ALTER TABLE

Changes the definition of a table.

Synopsis

ALTER TABLE [ONLY] name RENAME [COLUMN] column TO new_column

ALTER TABLE name RENAME TO new_name

ALTER TABLE name SET SCHEMA new_schema

ALTER TABLE [ONLY] name SET 
     DISTRIBUTED BY (column, [ ... ] ) 
   | DISTRIBUTED RANDOMLY 
   | WITH (REORGANIZE=true|false)

ALTER TABLE [ONLY] name
            action [, ... ]

ALTER TABLE name
   [ ALTER PARTITION { partition_name | FOR (RANK(number)) 
   | FOR (value) } partition_action [...] ] 
   partition_action

where action is one of:

  ADD [COLUMN] column_name type
      [ ENCODING ( storage_directive [,…] ) ]
      [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
  ADD table_constraint
  DROP CONSTRAINT constraint_name [RESTRICT | CASCADE]
  SET WITHOUT OIDS
  INHERIT parent_table
  NO INHERIT parent_table
  OWNER TO new_owner

where partition_action is one of:

  ALTER DEFAULT PARTITION
  DROP DEFAULT PARTITION [IF EXISTS]
  DROP PARTITION [IF EXISTS] { partition_name | 
      FOR (RANK(number)) | FOR (value) } [CASCADE]
  TRUNCATE DEFAULT PARTITION
  TRUNCATE PARTITION { partition_name | FOR (RANK(number)) | 
      FOR (value) }
  RENAME DEFAULT PARTITION TO new_partition_name
  RENAME PARTITION { partition_name | FOR (RANK(number)) | 
      FOR (value) } TO new_partition_name
  ADD DEFAULT PARTITION name [ ( subpartition_spec ) ]
  ADD PARTITION name
            partition_element
      [ ( subpartition_spec ) ]
  EXCHANGE DEFAULT PARTITION WITH TABLE table_name
        [ WITH | WITHOUT VALIDATION ]
  EXCHANGE PARTITION { partition_name | FOR (RANK(number)) | 
       FOR (value) } WITH TABLE table_name
        [ WITH | WITHOUT VALIDATION ]
  SET SUBPARTITION TEMPLATE (subpartition_spec)
  SPLIT DEFAULT PARTITION
    {  AT (list_value)
     | START([datatype] range_value) [INCLUSIVE | EXCLUSIVE] 
        END([datatype] range_value) [INCLUSIVE | EXCLUSIVE] }
    [ INTO ( PARTITION new_partition_name, 
             PARTITION default_partition_name ) ]
  SPLIT PARTITION { partition_name | FOR (RANK(number)) | 
     FOR (value) } AT (value) 
    [ INTO (PARTITION partition_name, PARTITION partition_name)]

where partition_element is:

    VALUES (list_value [,...] )
  | START ([datatype] 'start_value') [INCLUSIVE | EXCLUSIVE]
     [ END ([datatype] 'end_value') [INCLUSIVE | EXCLUSIVE] ]
  | END ([datatype] 'end_value') [INCLUSIVE | EXCLUSIVE]
[ WITH ( partition_storage_parameter=value [, ... ] ) ]
[ TABLESPACE tablespace ]

where subpartition_spec is:

            subpartition_element [, ...]

and subpartition_element is:

   DEFAULT SUBPARTITION subpartition_name
  | [SUBPARTITION subpartition_name] VALUES (list_value [,...] )
  | [SUBPARTITION subpartition_name] 
     START ([datatype] 'start_value') [INCLUSIVE | EXCLUSIVE]
     [ END ([datatype] 'end_value') [INCLUSIVE | EXCLUSIVE] ]
     [ EVERY ( [number | datatype] 'interval_value') ]
  | [SUBPARTITION subpartition_name] 
     END ([datatype] 'end_value') [INCLUSIVE | EXCLUSIVE]
     [ EVERY ( [number | datatype] 'interval_value') ]
[ WITH ( partition_storage_parameter=value [, ... ] ) ]
[ TABLESPACE tablespace ]

where storage_parameter is:

   APPENDONLY={TRUE}
   BLOCKSIZE={8192-2097152}
   ORIENTATION={ROW | PARQUET}
   COMPRESSTYPE={ZLIB|SNAPPY|GZIP|NONE}
   COMPRESSLEVEL={0-9}
   FILLFACTOR={10-100}
   OIDS[=TRUE|FALSE]

where storage_directive is:

   COMPRESSTYPE={ZLIB|SNAPPY|GZIP|NONE}} 
 | COMPRESSLEVEL={0-9} 
 | BLOCKSIZE={8192-2097152}

where column_reference_storage_directive is:

   COLUMN column_name ENCODING ( storage_directive [, … ] ), … 
 | DEFAULT COLUMN ENCODING ( storage_directive [, … ] )

Note: When using multi-level partition designs, the following operations are not supported with ALTER TABLE:

  • ADD DEFAULT PARTITION
  • ADD PARTITION
  • DROP DEFAULT PARTITION
  • DROP PARTITION
  • SPLIT PARTITION
  • All operations that involve modifying subpartitions.

