ALTER TABLE(7) PostgreSQL 10.1 Documentation ALTER TABLE(7)NAMEALTER_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 held unless
explicitly noted. When multiple subcommands are listed, the lock held
will be the strictest one required from 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 66.2 for more
information.
ADD table_constraint [ NOT VALID ]
This form adds a new constraint to a table using the same syntax as
CREATE TABLE (CREATE_TABLE(7)), plus the option NOT VALID, which is
currently only allowed for foreign key and CHECK 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, in the case of foreign
keys; and they'll fail unless the new row matches the specified
check constraints). 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.
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.
Validation can be a long process on larger tables. 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.
Note also that validation on its own does not prevent normal write
commands against the table while it runs.
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.
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 following planner
related parameters: effective_io_concurrency, parallel_workers,
seq_page_cost, random_page_cost, n_distinct and
n_distinct_inherited.
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. This
option has no effect except when logical replication is in use.
DEFAULT (the default for non-system tables) records the old values
of the columns of the primary key, if any. USING INDEX records the
old values of the columns covered by the named index, which must be
unique, not partial, not deferrable, and include only columns
marked NOT NULL. 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.) In all cases, no old values are
logged unless at least one of the columns that would be logged
differs between the old and new versions of the row.
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 is marked NO INHERIT, the command will
fail; such a constraint 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 no existing row in the table violates the
partition constraint. It is possible to avoid this scan by adding a
valid CHECK constraint to the table that would allow only the rows
satisfying the desired partition constraint before running this
command. It will be determined using such a constraint 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.
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))
PostgreSQL 10.1 2017 ALTER TABLE(7)
