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NAME

       ALTER TABLE - change the definition of a table


SYNOPSIS

       ALTER TABLE [ ONLY ] name [ * ]
           action [, ... ]
       ALTER TABLE [ ONLY ] name [ * ]
           RENAME [ COLUMN ] column TO new_column
       ALTER TABLE name
           RENAME TO new_name
       ALTER TABLE name
           SET SCHEMA new_schema

       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 ]
           DISABLE TRIGGER [ trigger_name | ALL | USER ]
           ENABLE TRIGGER [ trigger_name | ALL | USER ]
           CLUSTER ON index_name
           SET WITHOUT CLUSTER
           SET WITHOUT OIDS
           OWNER TO new_owner
           SET TABLESPACE new_tablespace


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 [create_table(l)].

       DROP COLUMN
              This  form  drops  a column from a table. Indexes and table con-
              straints 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  automati-
              cally  converted  to  use  the  new column type by reparsing the
              originally supplied expression. The optional USING clause speci-
              fies  how to compute the new column value from the old; if omit-
              ted, 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 val-
              ues 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 [analyze(l)] 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  the  docu-
              mentation.

       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  sub-
              string  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  the
              documentation for more information.

       ADD table_constraint
              This form adds a new constraint to a table using the same syntax
              as CREATE TABLE [create_table(l)].

       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 match-
              ing constraints will be dropped.

       DISABLE/ENABLE TRIGGER
              These forms disable or enable trigger(s) belonging to the table.
              A disabled trigger is still known to the system, but is not exe-
              cuted  when its triggering event occurs. For a deferred trigger,
              the enable status is checked when the event occurs, not when the
              trigger function is actually executed. One may disable or enable
              a single trigger specified by name, or all triggers on  the  ta-
              ble,  or  only user triggers (this option excludes triggers that
              are used to implement foreign  key  constraints).  Disabling  or
              enabling  constraint  triggers requires superuser privileges; it
              should be done with caution since of course the integrity of the
              constraint  cannot  be  guaranteed  if the triggers are not exe-
              cuted.

       CLUSTER
              This form selects the default index for  future  CLUSTER  [clus-
              ter(l)] operations. It does not actually re-cluster the table.

       SET WITHOUT CLUSTER
              This  form  removes  the most recently used CLUSTER [cluster(l)]
              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, 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  [cre-
              ate_tablespace(l)].

       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 ta-
              ble. There is no effect on the stored data.

       SET SCHEMA
              This form  moves  the  table  into  another  schema.  Associated
              indexes,  constraints,  and SERIAL-column sequences are moved as
              well.

       All the actions except 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 and/or alter the type of  several  col-
       umns  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.  To change the schema  of  a
       table, you must also have CREATE privilege on the new schema.  To alter
       the owner, you must also be a direct or indirect member of the new own-
       ing  role,  and  that  role  must  have CREATE privilege on the table's
       schema. (These restrictions enforce that altering the owner doesn't  do
       anything  you  couldn't  do by dropping and recreating the table.  How-
       ever, a superuser can alter ownership of any table anyway.)


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 indi-
              cate that descendant tables are to be altered, but in  the  cur-
              rent  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 depen-
              dent objects. This is the default behavior.

       trigger_name
              Name of a single trigger to disable or enable.

       ALL    Disable or enable all triggers belonging to  the  table.   (This
              requires superuser privilege if any of the triggers are for for-
              eign key constraints.)

       USER   Disable or enable all triggers belonging to the table except for
              foreign key constraint triggers.

       index_name
              The  index name on which the table should be marked for cluster-
              ing.

       new_owner
              The user 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.


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 tempo-
       rarily 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  some-
       times  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.

       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  match-
       ing the parent.

       A recursive DROP COLUMN operation will remove a descendant table's col-
       umn only if the descendant does not inherit that column from any  other
       parents and never had an independent definition of the column. A nonre-
       cursive DROP COLUMN (i.e., ALTER TABLE  ONLY  ...  DROP  COLUMN)  never
       removes any descendant columns, but instead marks them as independently
       defined rather than inherited.

       The TRIGGER, CLUSTER, OWNER, and TABLESPACE actions  never  recurse  to
       descendant  tables; that is, they always act as though ONLY were speci-
       fied.  Adding a constraint can recurse only for CHECK constraints.

       Changing any part of a system catalog table is not permitted.

       Refer to CREATE TABLE [create_table(l)] for a  further  description  of
       valid  parameters. the documentation has further information on inheri-
       tance.


EXAMPLES

       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 change an integer column containing  UNIX  timestamps  to  timestamp
       with time zone via a USING clause:

       ALTER TABLE foo
           ALTER COLUMN foo_timestamp TYPE timestamp with time zone
           USING
               timestamp with time zone 'epoch' + foo_timestamp * interval '1 second';

       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);

       To move a table to a different tablespace:

       ALTER TABLE distributors SET TABLESPACE fasttablespace;

       To move a table to a different schema:

       ALTER TABLE myschema.distributors SET SCHEMA yourschema;


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  disal-
       lows zero-column tables.

SQL - Language Statements         2005-11-05                     ALTER TABLE()

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