ALTER TABLE

Function

ALTER TABLE modifies tables, including modifying table definitions, renaming tables, renaming specified columns in tables, renaming table constraints, setting table schemas, enabling or disabling row-level access control, and adding or updating multiple columns.

Precautions

Only the table owner or a user granted with the ALTER permission can run the ALTER TABLE command. The system administrator has this permission by default. To modify the owner or schema of a table, you must be a table owner or system administrator and a member of the new owner role.
The tablespace of a partitioned table cannot be modified, but the tablespace of the partition can be modified.
The storage parameter ORIENTATION cannot be modified.
Currently, SET SCHEMA can only set schemas to user schemas. It cannot set a schema to a system internal schema.
Column-store tables support only PARTIAL CLUSTER KEY table-level constraints, but do not support primary and foreign key table-level constraints.
In a column-store table, you can perform ADD COLUMN, ALTER TYPE, SET STATISTICS, DROP COLUMN operations, and change table name and space. The types of new and modified columns should be the Data Types supported by column-store. The USING option of ALTER TYPE only supports constant expression and expression involved in the column.
The column constraints supported by column-store tables include NULL, NOT NULL, and DEFAULT constant values. Only the DEFAULT value can be modified (by using SET DEFAULT and DROP DEFAULT). Currently, NULL and NOT NULL constraints cannot be modified.
Auto-increment columns cannot be added, or a column whose DEFAULT value contains the nextval() expression cannot be added.
Row-access control cannot be enabled for foreign tables and temporary tables.
When you delete a PRIMARY KEY constraint by constraint name, the NOT NULL constraint is not deleted. If necessary, manually delete the NOT NULL constraint.
When JDBC is used, the DEFAULT value can be set through PrepareStatement.

