ALTER TABLE

Description

Modifies tables, including modifying table definitions, renaming tables, renaming specified columns in tables, renaming table constraints, setting table schemas, enabling or disabling row-level security policies, and adding or updating multiple columns.

Precautions

• If the base table is a hash-distributed table, to create a primary key or unique index that does not contain the distribution key of the base table, use a GSI (by specifying the BY GLOBAL INDEX column); to create a primary key or unique index that contains the distribution key of the base table, use a common index (without specifying the BY GLOBAL INDEX column). In single-DN deployment mode, both the GSI and common index can be successfully created. If the base table is a non-hash-distributed table, you can only create the primary key or unique index as a common index. That is, the index key must contain the distribution key of the base table.
• The owner of a table, users granted with the ALTER permission on the table, or users granted with the ALTER ANY TABLE permission can run the ALTER TABLE command. The system administrator has the permission to run the command by default. To modify the owner or schema of a table, you must be the 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.
• The distribution key (or column) of a table cannot be modified.

Syntax

• Modify the definition of a table.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14

  ALTER TABLE [ IF EXISTS ] { table_name [*] | ONLY table_name | ONLY ( table_name ) } action [, ... ]; ALTER TABLE [ IF EXISTS ] table_name ADD ( { column_name data_type [ compress_mode ] [ COLLATE collation ] [ column_constraint [ ... ] ]} [, ...] ); 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 } [, ...] ); ALTER TABLE [ IF EXISTS ] table_name RENAME TO new_table_name; ALTER TABLE [ IF EXISTS ] { table_name [*] | ONLY table_name | ONLY ( table_name )} RENAME [ COLUMN ] column_name TO new_column_name; ALTER TABLE [ IF EXISTS ] { table_name [*] | ONLY table_name | ONLY ( table_name )} RENAME CONSTRAINT constraint_name TO new_constraint_name; ALTER TABLE [ IF EXISTS ] table_name SET SCHEMA new_schema;

  There are several clauses of action:

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

  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 [, ... ] ) | UPDATE SLICE LIKE table_name | DISABLE TRIGGER [ trigger_name | ALL | USER ] | ENABLE TRIGGER [ trigger_name | ALL | USER ] | ENABLE REPLICA TRIGGER trigger_name | ENABLE ALWAYS TRIGGER trigger_name | ENABLE ROW LEVEL SECURITY | DISABLE ROW LEVEL SECURITY | FORCE ROW LEVEL SECURITY | NO FORCE ROW LEVEL SECURITY | ENCRYPTION KEY ROTATION | REPLICA IDENTITY { DEFAULT | USING INDEX index_name | FULL | NOTHING } | AUTO_INCREMENT [ = ] value | [ [ DEFAULT ] CHARACTER SET | CHARSET [ = ] default_charset ] [ [ DEFAULT ] COLLATE [ = ] default_collation ]

  

  • 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. If the specified index is a GSI, an error is reported. In this case, you need to use the BY GLOBAL INDEX syntax to add a GSI index constraint.

  • 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 ]

    Deletes 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. Deletes the most recently used CLUSTER index from the table.

  • SET ( {storage_parameter = value} [, ... ] )

    Changes one or more storage parameters for the table. If the value of table_name is an index name, ACTIVE_PAGES specifies the number of index pages, which may be less than the actual number of physical file pages and can be used for optimizer optimization. Currently, this parameter is valid only for the local index of the Ustore partitioned table and will be updated by VACUUM and ANALYZE (including AUTOVACUUM). You are advised not to manually set this parameter because it is invalid in distributed mode.

  • 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 dilatation tools. Common users should not use the mode. This command only modifies the logical mapping relationship of the table distribution nodes and does not migrate the table's metadata and data on the DN.

  • 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.

  • UPDATE SLICE LIKE table_name

    This syntax is used by internal scaling tools and cannot be used by common users.

  • 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 constraints 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 triggered only when the session is in replica mode.

  • | ENABLE ALWAYS TRIGGER trigger_name

    Determines that all triggers are fired regardless of the current replica mode.

  • | DISABLE/ENABLE ROW LEVEL SECURITY

    Enables or disables row-level security for a table.

    If row-level security is enabled for a data table but no row-level security policy is defined, the row-level access to the data table is not affected. If row-level security for a table is disabled, the row-level access to the table is not affected even if a row-level security policy has been defined. For details, see CREATE ROW LEVEL SECURITY POLICY.