Limitations

HAWQ does not support using ALTER TABLE to ADD or DROP a column in an existing Parquet table.

Parameters

ONLY
Only perform the operation on the table name specified. If the ONLY keyword is not used, the operation will be performed on the named table and any child table partitions associated with that table.

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.

Note: Constraints can only be added to an entire table, not to a partition. Because of that restriction, the name parameter can only contain a table name, not a partition name.

column
Name of a new or existing column. Note that HAWQ distribution key columns must be treated with special care. Altering or dropping these columns can change the distribution policy for the table.

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. If changing the data type of a HAWQ distribution key column, you are only allowed to change it to a compatible type (for example, text to varchar is OK, but text to int is not).

table_constraint
New table constraint for the table. Note that foreign key constraints are currently not supported in HAWQ. Also a table is only allowed one unique constraint and the uniqueness must be within the HAWQ distribution key.

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.

ALL
Disable or enable all triggers belonging to the table including constraint related triggers. This requires superuser privilege.

USER
Disable or enable all user-created triggers belonging to the table.

DISTRIBUTED RANDOMLY | DISTRIBUTED BY (column)
Specifies the distribution policy for a table. The default is RANDOM distribution. Changing a distribution policy will cause the table data to be physically redistributed on disk, which can be resource intensive. If you declare the same distribution policy or change from random to hash distribution, data will not be redistributed unless you declare SET WITH (REORGANIZE=true).

REORGANIZE=true|false
Use REORGANIZE=true when the distribution policy has not changed or when you have changed from a random to a hash distribution, and you want to redistribute the data anyways.

parent_table
A parent table to associate or de-associate with this table.

new_owner
The role 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.

parent_table_name
When altering a partitioned table, the name of the top-level parent table.

ALTER [DEFAULT] PARTITION
If altering a partition deeper than the first level of partitions, the ALTER PARTITION clause is used to specify which subpartition in the hierarchy you want to alter.

DROP [DEFAULT] PARTITION
Note: Cannot be used with multi-level partitions.

Drops the specified partition. If the partition has subpartitions, the subpartitions are automatically dropped as well.

TRUNCATE [DEFAULT] PARTITION
Truncates the specified partition. If the partition has subpartitions, the subpartitions are automatically truncated as well.

RENAME [DEFAULT] PARTITION
Changes the partition name of a partition (not the relation name). Partitioned tables are created using the naming convention: < parentname >_< level >_prt_< partition_name >.

ADD DEFAULT PARTITION
Note: Cannot be used with multi-level partitions.

Adds a default partition to an existing partition design. When data does not match to an existing partition, it is inserted into the default partition. Partition designs that do not have a default partition will reject incoming rows that do not match to an existing partition. Default partitions must be given a name.

ADD PARTITION
Note: Cannot be used with multi-level partitions.

partition_element - Using the existing partition type of the table (range or list), defines the boundaries of new partition you are adding.

name - A name for this new partition.

VALUES - For list partitions, defines the value(s) that the partition will contain.

START - For range partitions, defines the starting range value for the partition. By default, start values are INCLUSIVE. For example, if you declared a start date of ’2008-01-01’, then the partition would contain all dates greater than or equal to ’2008-01-01’. Typically the data type of the START expression is the same type as the partition key column. If that is not the case, then you must explicitly cast to the intended data type.

END - For range partitions, defines the ending range value for the partition. By default, end values are EXCLUSIVE. For example, if you declared an end date of ’2008-02-01’, then the partition would contain all dates less than but not equal to ’2008-02-01’. Typically the data type of the END expression is the same type as the partition key column. If that is not the case, then you must explicitly cast to the intended data type.

WITH - Sets the table storage options for a partition. For example, you may want older partitions to be append-only tables and newer partitions to be regular heap tables. See CREATE TABLE for a description of the storage options.

TABLESPACE - The name of the tablespace in which the partition is to be created.

subpartition_spec - Only allowed on partition designs that were created without a subpartition template. Declares a subpartition specification for the new partition you are adding. If the partitioned table was originally defined using a subpartition template, then the template will be used to generate the subpartitions automatically.

EXCHANGE [DEFAULT] PARTITION
Exchanges another table into the partition hierarchy into the place of an existing partition. In a multi-level partition design, you can only exchange the lowest level partitions (those that contain data).