Syntax

Modify the definition of a table.
```
ALTER TABLE [ IF EXISTS ] { table_name [*] | ONLY table_name | ONLY ( table_name ) }
    action [, ... ];
```
There are several clauses of action:
```
column_clause
    | ADD table_constraint [ NOT VALID ]
    | ADD table_constraint_using_index
    | VALIDATE CONSTRAINT constraint_name
    | DROP CONSTRAINT [ IF EXISTS ]  constraint_name [ RESTRICT | CASCADE ]
    | CLUSTER ON index_name
    | SET WITHOUT CLUSTER
    | SET ( {storage_parameter = value} [, ... ] )
    | RESET ( storage_parameter [, ... ] )
    | OWNER TO new_owner
    | SET TABLESPACE new_tablespace
    | SET {COMPRESS|NOCOMPRESS}
    | TO { GROUP groupname | NODE ( nodename [, ... ] ) }
    | ADD NODE ( nodename [, ... ] )
    | DELETE NODE ( nodename [, ... ] )
    | DISABLE TRIGGER [ trigger_name | ALL | USER ]
    | ENABLE TRIGGER [ trigger_name | ALL | USER ]
    | ENABLE REPLICA TRIGGER trigger_name
    | ENABLE ALWAYS TRIGGER trigger_name
    | DISABLE ROW LEVEL SECURITY
    | ENABLE ROW LEVEL SECURITY
    | FORCE ROW LEVEL SECURITY
    | NO FORCE ROW LEVEL SECURITY
```
NOTE:
- ADD table_constraint [ NOT VALID ] Adds a table constraint.
- ADD table_constraint_using_index Adds a primary key constraint or unique constraint to a table based on the existing unique index.
- VALIDATE CONSTRAINT constraint_name Validates a check-class constraint created with the NOT VALID option, and scans the entire table to ensure that all rows meet the constraint. Nothing happens if the constraint is already marked valid.
- DROP CONSTRAINT [ IF EXISTS ] constraint_name [ RESTRICT | CASCADE ] Drops a table constraint.
- CLUSTER ON index_name Selects the default index for future CLUSTER operations. Actually, the table is not re-clustered.
- SET WITHOUT CLUSTER Deletes the most recently used CLUSTER index from the table. This affects future CLUSTER operations that do not specify an index.
- SET ( {storage_parameter = value} [, … ] ) Changes one or more storage parameters for the table.
- RESET ( storage_parameter [, … ] ) Resets one or more storage parameters to their defaults. As with SET, a table rewrite might be needed to update the table entirely.
- OWNER TO new_owner Changes the owner of a table, sequence, or view to the specified user.
- SET TABLESPACE new_tablespace Changes the table's tablespace to the specified tablespace and moves the data files 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 option in ALTER INDEX.
- SET {COMPRESS|NOCOMPRESS} Sets the compression feature of a table. The table compression feature affects only the storage mode of data inserted in a batch subsequently and does not affect storage of existing data. Setting the table compression feature will result in the fact that there are both compressed and uncompressed data in the table. Row-store tables do not support compression.
- TO { GROUP groupname | NODE ( nodename [, … ] ) } The syntax is only available in extended mode (when GUC parameter support_extended_features is on). Exercise caution when enabling the mode. It is mainly used for tools like internal scale-out tools. Common users should not use the mode.
- ADD NODE ( nodename [, … ] ) It is only available for internal scale-out tools. Common users should not use the syntax.
- DELETE NODE ( nodename [, … ] ) It is only available for internal scale-in tools. Common users should not use the syntax.
- DISABLE TRIGGER [ trigger_name | ALL | USER ] Disables a single trigger specified by trigger_name, disables all triggers, or disables only user triggers (excluding internally generated constraint triggers, for example, deferrable unique constraint triggers and exclusion constraint triggers). Exercise caution when using this function because data integrity cannot be ensured as expected if the triggers are not executed.
- | ENABLE TRIGGER [ trigger_name | ALL | USER ] Enables a single trigger specified by trigger_name, enables all triggers, or enables only user triggers.
- | ENABLE REPLICA TRIGGER trigger_name Determines that the trigger firing mechanism is affected by the configuration variable session_replication_role. When the replication role is origin (default value) or local, a simple trigger is fired. When ENABLE REPLICA is configured for a trigger, it is fired only when the session is in replica mode.
- | ENABLE ALWAYS TRIGGER trigger_name Determines that all triggers are fired regardless of the current replication mode.
- | DISABLE/ENABLE ROW LEVEL SECURITY Enables or disables row-level access control for a table. If row-level access control is enabled for a data table but no row-level access control policy is defined, the row-level access to the data table is not affected. If row-level access control for a table is disabled, the row-level access to the table is not affected even if a row-level access control policy has been defined. For details, see CREATE ROW LEVEL SECURITY POLICY.
- | NO FORCE/FORCE ROW LEVEL SECURITY Forcibly enables or disables row-level access control for a table. By default, the table owner is not affected by the row-level access control feature. However, if row-level access control is forcibly enabled, the table owner (excluding system administrators) wil be affected. System administrators are not affected by any row-level access control policies.
- There are several clauses of column_clause:
```
ADD [ COLUMN ] column_name data_type [ compress_mode ] [ COLLATE collation ] [ column_constraint [ ... ] ]    
| MODIFY column_name data_type    
| MODIFY column_name [ CONSTRAINT constraint_name ] NOT NULL [ ENABLE ]
| MODIFY column_name [ CONSTRAINT constraint_name ] NULL
| DROP [ COLUMN ] [ IF EXISTS ] column_name [ RESTRICT | CASCADE ]    
| ALTER [ COLUMN ] column_name [ SET DATA ] TYPE data_type [ COLLATE collation ] [ USING expression ]    
| ALTER [ COLUMN ] column_name { SET DEFAULT expression | DROP DEFAULT }    
| ALTER [ COLUMN ] column_name { SET | DROP } NOT NULL    
| ALTER [ COLUMN ] column_name SET STATISTICS [PERCENT] integer    
| ADD STATISTICS (( column_1_name, column_2_name [, ...] ))    
