows in
the foreign table obey the partition constraint. (See the discussion in CREATE FOREIGN
TABLE (CREATE_FOREIGN_TABLE(7)) about constraints on the foreign table.)

When a table has a default partition, defining a new partition changes the partition
constraint for the default partition. The default partition can't contain any rows that
would need to be moved to the new partition, and will be scanned to verify that none are
present. This scan, like the scan of the new partition, can be avoided if an appropriate
CHECK constraint is present. Also like the scan of the new partition, it is always
skipped when the default partition is a foreign table.

Attaching a partition acquires a SHARE UPDATE EXCLUSIVE lock on the parent table, in
addition to the ACCESS EXCLUSIVE locks on the table being attached and on the default
partition (if any).

Further locks must also be held on all sub-partitions if the table being attached is
itself a partitioned table. Likewise if the default partition is itself a partitioned
table. The locking of the sub-partitions can be avoided by adding a CHECK constraint as
described in Section 5.11.2.2.

DETACH PARTITION partition_name [ CONCURRENTLY | FINALIZE ]
This form detaches the specified partition of the target table. The detached partition
continues to exist as a standalone table, but no longer has any ties to the table from
which it was detached. Any indexes that were attached to the target table's indexes are
detached. Any triggers that were created as clones of those in the target table are
removed. SHARE lock is obtained on any tables that reference this partitioned table in
foreign key constraints.

If CONCURRENTLY is specified, it runs using a reduced lock level to avoid blocking other
sessions that might be accessing the partitioned table. In this mode, two transactions
are used internally. During the first transaction, a SHARE UPDATE EXCLUSIVE lock is taken
on both parent table and partition, and the partition is marked as undergoing detach; at
that point, the transaction is committed and all other transactions using the partitioned
table are waited for. Once all those transactions have completed, the second transaction
acquires SHARE UPDATE EXCLUSIVE on the partitioned table and ACCESS EXCLUSIVE on the
partition, and the detach process completes. A CHECK constraint that duplicates the
partition constraint is added to the partition. CONCURRENTLY cannot be run in a
transaction block and is not allowed if the partitioned table contains a default
partition.

If FINALIZE is specified, a previous DETACH CONCURRENTLY invocation that was canceled or
interrupted is completed. At most one partition in a partitioned table can be pending
detach at a time.

All the forms of ALTER TABLE that act on a single table, except RENAME, SET SCHEMA, ATTACH
PARTITION, and DETACH PARTITION can be combined into a list of multiple alterations to be
applied together. 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 or tablespace of a table, you
must also have CREATE privilege on the new schema or tablespace. To add the table as a new
child of a parent table, you must own the parent table as well. Also, to attach a table as a
new partition of the table, you must own the table being attached. 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.) To add a column or alter a column type or
use the OF clause, you must also have USAGE privilege on the data type.

PARAMETERS
IF EXISTS
Do not throw an error if the table does not exist. A notice is issued in this case.

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
Name of a new or existing column.

new_column_name
New name for an existing column.

new_name
New name for the table.

data_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 a new or existing constraint.

CASCADE
Automatically drop objects that depend on the dropped column or constraint (for example,
views referencing the column), and in turn all objects that depend on those objects (see
Section 5.14).

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 name of an existing index.

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.

partition_name
The name of the table to attach as a new partition or to detach from this table.

partition_bound_spec
The partition bound specification for a new partition. Refer to CREATE TABLE
(CREATE_TABLE(7)) for more details on the syntax of the same.

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

When a column is added with ADD COLUMN and a non-volatile DEFAULT is specified, the default
is evaluated at the time of the statement and the result stored in the table's metadata. That
value will be used for the column for all existing rows. If no DEFAULT is specified, NULL is
used. In neither case is a rewrite of the table required.

Adding a column with a volatile DEFAULT or changing the type of an existing column will
require the entire table and its indexes to be rewritten. As an exception, when changing the
type of an existing column, 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. 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.

Similarly, when attaching a new partition it may be scanned to verify that existing rows meet
the partition 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.