  • | NO FORCE/FORCE ROW LEVEL SECURITY

    Forcibly enables or disables row-level security for a table.

    By default, the table owner is not affected by the row-level security feature. However, if row-level security is forcibly enabled, the table owner (excluding system administrators) will be affected. System administrators are not affected by any row-level security policies.

  • | ENCRYPTION KEY ROTATION

    Rotation of the transparent data encryption key.

    The data encryption key rotation of a table can be performed only when the TDE function is enabled for the database and enable_tde of the table is set to on. After the key rotation operation is performed, the system automatically applies for a new KMS key. After the key rotation, the data encrypted using the old key is decrypted using the old key, and the newly written data is encrypted using the new key. To ensure the security of encrypted data, you can periodically update the key based on the amount of new data in the encrypted table. It is recommended that the key be updated every two to three years.

  • REPLICA IDENTITY { DEFAULT | USING INDEX index_name | FULL | NOTHING }

    Specifies the record level of old tuples in UPDATE and DELETE statements on a table in logical replication scenarios.

    • DEFAULT records the old value of the primary key column. If there is no primary key, DEFAULT does not record the old value.
    • USING INDEX records the old values of columns covered by the named indexes. These values must be unique, non-local, and non-deferrable, and contain the values of columns marked NOT NULL.
    • FULL records the old values of all columns in the row.
    • NOTHING does not record information in old rows.

    In logical replication scenarios, when the UPDATE and DELETE statements of a table are parsed, the parsed old tuples consist of the information recorded in this method. For tables with primary keys, this option can be set to DEFAULT or FULL. For a table without a primary key, set this parameter to FULL. Otherwise, the old tuple will be parsed as empty during decoding. You are advised not to set this parameter to NOTHING in common scenarios because old tuples are always parsed as empty.

  • AUTO_INCREMENT [ = ] value

    Sets the next auto-increment value of the auto-increment column. The configured value takes effect only when it is greater than the current auto-increment counter.

    The value must be a non-negative integer and cannot be greater than 2¹²⁷ – 1.

    This clause takes effect only when sql_compatibility is set to 'B'.

  • [ [ DEFAULT ] CHARACTER SET | CHARSET [ = ] default_charset ] [ [ DEFAULT ] COLLATE [ = ] default_collation ]

    Modifies the default character set and default collation of a table to the specified values. The modification does not affect the existing columns in the table.

    This clause takes effect only when sql_compatibility is set to 'B'.

  • CONVERT TO CHARACTER SET | CHARSET charset [ COLLATE collation ]

    Modifies the default character set and default collation of a table to the specified values, sets the character set and collation of all columns with character type to the specified value and converts the data in the column to new character set encoding.

  • The column_clause can be one of the following clauses:

    1 2 3 4 5 6 7 8 9 10 11 12 13 14

    ADD [ COLUMN ] column_name data_type [ CHARACTER SET | CHARSET charset ] [ 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 }

    

    • ADD [ COLUMN ] column_name data_type [ compress_mode ] [ COLLATE collation ] [ column_constraint [ ... ] ]

      Adds a column to a table. If a column is added using 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. Running this command will clear the statistics of this column. You are advised to collect the statistics of this column again after the modification.

    • 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 (for example, view), CASCADE must be specified.

      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 recycled. The space will be recycled 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.

      If the original data type of a column and the modified data type are binary compatible, you do not need to rewrite the entire table when running this statement. In other scenarios, the entire table is rewritten. You can check whether the original type and target type are binary compatible in the PG_CAST system catalog. If castmethod is 'b', they are binary compatible. For example, if the data type of the source table is text and is converted to int, table rewriting is triggered. If it is converted to clob, table rewriting is not triggered. If table rewriting is triggered, the deleted space on the table is recycled immediately.

      Running this command will clear the statistics of this column. You are advised to collect the statistics of this column again after the modification.

    • 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.

    • {ENABLE | DISABLE } STATISTICS ((column_1_name, column_2_name [, ...]))

      Enables or disables multi-column statistics. When automatic statistics creation is enabled (the GUC parameter auto_statistic_ext_columns is required), you can disable specific multi-column combinations to prevent them from being automatically created and used.

    • 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. It specifies whether this column is held inline or in an attached table, and whether the data should be compressed. SET STORAGE does not change a table. It only specifies the recommended strategy for future table updates.