| DELETE STATISTICS (( column_1_name, column_2_name [, ...] ))    
| ALTER [ COLUMN ] column_name SET ( {attribute_option = value} [, ... ] )    
| ALTER [ COLUMN ] column_name RESET ( attribute_option [, ... ] )    
| ALTER [ COLUMN ] column_name SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
```
  NOTE:
  - ADD [ COLUMN ] column_name data_type [ compress_mode ] [ COLLATE collation ] [ column_constraint [ … ] ] Adds a column to a table. If 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).
  - ADD ( { column_name data_type [ compress_mode ] } [, …] ) Adds columns in the table.
  - MODIFY ( { column_name data_type | column_name [ CONSTRAINT constraint_name ] NOT NULL [ ENABLE ] | column_name [ CONSTRAINT constraint_name ] NULL } [, …] ) Modifies the data type of an existing column in the table.
  - DROP [ COLUMN ] [ IF EXISTS ] column_name [ RESTRICT | CASCADE ] Drops a column from a table. Indexes and constraints related to the column are automatically dropped. If an object not belonging to the table depends on the column, CASCADE must be specified, such as a view. The DROP COLUMN statement 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. Therefore, column deletion takes a short period of time but does not immediately release the tablespace on the disks, because the space occupied by the deleted column is not reclaimed. The space will be reclaimed when VACUUM is executed.
  - ALTER [ COLUMN ] column_name [ SET DATA ] TYPE data_type [ COLLATE collation ] [ USING expression ] Modifies the type of a column in a table. Indexes and simple table constraints on the column will automatically use the new data type by reparsing the originally supplied expression. ALTER TYPE requires an entire table be rewritten. This is an advantage sometimes, because it frees up unnecessary space from a table. For example, to reclaim the space occupied by a deleted column, the fastest method is to use the following statement.
```
ALTER TABLE table ALTER COLUMN anycol TYPE anytype;
```
  In this statement, anycol indicates any column existing in the table and anytype indicates the type of the prototype of the column. ALTER TYPE does not change the table except that the table is forcibly rewritten. In this way, the data that is no longer used is deleted.
  - ALTER [ COLUMN ] column_name { SET DEFAULT expression | DROP DEFAULT } Sets or removes the default value for a column. The default values only apply to subsequent INSERT operations; they do not cause rows already in the table to change. Defaults can 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.
  - ALTER [ COLUMN ] column_name { SET | DROP } NOT NULL Changes 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.
  - ALTER [ COLUMN ] column_name SET STATISTICS [PERCENT] integer Specifies the per-column statistics-gathering target for subsequent ANALYZE operations. The target can be set in the range from 0 to 10000. Set it to -1 to revert to using the default system statistics target.
  - {ADD | DELETE} STATISTICS ((column_1_name, column_2_name [, …])) Adds or deletes the declaration of collecting multi-column statistics to collect multi-column statistics as needed when ANALYZE is performed for a table or a database. The statistics about a maximum of 32 columns can be collected at a time. You are not allowed to add or delete such declaration for system catalogs or foreign tables.
  - ALTER [ COLUMN ] column_name SET ( {attribute_option = value} [, … ] ) ALTER [ COLUMN ] column_name RESET ( attribute_option [, … ] ) Sets or resets per-attribute options. Currently, the only defined per-attribute options are n_distinct and n_distinct_inherited. n_distinct affects statistics of a table, while n_distinct_inherited affects the statistics of the table and its subtables. Currently, only SET/RESET n_distinct is supported, and SET/RESET n_distinct_inherited is forbidden.
  - ALTER [ COLUMN ] column_name SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN } Sets the storage mode for a column. This clause specifies whether this column is held inline or in a secondary TOAST table, and whether the data should be compressed. It is set only for row-store tables and is invalid for column-store tables. If it is set for column-store tables, an error will be displayed when the statement is executed. SET STORAGE itself does not change anything in the table. It sets the strategy to be pursued during future table updates.
  - column_constraint is as follows:
```
[ CONSTRAINT constraint_name ]
    { NOT NULL |
      NULL |
      CHECK ( expression ) |
      DEFAULT default_expr  |
      UNIQUE index_parameters |
      PRIMARY KEY index_parameters |
      ENCRYPTEDWITH(COLUMN_ENCRYPTION_KEY=column_encryption_key,ENCRYPTION_TYPE=encryption_type_value) |
      REFERENCES reftable [ ( refcolumn ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
          [ ON DELETE action ] [ ON UPDATE action ] }}
    [ DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
```
  - compress_mode of a column is as follows:
```
[ DELTA | PREFIX | DICTIONARY | NUMSTR | NOCOMPRESS ]
```
- table_constraint_using_index used to add the primary key constraint or unique constraint based on the unique index is as follows:
```
[ CONSTRAINT constraint_name ]
    { UNIQUE | PRIMARY KEY } USING INDEX index_name
    [ DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
```
- table_constraint is as follows:
```
[ CONSTRAINT constraint_name ]
    { CHECK ( expression ) |
      UNIQUE ( column_name [, ... ] ) index_parameters |
      PRIMARY KEY ( column_name [, ... ] ) index_parameters |
      PARTIAL CLUSTER KEY ( column_name [, ... ] |
      FOREIGN KEY ( column_name [, ... ] ) REFERENCES reftable [ ( refcolumn [, ... ] ) ]
          [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE action ] [ ON UPDATE action ] }
    [ DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
```
  index_parameters is as follows:
```
[ WITH ( {storage_parameter = value} [, ... ] ) ]
    [ USING INDEX TABLESPACE tablespace_name ]
```

