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Help Center/ GaussDB(DWS)/ Developer Guide/ Hybrid Data Warehouse (8.1.3.x）/ Hybrid Data Warehouse Syntax/ ALTER TABLE

ALTER TABLE

Updated on 2024-11-05 GMT+08:00

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Function

Modify a table, including modifying the definition of a table, renaming a table, renaming a specified column in a table, adding or updating multiple columns, and changing a column-store table to an HStore table.

Precautions

You can set enable_hstore by using ALTER to change a column-store table to an HStore table, or to change it back. If enable_delta is set to on, enable_hstore cannot be set to on.
For some ALTER operations (such as modifying column types, merging partitions, adding NOT NULL constraints, and adding primary key constraints), HStore tables need to merge data to the primary table and then perform ALTER, which may cause extra performance overhead. The impact on performance depends on the data volume in the delta table.
When you add a column, do not use ALTER to specify other operations (for example, modifying the column type). An ALTER statement with only the ADD COLUMN parameter can achieve high performance, because it does not require FULL MERGE.
The storage parameter ORIENTATION cannot be modified.

Modifying Table Attributes

Syntax:

    
       ALTER TABLE [ IF EXISTS ] <table_name> SET  ( {ENABLE_HSTORE = ON} [, ... ] );

To change a column-store table to an HStore table, run the following command:

    
       CREATE TABLE alter_test(a int, b int) WITH(ORIENTATION = COLUMN);
ALTER TABLE alter_test SET (ENABLE_HSTORE = ON);

NOTICE:

To use HStore tables, set the following parameters, or the HStore performance will deteriorate severely. The recommended settings are as follows:

autovacuum_max_workers_hstore=3, autovacuum_max_workers=6, autovacuum=true

Adding a Column

Syntax:

    
       ALTER TABLE [ IF EXISTS ] <table_name> ADD COLUMN <new_column> <data_type> [ DEFAULT <default_value>];

Example:

Create the alter_test2 table and add a column to it.

    
       CREATE TABLE alter_test2(a int, b int) WITH(ORIENTATION = COLUMN,ENABLE_HSTORE = ON);
ALTER TABLE alter_test ADD COLUMN c int;

NOTE:

When adding a column, you are not advised to use ALTER to specify other operations in the same SQL statement.

Renaming

Syntax:

    
       ALTER TABLE [ IF EXISTS ] <table_name> RENAME TO <new_table_name>;

Example:

Create table alter_test3 and rename it as alter_new.

    
       CREATE TABLE alter_test3(a int, b int) WITH(ORIENTATION = COLUMN,ENABLE_HSTORE = ON);
ALTER TABLE alter_test3 RENAME TO alter_new;

Parent topic: Hybrid Data Warehouse Syntax

Previous topic: SELECT

Next topic: Hybrid Data Warehouse Functions

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