    • column_constraint is as follows:

      1 2 3 4 5 6 7 8 9 10 11 12 13

      [ CONSTRAINT constraint_name ] { NOT NULL | NULL | CHECK ( expression ) | DEFAULT default_expr | GENERATED ALWAYS AS ( generation_expr ) [STORED] | AUTO_INCREMENT | UNIQUE [KEY] index_parameters | PRIMARY KEY index_parameters | ENCRYPTED WITH ( 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 ]

      • index_parameters is as follows:

        1 2	[ WITH ( {storage_parameter = value} [, ... ] ) ] [ USING INDEX TABLESPACE tablespace_name ]

    • compress_mode of a column is as follows:

      1	{ 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:

    1 2 3

    [ CONSTRAINT constraint_name ] { UNIQUE | PRIMARY KEY } USING INDEX index_name [ DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

  • table_constraint is as follows:

    1 2 3 4 5 6

    [ CONSTRAINT constraint_name ] { CHECK ( expression ) | UNIQUE ( column_name [, ... ] ) index_parameters | PRIMARY KEY ( column_name [, ... ] ) index_parameters } [ DEFERRABLE | NOT DEFERRABLE | INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

    • index_parameters is as follows:

      1 2	[ WITH ( {storage_parameter = value} [, ... ] ) ] [ USING INDEX TABLESPACE tablespace_name ][BY GLOBAL INDEX]

    

    If index_parameters is set to BY GLOBAL INDEX, the global secondary index is used to create constraints.

Parameters

• 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 are 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 index name.

  

  In the ADD CONSTRAINT operation:

  • index_name is supported only in B-compatible databases (that is, sql_compatibility set to 'B').
  • For foreign key constraints, if constraint_name and index_name are specified at the same time, constraint_name is used as the index name.
  • For a unique key constraint, if both constraint_name and index_name are specified, index_name is used as the index name.
• USING method

  Specifies the name of the index method to be used.

  For details about the value range, see USING method.

  

  In the ADD CONSTRAINT operation:

  • The USING method is supported only in B-compatible databases (that is, sql_compatibility set to 'B').
  • In B-compatible mode, if USING method is not specified, the default index method is btree for Astore or UB-tree for Ustore.
• ASC | DESC

  ASC specifies an ascending (default) sort order. DESC specifies a descending sort order.

  

  In ADD CONSTRAINT, ASC|DESC is supported only in B-compatible databases (that is, sql_compatibility set to 'B').

• expression

  Specifies an expression index constraint based on one or more columns of the table. It must be written in parentheses.

  

  Expression indexes in the UNIQUE constraint are supported only in B-compatible databases (that is, sql_compatibility set to 'B').

• storage_parameter

  Specifies the name of a storage parameter.

  The following options are added for online scaling:

  • append_mode (enumerated type)

    Scales out a table online or offline, or stops scaling it. You can modify certain content in the table during online scaling but cannot do so during offline scaling.

    To modify a table that is being scaled, append new data so that they can be recorded as incremental data.

    • on: scales out a table online. New data will be appended.
    • off: stops scaling. New data will be written in normal mode, and options for online scaling will not be displayed in pg_class.reloptions.
    • read_only: scales a table offline, during which no other operations can be performed on the table.
    • end_catchup: reports errors for the write service in the last round of data increment. The read service is executed normally.
  • rel_cn_oid (OID type)

    Records the OID of tables on the current CN to generate delete_delta on the DNs.

    If append_mode is set to on, rel_cn_oid must be specified.

    The append_mode and rel_cn_oid options are used only for online scale-out tools.

  • exec_step (integer)

    Records resumable transmission steps in relOptions of the temporary table.

    Value range: [1,4]

    It can be used only for data redistribution.

  • create_time (long integer)

    Records the time when the temporary table is created during resumable transmission in relOptions of the temporary table.

    Only the data redistribution tool is supported.

  • wait_clean_cbi (string type)

    Specifies whether the current global index contains the residual tuple generated during bucket migration for scaling. After scaling, wait_clean_cbi is set to y. After the residual tuple is cleared in the vacuum process, wait_clean_cbi is set to n.

    This option is used only in scaling tools.

  The following option is added for creating an index:

  • parallel_workers (int type)

    Number of bgworker threads started when an index is created. For example, value 2 indicates that two bgworker threads are started to create indexes concurrently.

