ALTER TABLE

Synopsis

Use the ALTER TABLE statement to change the definition of a table.

Syntax

• Grammar
• Diagram

alter_table ::= ALTER TABLE table_expr alter_table_action [ , ... ]

alter_table_action ::= ADD [ COLUMN ] column_name data_type 
                       [ alter_column_constraint [ ... ] ]
                       | RENAME TO table_name
                       | DROP [ COLUMN ] column_name 
                         [ RESTRICT | CASCADE ]
                       | ADD alter_table_constraint
                       | DROP CONSTRAINT constraint_name 
                         [ RESTRICT | CASCADE ]
                       | RENAME [ COLUMN ] column_name TO column_name
                       | RENAME CONSTRAINT constraint_name TO 
                         constraint_name
                       | DISABLE ROW LEVEL SECURITY
                       | ENABLE ROW LEVEL SECURITY
                       | FORCE ROW LEVEL SECURITY
                       | NO FORCE ROW LEVEL SECURITY

alter_table_constraint ::= [ CONSTRAINT constraint_name ] 
                           { CHECK ( expression )
                             | UNIQUE ( column_names ) 
                               index_parameters
                             | FOREIGN KEY ( column_names ) 
                               references_clause } 
                           [ DEFERRABLE | NOT DEFERRABLE ] 
                           [ INITIALLY DEFERRED
                             | INITIALLY IMMEDIATE ]

alter_column_constraint ::= [ CONSTRAINT constraint_name ] 
                            { NOT NULL
                              | NULL
                              | CHECK ( expression )
                              | DEFAULT expression
                              | UNIQUE index_parameters
                              | references_clause } 
                            [ DEFERRABLE | NOT DEFERRABLE ] 
                            [ INITIALLY DEFERRED
                              | INITIALLY IMMEDIATE ]

table_expr ::= [ ONLY ] table_name [ * ]

alter table t * add column y text;

alter table only t add column y text;

Semantics

alter_table_action

Specify one of the following actions.

ADD [ COLUMN ] column_name data_type constraint

Add the specified column with the specified data type and constraint.

RENAME TO table_name

Rename the table to the specified table name.

DROP [ COLUMN ] column_name [ RESTRICT | CASCADE ]

Drop the named column from the table.

• RESTRICT — Remove only the specified column.
• CASCADE — Remove the specified column and any dependent objects.

Example

Set up and populate a parents-children pair of tables:

drop table if exists children cascade;
drop table if exists parents  cascade;

-- The column "b" models a (natural) business unique key.
create table parents(
  k int primary key,
  b int not null,
  v text not null,
  constraint parents_b_unq unique(b));

create table children(
  parents_b  int  not null,
  k          int  not null,
  v          text not null,

  constraint children_pk primary key(parents_b, k),

  constraint children_fk foreign key(parents_b)
    references parents(b)
    match full
    on delete cascade
    on update restrict);

insert into parents(k, b, v) values (1, 10, 'dog'), (2, 20, 'cat'), (3, 30, 'frog');

insert into children(parents_b, k, v) values
  (10, 1, 'dog-child-a'),
  (10, 2, 'dog-child-b'),
  (10, 3, 'dog-child-c'),
  (20, 1, 'cat-child-a'),
  (20, 2, 'cat-child-b'),
  (20, 3, 'cat-child-c'),
  (30, 1, 'frog-child-a'),
  (30, 2, 'frog-child-b'),
  (30, 3, 'frog-child-c');

select p.v as "p.v", c.v as "c.v"
from parents p inner join children c on c.parents_b = p.b
order by p.b, c.k;

This is the result:

 p.v  |     c.v
------+--------------
 dog  | dog-child-a
 dog  | dog-child-b
 dog  | dog-child-c
 cat  | cat-child-a
 cat  | cat-child-b
 cat  | cat-child-c
 frog | frog-child-a
 frog | frog-child-b
 frog | frog-child-c

The \d children meta-command shows that it has a foreign key that's a dependent object on the column b in the parents table:

Indexes:
    "children_pk" PRIMARY KEY, lsm (parents_b HASH, k ASC)
Foreign-key constraints:
    "children_fk" FOREIGN KEY (parents_b) REFERENCES parents(b) MATCH FULL ON UPDATE RESTRICT ON DELETE CASCADE

This is a contrived example. It is unusual practice (and normally bad practice) to make a foreign key constraint target anything but the column list upon which the parent table's primary key constraint is defined. But there are sometimes defensible reasons to do this.

Now try to drop the column parents.b:

do $body$
declare
  message  text not null := '';
  detail   text not null := '';
begin
  -- Causes error 'cos "cascade" is required.
  alter table parents drop column b;
  assert false, 'Should not get here';
exception
  -- Error 2BP01
  when dependent_objects_still_exist then
    get stacked diagnostics
      message  = message_text,
      detail   = pg_exception_detail;
    assert message = 'cannot drop column b of table parents because other objects depend on it',      'Bad message';
    assert detail  = 'constraint children_fk on table children depends on column b of table parents', 'Bad detail';
end;
$body$;

It finishes without error, showing that the bare alter table parents drop column b, without cascade, fails and causes the message and hint that the code presents. Now repeat the attempt with cascade and observe the result:

alter table parents drop column b cascade;

It quietly succeeds. Now \d children shows that the foreign key constraint children_fk has been transitively dropped.

ADD alter_table_constraint

Add the specified constraint to the table.

DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ]

Drop the named constraint from the table.

• RESTRICT — Remove only the specified constraint.
• CASCADE — Remove the specified constraint and any dependent objects.

RENAME [ COLUMN ] column_name TO column_name

Rename a column to the specified name.

RENAME CONSTRAINT constraint_name TO constraint_name

Rename a constraint to the specified name.

Example

Create a table with a constraint and rename the constraint:

CREATE TABLE test(id BIGSERIAL PRIMARY KEY, a TEXT);
ALTER TABLE test ADD constraint vague_name unique (a);
ALTER TABLE test RENAME CONSTRAINT vague_name TO unique_a_constraint;

ENABLE / DISABLE ROW LEVEL SECURITY

This enables or disables row level security for the table. If enabled and no policies exist for the table, then a default-deny policy is applied. If disabled, then existing policies for the table will not be applied and will be ignored. See CREATE POLICY for details on how to create row level security policies.

FORCE / NO FORCE ROW LEVEL SECURITY

This controls the application of row security policies for 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 CREATE POLICY for details on how to create row level security policies.

Constraints

Specify a table or column constraint.

CONSTRAINT constraint_name

Specify the name of the constraint.

Foreign key

FOREIGN KEY and REFERENCES specify that the set of columns can only contain values that are present in the referenced columns of the referenced table. It is used to enforce referential integrity of data.

Unique

This enforces that the set of columns specified in the UNIQUE constraint are unique in the table, that is, no two rows can have the same values for the set of columns specified in the UNIQUE constraint.

Check

This is used to enforce that data in the specified table meets the requirements specified in the CHECK clause.

Default

This is used to specify a default value for the column. If an INSERT statement does not specify a value for the column, then the default value is used. If no default is specified for a column, then the default is NULL.

Deferrable constraints

Constraints can be deferred using the DEFERRABLE clause. Currently, only foreign key constraints can be deferred in YugabyteDB. A constraint that is not deferrable will be checked after every row within a statement. In the case of deferrable constraints, the checking of the constraint can be postponed until the end of the transaction.

Constraints marked as INITIALLY IMMEDIATE will be checked after every row within a statement.

Constraints marked as INITIALLY DEFERRED will be checked at the end of the transaction.

See also

• CREATE TABLE