ALTER TABLE

## Name

ALTER TABLE — change the definition of a table

## Synopsis

    ALTER TABLE [ ONLY ] name [ * ]
        action [, ... ]
    ALTER TABLE [ ONLY ] name [ * ]
        RENAME [ COLUMN ] column TO new_column
    ALTER TABLE name
        RENAME TO new_name
    ALTER TABLE name
        SET SCHEMA new_schema

    where action is one of:

        ADD [ COLUMN ] column type [ column_constraint [ ... ] ]
        DROP [ COLUMN ] column [ RESTRICT | CASCADE ]
        ALTER [ COLUMN ] column TYPE type [ USING expression ]
        ALTER [ COLUMN ] column SET DEFAULT expression
        ALTER [ COLUMN ] column DROP DEFAULT
        ALTER [ COLUMN ] column { SET | DROP } NOT NULL
        ALTER [ COLUMN ] column SET STATISTICS integer
        ALTER [ COLUMN ] column SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
        ADD table_constraint
        DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ]
        DISABLE TRIGGER [ trigger_name | ALL | USER ]
        ENABLE TRIGGER [ trigger_name | ALL | USER ]
        CLUSTER ON index_name
        SET WITHOUT CLUSTER
        SET WITHOUT OIDS
        OWNER TO new_owner
        SET TABLESPACE new_tablespace

## Description

`ALTER TABLE` changes the definition of an existing table. There are several subforms:

`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. 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 or views.

`ALTER COLUMN 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 `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. The default values 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 can only use `SET NOT NULL` when the column contains no null values.

`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 (default_statistics_target). For more information on the use of statistics by the PostgreSQL query planner, refer to Section 13.2, “Statistics Used by the Planner”.

`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 most data types that support non-`PLAIN` storage. Use of `EXTERNAL` will make substring operations on `text` and `bytea` columns 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 50.2, “TOAST” for more information.

`ADD `table_constraint``

  This form adds a new constraint to a table using the same syntax as CREATE TABLE.

`DROP CONSTRAINT`

  This form drops the specified constraint on a table.

`DISABLE`/`ENABLE TRIGGER`

  These forms disable or enable 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 may disable or enable a single trigger specified by name, or all triggers on the table, or only user triggers (this option excludes triggers that are used to implement foreign key constraints). Disabling or enabling 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.

`CLUSTER`

  This form selects the default index for future CLUSTER operations. It does not actually re-cluster the table.

`SET WITHOUT CLUSTER`

  This form removes the most recently used CLUSTER index specification from the table. This affects future cluster operations that don't specify an index.

`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.

  Note that there is no variant of `ALTER TABLE` that allows OIDs to be restored to a table once they have been removed.

`OWNER`

  This form changes the owner of the table, sequence, or view to the specified user.

`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. See also CREATE TABLESPACE.

`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.

`SET SCHEMA`

  This form moves the table into another schema. Associated indexes, constraints, and SERIAL-column sequences are moved as well.

All the actions except `RENAME` and `SET SCHEMA` can be combined into a list of multiple alterations to apply in parallel. 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 of a table, you must also have `CREATE` privilege on the new schema. 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.)

## Parameters

`name`

  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 altered, 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 configuration parameter sql_inheritance.)

`column`

  Name of a new or existing column.

`new_column`

  New name for an existing column.

`new_name`

  New name for the table.

`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 an existing constraint to drop.

`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.

`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 for foreign key constraints.)

`USER`

  Disable or enable all triggers belonging to the table except for foreign key constraint triggers.

`index_name`

  The index name on which the table should be marked for clustering.

`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.

## 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).

Adding a column with a non-null default or changing the type of an existing column will require the entire table to be rewritten. This may take a significant amount of time for a large table; and it will temporarily require double the disk space.

Adding a `CHECK` or `NOT NULL` constraint requires scanning the table to verify that existing rows meet the 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.

The fact that `ALTER TYPE` requires rewriting the whole table is sometimes an advantage, because the rewriting process eliminates any dead space in the table. For example, to reclaim the space occupied by a dropped column immediately, the fastest way is

    ALTER TABLE table ALTER COLUMN anycol TYPE anytype;

where `anycol` is any remaining table column and `anytype` is the same type that column already has. This results in no semantically-visible change in the table, but the command forces rewriting, which gets rid of no-longer-useful data.

The `USING` option of `ALTER 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 `ALTER 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, `ALTER TYPE` may 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 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.

The `TRIGGER`, `CLUSTER`, `OWNER`, and `TABLESPACE` actions never recurse to descendant tables; that is, they always act as though `ONLY` were specified. Adding a constraint can recurse only for `CHECK` constraints.

Changing any part of a system catalog table is not permitted.

Refer to CREATE TABLE for a further description of valid parameters. Chapter 5, Data Definition 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 TYPE timestamp with time zone
        USING
            timestamp with time zone 'epoch' + foo_timestamp * interval '1 second';

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);

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;

## Compatibility

The `ADD`, `DROP`, and `SET DEFAULT` forms 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.