    Value range: [0,32]. The value 0 indicates that concurrent index creation is disabled.

    Default value: If this parameter is not set, the concurrent index creation function is disabled.

  The following option is added to the replication table:

  • primarynode (Boolean type)

    Default value: off

    When primarynode is set to on, the primary node is selected for the replication table. Generally, the primary node is the first node recorded in the nodeoids column in the pgxc_class table. When the IUD operation is performed on the replication table, the operation is delivered to the primary node first. After the result is received, the operation is delivered to other DNs.

  • logical_repl_node (string type)

    Name of the DN that returns logical logs to the CN during logical decoding of a distributed replication table. For the replication table, if this parameter is not specified, the first node in the node group where the current table is located is used by default. When the RESET operation is performed on this option, logical_repl_node is reset to the first node of current table.

    Value range: a string

    Default value: For the non-replication table, this parameter is empty by default. For the replication table, this parameter is set to the name of the first node by default.

  TDE options:

  • enable_tde (bool type)

    Specifies whether transparent data encryption is enabled for a table. Before enabling this function, ensure that the GUC parameter enable_tde has been enabled, the KMS has been enabled, and the cluster master key ID in the GUC parameter tde_cmk_id has been correctly configured.

    This parameter does not support temporary tables. The Ustore storage engine is not supported. The Ustore storage engine is not supported. This parameter can be modified only when enable_tde is specified during table creation. Switching the encryption switch status does not change the encryption algorithm and key information.

    Value range: on and off. on indicates that transparent data encryption is enabled. After the value is switched from off to on, new data is automatically encrypted when being written to the data page, and old data is automatically encrypted when the data page is updated. off indicates that transparent data encryption is disabled. After the value is switched from on to off, newly written data is not encrypted, old encrypted data can be automatically decrypted when being read, and data is not encrypted when being written back to the data page.

    Default value: off

  • hasuids (Boolean type)

    Default value: off

    If this parameter is set to on, a unique table-level ID is allocated to a tuple when the tuple is updated.

• 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

  Specifies whether to compress a table column. 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.

  

  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 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.

• ENABLE

  Specifies that the constraint is enabled. By default, the constraint is enabled.

• 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-store 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 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 to 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.

• COLUMN_ENCRYPTION_KEY = column_encryption_key

  Specifies the name of the column encryption key in the ENCRYPTED WITH constraint.

  Value range: a string. It must comply with the naming convention.

• ENCRYPTION_TYPE = encryption_type_value

  For the encryption type in the ENCRYPTED WITH constraint, the value of encryption_type_value is DETERMINISTIC or RANDOMIZED.

• UNIQUE [KEY] index_parameters

  UNIQUE ( column_name [, ... ] ) index_parameters

  UNIQUE specifies that a group of one or more columns of a table can contain only unique values.

  UNIQUE KEY can be used only when sql_compatibility is set to 'MYSQL', which has the same semantics as UNIQUE.

• PRIMARY KEY index_parameters

  PRIMARY KEY ( column_name [, ... ] ) index_parameters

  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 checked 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 if the column is referenced by other columns or constraints. RESTRICT is the default option. If CASCADE is not specified, the value is RESTRICT. An example of the statement is as follows:

  alter table <Table name>[drop [column] <Column name> [cascade | restrict]];

• schema_name

  Specifies the schema name of a table.

Examples of Modifying a Table

• Rename a table.

  gaussdb=# CREATE TABLE aa(c1 int, c2 int);
  gaussdb=# ALTER TABLE IF EXISTS aa RENAME TO test_alt1;

• Modify the schema of a table.

  -- Create the test_schema schema.
  gaussdb=# CREATE SCHEMA test_schema;
  
  -- Change the schema of the test_alt1 table to test_schema.
  gaussdb=# ALTER TABLE test_alt1 SET SCHEMA test_schema;
  
  -- Query table information.
  gaussdb=# SELECT schemaname,tablename FROM pg_tables WHERE tablename = 'test_alt1';
   schemaname  | tablename 
  -------------+-----------
   test_schema | test_alt1
  (1 row)

• Change the owner of a table.