Scanning a large table to verify a new foreign key or check constraint can take a long time,
and other updates to the table are locked out until the ALTER TABLE ADD CONSTRAINT command is
committed. The main purpose of the NOT VALID constraint option is to reduce the impact of
adding a constraint on concurrent updates. With NOT VALID, the ADD CONSTRAINT command does
not scan the table and can be committed immediately. After that, a VALIDATE CONSTRAINT
command can be issued to verify that existing rows satisfy the constraint. The validation
step does not need to lock out concurrent updates, since it knows that other transactions
will be enforcing the constraint for rows that they insert or update; only pre-existing rows
need to be checked. Hence, validation acquires only a SHARE UPDATE EXCLUSIVE lock on the
table being altered. (If the constraint is a foreign key then a ROW SHARE lock is also
required on the table referenced by the constraint.) In addition to improving concurrency, it
can be useful to use NOT VALID and VALIDATE CONSTRAINT in cases where the table is known to
contain pre-existing violations. Once the constraint is in place, no new violations can be
inserted, and the existing problems can be corrected at leisure until VALIDATE CONSTRAINT
finally succeeds.

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 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. This ensures that
the descendants always have columns matching the parent. Similarly, a CHECK constraint cannot
be renamed in the parent without also renaming it in all descendants, so that CHECK
constraints also match between the parent and its descendants. (That restriction does not
apply to index-based constraints, however.) Also, because selecting from the parent also
selects from its descendants, a constraint on the parent cannot be marked valid unless it is
also marked valid for those descendants. In all of these cases, ALTER TABLE ONLY will be
rejected.

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. A nonrecursive DROP COLUMN command will fail for a partitioned table, because
all partitions of a table must have the same columns as the partitioning root.

The actions for identity columns (ADD GENERATED, SET etc., DROP IDENTITY), as well as the
actions TRIGGER, CLUSTER, OWNER, and TABLESPACE never recurse to descendant tables; that is,
they always act as though ONLY were specified. Adding a constraint recurses only for CHECK
constraints that are not marked NO INHERIT.

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.
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);

That will cause all existing rows in the table to be filled with null values for the new
column.

To add a column with a non-null default:

ALTER TABLE measurements
ADD COLUMN mtime timestamp with time zone DEFAULT now();

Existing rows will be filled with the current time as the value of the new column, and then
new rows will receive the time of their insertion.

To add a column and fill it with a value different from the default to be used later:

ALTER TABLE transactions
ADD COLUMN status varchar(30) DEFAULT 'old',
ALTER COLUMN status SET default 'current';

Existing rows will be filled with old, but then the default for subsequent commands will be
current. The effects are the same as if the two sub-commands had been issued in separate
ALTER TABLE commands.

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 rename an existing constraint:

ALTER TABLE distributors RENAME CONSTRAINT zipchk TO zip_check;

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 add a check constraint only to a table and not to its children:

ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5) NO INHERIT;

(The check constraint will not be inherited by future children, either.)

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);

To add a foreign key constraint to a table with the least impact on other work:

ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) NOT VALID;
ALTER TABLE distributors VALIDATE CONSTRAINT distfk;

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;

To attach a partition to a range-partitioned table:

ALTER TABLE measurement
ATTACH PARTITION measurement_y2016m07 FOR VALUES FROM ('2016-07-01') TO ('2016-08-01');

To attach a partition to a list-partitioned table:

ALTER TABLE cities
ATTACH PARTITION cities_ab FOR VALUES IN ('a', 'b');

To attach a partition to a hash-partitioned table:

ALTER TABLE orders
ATTACH PARTITION orders_p4 FOR VALUES WITH (MODULUS 4, REMAINDER 3);

To attach a default partition to a partitioned table:

ALTER TABLE cities
ATTACH PARTITION cities_partdef DEFAULT;

To detach a partition from a partitioned table:

ALTER TABLE measurement
DETACH PARTITION measurement_y2015m12;

COMPATIBILITY
The forms ADD (without USING INDEX), DROP [COLUMN], DROP IDENTITY, RESTART, SET DEFAULT, SET
DATA TYPE (without USING), SET GENERATED, and SET sequence_option 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.