Rename a table. The renaming does not affect stored data.

ALTER TABLE [ IF EXISTS ] table_name 
    RENAME TO new_table_name;

Rename the specified column in the table.

ALTER TABLE [ IF EXISTS ] { table_name [*] | ONLY table_name | ONLY ( table_name )}
    RENAME [ COLUMN ] column_name TO new_column_name;

Rename the constraint of the table.

ALTER TABLE [ IF EXISTS ] { table_name [*] | ONLY table_name | ONLY ( table_name ) }
    RENAME CONSTRAINT constraint_name TO new_constraint_name;

Set the schema of the table.
```
ALTER TABLE [ IF EXISTS ] table_name 
    SET SCHEMA new_schema;
```
NOTE:
- The schema setting moves the table into another schema. Associated indexes and constraints owned by table columns are migrated as well. Currently, the schema for sequences cannot be changed. If the table has sequences, delete the sequences, and create them again or delete the ownership between the table and sequences. In this way, the table schema can be changed.
- To change the schema of a table, you must also have the CREATE permission on the new schema. To add the table as a new child of a parent table, you must own the parent table as well. To alter the owner, you must also be a direct or indirect member of the new owning role, and that role must have the CREATE permission on the table's schema. These restrictions enforce that the user can only recreate and delete the table. However, a system administrator can alter the ownership of any table anyway.
- All the actions except for 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 or alter the type of several columns in a single statement. This is useful with large tables, since only one pass over the tables needs to be made.
- Adding a CHECK or NOT NULL constraint will scan the table to validate that existing rows meet the constraint.
- Adding a column with a non-NULL default or changing the type of an existing column will rewrite the entire table. Rewriting a large table may take much time and temporarily needs doubled disk space.

Add columns.

ALTER TABLE [ IF EXISTS ] table_name
    ADD ( { column_name data_type [ compress_mode ] [ COLLATE collation ] [ column_constraint [ ... ] ]} [, ...] );

Update columns.

ALTER TABLE [ IF EXISTS ] table_name 
    MODIFY ( { column_name data_type | column_name [ CONSTRAINT constraint_name ] NOT NULL [ ENABLE ] | column_name [ CONSTRAINT constraint_name ] NULL } [, ...] );

