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ALTER TABLE

The ALTER TABLE command modifies the definition of a table.

Syntax

ALTER TABLE table_name 
alter_option;

alter_option depends on the operation you want to perform on the table. For all supported clauses, see the sections below.

Clauses

ADD COLUMN

Syntax
ALTER TABLE table_name 
ADD [ COLUMN ] column_name data_type [ PRIMARY KEY ] [ DEFAULT default_expr ];
Parameter or clauseDescription
ADD [ COLUMN ]This clause adds a new column to the table. COLUMN is optional.
column_nameSpecify the name of the column you want to add.
data_typeThe data type of the new column.
DEFAULTThe DEFAULT clause allows you to assign a default value to a column. This default value is used when a new row is inserted, and no explicit value is provided for that column.
default_exprdefault_expr is any constant value or variable-free expression that does not reference other columns in the current table or involve subqueries. The data type of default_expr must match the data type of the column.
If default_expr is impure, such as using a function like now(), all historical data will be filled with the result of the expression evaluated at the time the statement was executed. For future insertions, the default expression will be evaluated at the time of each respective insertion.
Example
-- Add a column named "age" to a table named "employees" with a data type of integer
ALTER TABLE employees ADD age int;
note
  • If your table is defined with a schema registry, its columns can not be altered.

  • Columns added by this command cannot be used by any existing materialized views or indexes. You must create new materialized views or indexes to reference it.

DROP COLUMN

Syntax
ALTER TABLE table_name 
DROP [ COLUMN ] [ IF EXISTS ] column_name;
Parameter or clauseDescription
DROP [ COLUMN ]This clause drops an existing column from a table. COLUMN is optional.
IF EXISTSDo not return an error if the specified column does not exist. A notice is issued instead.
column_nameSpecify the column you want to remove.
Example
-- Remove a column named "fax" from the "employees" table
ALTER TABLE employees DROP fax;
note
  • If your table is defined with a schema registry, its column can not be altered.

  • You cannot drop columns referenced by materialized views or indexes.

OWNER TO

Syntax
ALTER TABLE table_name 
OWNER TO new_user;
Parameter or clauseDescription
OWNER TOThis clause changes the owner of the table to the specified user. It will cascadingly change all related internal objects as well, and the associated indexes will be changed too.
new_userSpecify the user you want to assign to the table.
Example
-- Change the owner of the table named "t" to the user "user1"
ALTER TABLE t OWNER TO user1;

After setting, you can observe the parallelism status within the internal rw_table_fragments table.

SET SCHEMA

Syntax
ALTER TABLE table_name 
SET SCHEMA schema_name;
Parameter or clauseDescription
SET SCHEMAThis clause moves the table into another schema. Associated indexes, constraints, and sequences owned by table columns are moved as well.
schema_nameSpecify the schema to which the table will be moved.
Example
-- Move a table named "test_table" into a schema named "test_schema"
ALTER TABLE test_table SET SCHEMA test_schema;

SET PARALLELISM

Syntax
ALTER TABLE table_name 
SET PARALLELISM = parallelism_number;
Parameter or clauseDescription
SET PARALLELISMThis clause controls the degree of parallelism for the targeted streaming job.
parallelism_numberThis parameter can be AUTO or a fixed number, like 1, 2, 3, etc. Altering the parameter to AUTO will expand the streaming job's degree of parallelism to encompass all available units, whereas setting it to a fixed number will lock the job's parallelism at that specific figure. Setting it to 0 is equivalent to AUTO.

After setting the parallelism, the parallelism status of a table can be observed within the internal rw_table_fragments table or the rw_fragmentstable.
Example
ALTER TABLE test_table SET PARALLELISM = 8;

Here is a more detailed example for you to practise this clause:

First, let's set the parallelism to 3 by the SET command.

Example
SET streaming_parallelism = 3;

Then let's create a table to view the parallelism we set. As mentioned, the parallelism status of a table can be observed within the rw_fragments table.

Example
-- Create a table.
CREATE TABLE t(v int);
-- View parrellelism by rw_fragments table.
SELECT fragment_id, parallelism FROM rw_fragments;

------RESULTS
fragment_id | parallelism
-------------+-------------
1 | 3
2 | 3
(2 rows)

Now we can use SET PARALLELISM to change the parallelism and view the change:

Example
-- Set to a fixed number.
ALTER TABLE t SET PARALLELISM = 8;
SELECT fragment_id, parallelism FROM rw_fragments;

------RESULTS
fragment_id | parallelism
-------------+-------------
1 | 8
2 | 8
(2 rows)
Example
-- Set to AUTO
ALTER TABLE t SET PARALLELISM = auto;
SELECT fragment_id, parallelism FROM rw_fragments;

------RESULTS
fragment_id | parallelism
-------------+-------------
1 | 12
2 | 12
(2 rows)

RENAME TO

Syntax
ALTER TABLE table_name
RENAME TO new_name;
Parameter or clauseDescription
RENAME TOThis clause changes the name of the table.
new_nameThe new name of the table.
Example
-- Change the name of the table named "table0" to "table1"
ALTER TABLE table0 RENAME TO table1;
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