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ALTER
NAME
ALTER TABLE - change the definition of a table
SYNOPSIS
ALTER TABLE [ ONLY ] table [ * ]
ADD [ COLUMN ] column type [ column_constraint [ ... ] ]
ALTER TABLE [ ONLY ] table [ * ]
DROP [ COLUMN ] column [ RESTRICT | CASCADE ]
ALTER TABLE [ ONLY ] table [ * ]
ALTER [ COLUMN ] column { SET DEFAULT value | DROP DEFAULT }
ALTER TABLE [ ONLY ] table [ * ]
ALTER [ COLUMN ] column { SET | DROP } NOT NULL
ALTER TABLE [ ONLY ] table [ * ]
ALTER [ COLUMN ] column SET STATISTICS integer
ALTER TABLE [ ONLY ] table [ * ]
ALTER [ COLUMN ] column SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
ALTER TABLE [ ONLY ] table [ * ]
RENAME [ COLUMN ] column TO new_column
ALTER TABLE table
RENAME TO new_table
ALTER TABLE [ ONLY ] table [ * ]
ADD table_constraint
ALTER TABLE [ ONLY ] table [ * ]
DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ]
ALTER TABLE table
OWNER TO new_owner
INPUTS
table
The name (possibly schema-qualified) of an existing table to alter. If
ONLY is specified, only that table is altered. If ONLY is not
specified, the table and all its descendant tables (if any) are
updated. * can be appended to the table name to indicate that
descendant tables are to be scanned, but in the current version, this
is the default behavior. (In releases before 7.1, ONLY was the default
behavior.) The default can be altered by changing the SQL_INHERITANCE
configuration option.
column
Name of a new or existing column.
type
Type of the new column.
new_column
New name for an existing column.
new_table
New name for the table.
table_constraint
New table constraint for the table.
constraint_name
Name of an existing constraint to drop.
new_owner
The user name of the new owner of the table.
CASCADE
Automatically drop objects that depend on the dropped column or
constraint (for example, views referencing the column).
RESTRICT
Refuse to drop the column or constraint if there are any dependent
objects. This is the default behavior.
OUTPUTS
ALTER TABLE
Message returned from column or table renaming.
ERROR
Message returned if table or column is not available.
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 [create_table(5)].
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 [analyze(5)] 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 [create_table(5)].
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 [update(5)].)
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);
COMPATIBILITY
SQL92
The ADD COLUMN form is compliant with the exception that it does not
support defaults and NOT NULL constraints, as explained above. The ALTER
COLUMN form is in full compliance.
The clauses to rename tables, columns, indexes, and sequences are
PostgreSQL extensions from SQL92.
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Index for
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