Parameter Description

IF EXISTS
Sends a notice instead of an error if no tables have identical names. The notice prompts that the table you are querying does not exist.
table_name [*] | ONLY table_name | ONLY ( table_name )
table_name is the name of the table that you need to modify.
If ONLY is specified, only the table is modified. If ONLY is not specified, the table and all subtables will be modified. You can add the asterisk (*) option following the table name to specify that all subtables are scanned, which is the default operation.
constraint_name
Specifies the name of an existing constraint to drop.
index_name
Specifies the name of this index.
storage_parameter
Specifies the name of a storage parameter.
new_owner
Specifies the name of the new table owner.
new_tablespace
Specifies the new name of the tablespace to which the table belongs.
column_name, column_1_name, column_2_name
Specifies the name of a new or existing column.
data_type
Specifies the type of a new column or a new type of an existing column.
compress_mode
Compression option of a table field. The clause specifies the compression algorithm preferentially used by the column. Row-store tables do not support compression.
collation
Specifies the collation rule name of a column. The optional COLLATE clause specifies a collation for the new column; if omitted, the collation is the default for the new column. You can run the select * from pg_collation; command to query collation rules from the pg_collation system catalog. The default collation rule is the row starting with default in the query result.
USING expression
Specifies how to compute the new column value from the old; if omitted, the default conversion is an assignment cast from old data type to new. A USING clause must be provided if there is no implicit or assignment cast from the old to new type.
NOTE: USING in ALTER TYPE can specify any expression involving the old values of the row; that is, it can refer to any columns other than the one being cast. This allows general casting 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 might 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.
NOT NULL | NULL
Sets whether the column allows null values.
integer
Specifies the constant value of a signed integer. When using PERCENT, the range of integer is from 0 to 100.
attribute_option
Specifies an attribute option.
PLAIN | EXTERNAL | EXTENDED | MAIN
Specifies a column storage mode.
- PLAIN must be used for fixed-length values (such as integers). It must be inline and uncompressed.
- MAIN is for inline, compressible data.
- EXTERNAL is for external, uncompressed data. Use of EXTERNAL will make substring operations on text and bytea values run faster, at the penalty of increased storage space.
- EXTENDED is for external, compressed data. EXTENDED is the default for most data types that support non-PLAIN storage.
CHECK ( expression )
New rows or rows to be updated must satisfy for an expression to be true. If any row produces a false result, an error is raised and the database is not modified.
A check constraint specified as a column constraint should reference only the column's values, while an expression appearing in a table constraint can reference multiple columns.
Currently, CHECK ( expression ) does not include subqueries and cannot use variables apart from the current column.
DEFAULT default_expr
Assigns a default data value for a column.
The data type of the default expression must match the data type of the column.
The default expression will be used in any insert operation that does not specify a value for the column. If there is no default value for a column, then the default value is null.
UNIQUE index_parameters
UNIQUE ( column_name [, … ] ) index_parameters
Specifies that a group of one or more columns of a table can contain only unique values.
PRIMARY KEY index_parameters
PRIMARY KEY ( column_name [, … ] ) index_parameters
The primary key constraint specifies that a column or columns of a table can contain only unique (non-duplicate) and non-null values.
DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE
Sets whether the constraint can be deferrable.
- DEFERRABLE: deferrable to the end of the transaction and checks using SET CONSTRAINTS.
- NOT DEFERRABLE: checks immediately after the execution of each command.
- INITIALLY IMMEDIATE: checks immediately after the execution of each statement.
- INITIALLY DEFERRED: checks when the transaction ends.
WITH ( {storage_parameter = value} [, … ] )
Specifies an optional storage parameter for a table or an index.
tablespace_name
Specifies the name of the tablespace where the index locates.
COMPRESS|NOCOMPRESS
- NOCOMPRESS: If the NOCOMPRESS keyword is specified, the existing compression feature of the table will not be changed.
- COMPRESS: If the COMPRESS keyword is specified, the table compression feature will be triggered by batch tuple insertion. Row-store tables do not support compression.
new_table_name
Specifies the new table name.
new_column_name
Specifies the new name of a specific column in a table.
new_constraint_name
Specifies the new name of a table constraint.
new_schema
Specifies the new schema name.
CASCADE
Automatically drops objects that depend on the dropped column or constraint (for example, views referencing the column).
RESTRICT
Refuses to drop the column or constraint if there are any dependent objects. This is the default behavior.
schema_name
Specifies the schema name of a table.

Examples

See Example: in CREATE TABLE.

Helpful Links

CREATE TABLE and DROP TABLE