  -- Create user test_user.
  gaussdb=# CREATE USER test_user PASSWORD 'XXXXXXXXXX';
  
  -- Change the owner of the test_alt1 table to test_user.
  gaussdb=# ALTER TABLE IF EXISTS test_schema.test_alt1 OWNER TO test_user;
  
  -- Query.
  gaussdb=# SELECT tablename, schemaname, tableowner FROM pg_tables WHERE tablename = 'test_alt1';
   tablename | schemaname  | tableowner 
  -----------+-------------+------------
   test_alt1 | test_schema | test_user
  (1 row)

• Modify the tablespace of a table.

  -- Create the tbs_data1 tablespace.
  gaussdb=# CREATE TABLESPACE tbs_data1 RELATIVE LOCATION 'tablespace1/tbs_data1';
  
  -- Change the tablespace of the test_alt1 table to tbs_data1.
  gaussdb=# ALTER TABLE test_schema.test_alt1 SET TABLESPACE tbs_data1;
  
  -- Query.
  gaussdb=# SELECT tablename, tablespace FROM pg_tables WHERE tablename = 'test_alt1';
   tablename | tablespace 
  -----------+------------
   test_alt1 | tbs_data1
  (1 row)
  
  -- Delete.
  gaussdb=# DROP TABLE test_schema.test_alt1; 
  gaussdb=# DROP TABLESPACE tbs_data1; 
  gaussdb=# DROP SCHEMA test_schema; 
  gaussdb=# DROP USER test_user;

Examples of Modifying a Column

• Change column names.

  -- Create a table.
  gaussdb=# CREATE TABLE test_alt2(c1 INT,c2 INT);
  
  -- Change column names.
  gaussdb=# ALTER TABLE test_alt2 RENAME c1 TO id;
  gaussdb=# ALTER TABLE test_alt2 RENAME COLUMN c2 to areaid;
  
  -- Query.
  \d test_alt1
     Table "public.test_alt1"
   Column |  Type   | Modifiers 
  --------+---------+-----------
   id     | integer | 
   areaid | integer | 

• Add columns.

  -- Add a column to the test_alt1 table.
  gaussdb=# ALTER TABLE IF EXISTS test_alt2 ADD COLUMN name VARCHAR(20);
  
  -- Query.
  gaussdb=# \d test_alt2
             Table "public.test_alt1"
    Column  |         Type          | Modifiers 
  ----------+-----------------------+-----------
   id       | integer               | 
   areacode | integer               | 
   name     | character varying(20) |

• Modify the data type of a column.

  -- Change the type of the name column in the test_alt1 table.
  gaussdb=# ALTER TABLE test_alt1 MODIFY name VARCHAR(50);
  
  -- Query.
  gaussdb=# \d test_alt1
            Table "public.test_alt2"
   Column |         Type          | Modifiers 
  --------+-----------------------+-----------
   id     | integer               | 
   areaid | integer               | 
   name   | character varying(50) | 

  -- Change the type of the name column in the test_alt1 table.
  gaussdb=# ALTER TABLE test_alt2 ALTER COLUMN name TYPE VARCHAR(25);
  
  -- Query.
  gaussdb=# \d test_alt2
            Table "public.test_alt2"
   Column |         Type          | Modifiers 
  --------+-----------------------+-----------
   id     | integer               | 
   areaid | integer               | 
   name   | character varying(25) | 

• Delete a column.

  -- Delete the areaid column from test_alt1.
  gaussdb=# ALTER TABLE test_alt2 DROP COLUMN areaid;
  
  -- Query.
  gaussdb=# \d test_alt2
            Table "public.test_alt2"
   Column |         Type          | Modifiers 
  --------+-----------------------+-----------
   id     | integer               | 
   name   | character varying(25) | 

• Modify the column-store mode.

  -- View table details.
  gaussdb=# \d+ test_alt2
                                Table "public.test_alt2"
   Column |         Type          | Modifiers | Storage  | Stats target | Description 
  --------+-----------------------+-----------+----------+--------------+-------------
   id     | integer               |           | plain    |              | 
   name   | character varying(25) |           | extended |              | 
  Has OIDs: no
  Options: orientation=row, compression=no, storage_type=USTORE
  
  -- Change the storage mode of the name column in the test_alt2 table.
  gaussdb=# ALTER TABLE test_alt2 ALTER COLUMN name SET STORAGE PLAIN;
  
  -- Query.
  gaussdb=# \d+ test_alt2
                               Table "public.test_alt2"
   Column |         Type          | Modifiers | Storage | Stats target | Description 
  --------+-----------------------+-----------+---------+--------------+-------------
   id     | integer               |           | plain   |              | 
   name   | character varying(25) |           | plain   |              | 
  Has OIDs: no
  Options: orientation=row, compression=no, storage_type=USTORE
  
  -- Delete.
  gaussdb=# DROP TABLE test_alt2;

Examples of Modifying a Constraint

• Add a not-null constraint to a column.

