ALTER TABLE SQL - Language Statements ALTER TABLE change the definition of a table ALTER TABLE ALTER TABLE [ ONLY ] name [ * ] action [, ... ] ALTER TABLE [ ONLY ] name [ * ] RENAME [ COLUMN ] column TO new_column ALTER TABLE name RENAME TO new_name 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 ] SET WITHOUT OIDS OWNER TO new_owner CLUSTER ON index_name SET WITHOUT CLUSTER 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 . 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 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. For more information on the use of statistics by the PostgreSQL query planner, refer to . TOAST per-column storage settings 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, for example, make substring operations on a text column faster, at the penalty of increased storage space. ADD table_constraint This form adds a new constraint to a table using the same syntax as . 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 matching constraints will be dropped. 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, index, sequence, or view to the specified user. CLUSTER This form selects the default index for future operations. SET WITHOUT CLUSTER This form removes the most recently used index specification from the table. 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. All the actions except RENAME 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; except for ALTER TABLE OWNER, which may only be executed by a superuser. 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 .) column Name of a new or existing column. type Data type of the new column, or new data type for an existing column. new_column New name for an existing column. new_name 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. index_name The index name on which the table should be marked for clustering. 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. 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. 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. Changing any part of a system catalog table is not permitted. Refer to for a further description of valid parameters. has further information on inheritance. 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 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); 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.