ALTER TABLE
Synopsis
Use the ALTER TABLE statement to change the definition of a table.
Syntax
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
| 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
alter_table_action
alter_table_constraint
alter_column_constraint
table_expr
Table inheritance is not yet supported
YSQL in the present "latest" YugabyteDB does not yet support the "table inheritance" feature that is described in the PostgreSQL documentation. The attempt to create a table that inherits another table causes the 0A000 (feature_not_supported) error with the message "INHERITS not supported yet". This means that the syntax that the table_expr rule allows doesn't not yet bring any useful meaning.
It says that you can write, for example, this:
alter table t * add column y text;
or this:
alter table only t add column y text;
These variants are useful only when at least one other table inherits t. But as yet, no table can inherit t. This means that if the unadorned variant alter table t... runs without error, then each of these variants will run without error too. But the effect of each is the same as that of the unadorned variant. Until inheritance is supported, use a bare table_name.
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.
Note
Renaming a table is a non blocking metadata change operation.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 metacommand 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.
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.