  -- Create a table.
  gaussdb=# CREATE TABLE test_alt3(pid INT, areaid CHAR(5), name VARCHAR(20));
  
  Add a not-null constraint to pid.
  gaussdb=# ALTER TABLE test_alt3 MODIFY pid NOT NULL;
  
  -- Query.
  gaussdb=# \d test_alt3
            Table "public.test_alt3"
   Column |         Type          | Modifiers 
  --------+-----------------------+-----------
   pid    | integer               | not null
   areaid | character(5)          | 
   name   | character varying(20) |

• Cancel the not-null constraint on a column.

  gaussdb=# ALTER TABLE test_alt3 MODIFY pid NULL;
  -- Query.
  gaussdb=# \d test_alt3
            Table "public.test_alt3"
   Column |         Type          | Modifiers 
  --------+-----------------------+-----------
   pid    | integer               | 
   areaid | character(5)          | 
   name   | character varying(20) | 

• Modify the default value of a column.

  -- Modify the default value of id in the test_alt1 table.
  gaussdb=# ALTER TABLE test_alt3 ALTER COLUMN areaid SET DEFAULT '00000';
  
  -- Query.
  gaussdb=# \d test_alt3
                   Table "public.test_alt3"
   Column |         Type          |        Modifiers        
  --------+-----------------------+-------------------------
   pid    | integer               | 
   areaid | character(5)          | default '00000'::bpchar
   name   | character varying(20) |

  -- Delete the default value of id.
  gaussdb=# ALTER TABLE test_alt3 ALTER COLUMN areaid DROP DEFAULT;
  
  -- Query.
  gaussdb=# \d test_alt3
            Table "public.test_alt3"
   Column |         Type          | Modifiers 
  --------+-----------------------+-----------
   pid    | integer               | 
   areaid | character(5)          | 
   name   | character varying(20) |

• Add a table-level constraint.
  • Directly add a constraint.

    -- Add a primary key constraint to the table.
    gaussdb=# ALTER TABLE test_alt3 ADD CONSTRAINT pk_test3_pid PRIMARY KEY (pid);
    
    -- Query.
    gaussdb=# \d test_alt3
              Table "public.test_alt3"
     Column |         Type          | Modifiers 
    --------+-----------------------+-----------
     pid    | integer               | not null
     areaid | integer               | 
     name   | character varying(20) | 
    Indexes:
        "pk_test3_pid" PRIMARY KEY, btree (pid) TABLESPACE pg_default

  • Create an index and then add constraints.

    -- Create a table.
    gaussdb=# CREATE TABLE test_alt4(c1 INT, c2 INT);
    
    -- Create an index.
    gaussdb=# CREATE UNIQUE INDEX pk_test4_c1 ON test_alt4(c1);
    
    -- Associate the created index when adding a constraint.
    gaussdb=# ALTER TABLE test_alt4 ADD CONSTRAINT pk_test4_c1 PRIMARY KEY USING INDEX pk_test4_c1;
    
    -- Query.
    gaussdb=# \d test_alt4
       Table "public.test_alt4"
     Column |  Type   | Modifiers 
    --------+---------+-----------
     c1     | integer | not null
     c2     | integer | 
    Indexes:
        "pk_test4_c1" PRIMARY KEY, btree (c1) TABLESPACE pg_default
    
    -- Delete.
    gaussdb=# DROP TABLE test_alt4;

• Delete a table-level constraint.

  -- Delete a constraint.
  gaussdb=# ALTER TABLE test_alt3 DROP CONSTRAINT IF EXISTS pk_test3_pid;
  -- Query.
  gaussdb=# \d test_alt3
            Table "public.test_alt3"
   Column |         Type          | Modifiers 
  --------+-----------------------+-----------
   pid    | integer               | not null
   areaid | integer               | 
   name   | character varying(20) | 
  
  -- Delete.
  gaussdb=# DROP TABLE test_alt3;

Helpful Links

CREATE TABLE and DROP TABLE