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ALTER TABLE
Synopsis
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.
- 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 49.2 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 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.
- 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, index, 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.
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.
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.
- 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.
- tablespace_name
The tablespace name 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
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.
Changing any part of a system catalog table is not permitted.
Refer to CREATE TABLE for a further description of valid parameters. Chapter 5 has further information on inheritance.
Examples
To add a column of type varchar to a table:
To drop a column from a table:
To change the types of two existing columns in one operation:
To change an integer column containing UNIX timestamps to timestamp with time zone via a USING clause:
To rename an existing column:
To rename an existing table:
To add a not-null constraint to a column:
To remove a not-null constraint from a column:
To add a check constraint to a table:
To remove a check constraint from a table and all its children:
To add a foreign key constraint to a table:
To add a (multicolumn) unique constraint to a table:
To add an automatically named primary key constraint to a table, noting that a table can only ever have one primary key:
To move a table to a different tablespace:
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.