WITH TABLE table_name - The name of the table you are swapping in to the partition design.

WITH | WITHOUT VALIDATION - Validates that the data in the table matches the CHECK constraint of the partition you are exchanging. The default is to validate the data against the CHECK constraint.

SET SUBPARTITION TEMPLATE
Modifies the subpartition template for an existing partition. After a new subpartition template is set, all new partitions added will have the new subpartition design (existing partitions are not modified).

SPLIT DEFAULT PARTITION
Note: Cannot be used with multi-level partitions.

Splits a default partition. In a multi-level partition design, you can only split the lowest level default partitions (those that contain data). Splitting a default partition creates a new partition containing the values specified and leaves the default partition containing any values that do not match to an existing partition.

AT - For list partitioned tables, specifies a single list value that should be used as the criteria for the split.

START - For range partitioned tables, specifies a starting value for the new partition.

END - For range partitioned tables, specifies an ending value for the new partition.

INTO - Allows you to specify a name for the new partition. When using the INTO clause to split a default partition, the second partition name specified should always be that of the existing default partition. If you do not know the name of the default partition, you can look it up using the pg_partitions view.

SPLIT PARTITION
Note: Cannot be used with multi-level partitions.

Splits an existing partition into two partitions. In a multi-level partition design, you can only split the lowest level partitions (those that contain data).

AT - Specifies a single value that should be used as the criteria for the split. The partition will be divided into two new partitions with the split value specified being the starting range for the latter partition.

INTO - Allows you to specify names for the two new partitions created by the split.

partition_name
The given name of a partition.

FOR (RANK(number))
For range partitions, the rank of the partition in the range.

FOR (’value’)
Specifies a partition by declaring a value that falls within the partition boundary specification. If the value declared with FOR matches to both a partition and one of its subpartitions (for example, if the value is a date and the table is partitioned by month and then by day), then FOR will operate on the first level where a match is found (for example, the monthly partition). If your intent is to operate on a subpartition, you must declare so as follows:

ALTER TABLE name ALTER PARTITION FOR ('2008-10-01') DROP PARTITION FOR ('2008-10-01');

Notes

Take special care when altering or dropping columns that are part of the HAWQ distribution key as this can change the distribution policy for the table. HAWQ does not currently support foreign key constraints.

Note: Note: The table name specified in the ALTER TABLE command cannot be the name of a partition within a table.

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

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.

You can specify multiple changes in a single ALTER TABLE command, which will be done in 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 sametype; Where anycol is any remaining table column and sametype 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 is partitioned or 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. 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 (ALTER TABLE ONLY ... DROP COLUMN) never removes any descendant columns, but instead marks them as independently defined rather than inherited.

The OWNER action 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.

Examples

Add a column to a table:

ALTER TABLE distributors ADD COLUMN address varchar(30);

Rename an existing column:

ALTER TABLE distributors RENAME COLUMN address TO city;

Rename an existing table:

ALTER TABLE distributors RENAME TO suppliers;

Add a not-null constraint to a column:

ALTER TABLE distributors ALTER COLUMN street SET NOT NULL;

Add a check constraint to a table:

ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);

Move a table to a different schema:

ALTER TABLE myschema.distributors SET SCHEMA yourschema;

Add a new partition to a partitioned table:

ALTER TABLE sales ADD PARTITION
        START (date '2009-02-01') INCLUSIVE 
        END (date '2009-03-01') EXCLUSIVE; 

Add a default partition to an existing partition design:

ALTER TABLE sales ADD DEFAULT PARTITION other;

Rename a partition:

ALTER TABLE sales RENAME PARTITION FOR ('2008-01-01') TO jan08;

Drop the first (oldest) partition in a range sequence:

ALTER TABLE sales DROP PARTITION FOR (RANK(1));

Exchange a table into your partition design:

ALTER TABLE sales EXCHANGE PARTITION FOR ('2008-01-01') WITH TABLE jan08;

Split the default partition (where the existing default partition’s name is other) to add a new monthly partition for January 2009:

ALTER TABLE sales SPLIT DEFAULT PARTITION
    START ('2009-01-01') INCLUSIVE
    END ('2009-02-01') EXCLUSIVE
    INTO (PARTITION jan09, PARTITION other);

Split a monthly partition into two with the first partition containing dates January 1-15 and the second partition containing dates January 16-31:

ALTER TABLE sales SPLIT PARTITION FOR ('2008-01-01')
    AT ('2008-01-16')
    INTO (PARTITION jan081to15, PARTITION jan0816to31);

Compatibility

The ADD, DROP, and SET DEFAULT forms conform with the SQL standard. The other forms are HAWQ 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.

See Also

CREATE TABLE, DROP TABLE