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

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 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 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