Provided by: postgresql-client-10_10.23-0ubuntu0.18.04.2_amd64 
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
ALTER TABLE [ IF EXISTS ] name
DETACH PARTITION partition_name
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 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 }
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 WITH OIDS
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_USER | SESSION_USER }
REPLICA IDENTITY { DEFAULT | USING INDEX index_name | FULL | NOTHING }
and 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
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
(CREATE_TABLE(7)). 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. Default values only apply in
subsequent INSERT or UPDATE commands; they do 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. You can only use SET NOT NULL when the column contains no null values.
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.
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 (CREATE_TABLE(7)) for
details.
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 (ALTER_SEQUENCE(7)) such as
INCREMENT BY.
SET STATISTICS
This form sets the per-column statistics-gathering target for subsequent ANALYZE(7)
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(7) 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.
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 67.2 for more information.
ADD table_constraint [ NOT VALID ]
This form adds a new constraint to a table using the same constraint syntax as CREATE
TABLE (CREATE_TABLE(7)), 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.
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.
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. 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.
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.
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 (CREATE_POLICY(7)).
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
(CREATE_POLICY(7)).
CLUSTER ON
This form selects the default index for future CLUSTER(7) 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(7) 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 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 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. 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 (CREATE_TABLESPACE(7)).
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
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(7) 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 and autovacuum storage
parameters, as well as the planner parameter parallel_workers.
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, 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 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, 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. 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
This form attaches an existing table (which might itself be partitioned) as a
partition of the target table using the same syntax for partition_bound_spec as CREATE
TABLE (CREATE_TABLE(7)). 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. Currently UNIQUE, PRIMARY KEY, and FOREIGN KEY constraints are not
considered. If any of the CHECK constraints of the table being attached are marked NO
INHERIT, the command will fail; such constraints must be recreated without the NO
INHERIT clause.
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 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.)
DETACH PARTITION partition_name
This form detaches 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.
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 5.13).
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, all existing rows in the table are initialized
with the column's default value (NULL if no DEFAULT clause is specified). If there is no
DEFAULT clause, this is merely a metadata change and does not require any immediate update
of the table's data; the added NULL values are supplied on readout, instead.
Adding a column with a DEFAULT clause 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. 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.
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. (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 same to the descendants. This ensures
that the descendants always have columns matching the parent. Similarly, a constraint
cannot be renamed in the parent without also renaming it in all descendants, so that
constraints also match between the parent and its descendants. 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);
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;
Attach a partition to range partitioned table:
ALTER TABLE measurement
ATTACH PARTITION measurement_y2016m07 FOR VALUES FROM ('2016-07-01') TO ('2016-08-01');
Attach a partition to list partitioned table:
ALTER TABLE cities
ATTACH PARTITION cities_ab FOR VALUES IN ('a', 'b');
Detach a partition from 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.
SEE ALSO
CREATE TABLE (CREATE_TABLE(7))