Description
ALTER TABLE changes the definition of an existing table.
There are several sub-forms:
- ADD COLUMN
This form adds a new column to the table using the same syntax as
CREATE TABLE.
- DROP COLUMN
This form drops a column from a table. Note that indexes and
table constraints involving the column will be automatically
dropped as well. You will need to say CASCADE if
anything outside the table depends on the column --- for example,
foreign key references, views, etc.
- SET/DROP DEFAULT
These forms set or remove the default value for a column. Note
that defaults only apply to subsequent INSERT
commands; they do not cause rows already in the table to change.
Defaults may also be created for views, in which case they are
inserted into INSERT statements on the view before
the view's ON INSERT rule is applied.
- SET/DROP NOT NULL
These forms change whether a column is marked to allow NULL
values or to reject NULL values. You may only SET NOT NULL
when the table contains no null values in the column.
- 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 1000; alternatively, set it
to -1 to revert to using the system default statistics target.
- SET STORAGE
This form sets the storage mode for a column. This controls whether this
column is held inline or in a supplementary 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 all
data types that support it. The use of EXTERNAL will
make substring operations on a TEXT column faster, at the penalty of
increased storage space.
- RENAME
The RENAME forms change the name of a table
(or an index, sequence, or view) or the name of an individual column in
a table. There is no effect on the stored data.
- ADD table_constraint
This form adds a new constraint to a table using the same syntax as
CREATE TABLE.
- DROP CONSTRAINT
This form drops constraints on a table.
Currently, constraints on tables are not required to have unique
names, so there may be more than one constraint matching the specified
name. All such constraints will be dropped.
- OWNER
This form changes the owner of the table, index, sequence or view to the
specified user.
You must own the table to use ALTER TABLE; except for
ALTER TABLE OWNER, which may only be executed by a superuser.
Notes
The keyword COLUMN is noise and can be omitted.
In the current implementation of ADD COLUMN,
default and NOT NULL clauses for the new column are not supported.
The new column always comes into being with all values NULL.
You can use the SET DEFAULT form
of ALTER TABLE to set the default afterwards.
(You may also want to update the already existing rows to the
new default value, using
UPDATE.)
If you want to mark the column non-null, use the SET NOT NULL
form after you've entered non-null values for the column in all rows.
The DROP COLUMN command does not physically remove
the column, but simply makes it invisible to SQL operations. Subsequent
inserts and updates of the table will store a NULL 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 reclaim the space at once, do a dummy UPDATE of all rows
and then vacuum, as in:
UPDATE table SET col = col;
VACUUM FULL table;
If a table has any descendant tables, it is not permitted to ADD or
RENAME a column in the parent table without doing the same to the
descendants --- that is, ALTER TABLE ONLY will be rejected. This
ensures that the descendants always have columns matching the parent.
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.
Changing any part of the schema of a system
catalog is not permitted.
Refer to CREATE TABLE for a further description
of valid arguments.
The PostgreSQL User's Guide has further
information on inheritance.
Usage
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 rename an existing column:
ALTER TABLE distributors RENAME COLUMN address TO city;
To rename an existing table:
ALTER TABLE distributors RENAME TO suppliers;
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:
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);
To remove a check constraint from a table and all its children:
ALTER TABLE distributors DROP CONSTRAINT zipchk;
To add a foreign key constraint to a table:
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses(address) MATCH FULL;
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);
