Alter table is the most frequently used command by development DBA's.
Altering a Table
The ALTER TABLE command changes the definition of a table. Use ALTER TABLE to change table attributes such as column definitions, distribution policy, storage model, and partition structure. For example, to add a not-null constraint to a table column:
ALTER TABLE sachi ALTER COLUMN name SET NOT NULL;
Altering Table Distribution
ALTER TABLE provides options to change a table’s distribution policy . When the table distribution options change, the table data is redistributed on disk, which can be resource intensive. You can also redistribute table data using the existing distribution policy.
Changing the Distribution Policy
For partitioned tables, changes to the distribution policy apply recursively to the child partitions. This operation preserves the ownership and all other attributes of the table. For example, the following command redistributes the table sales across all segments using the customer_id column as the distribution key:
ALTER TABLE sales SET DISTRIBUTED BY (customer_id);
When you change the hash distribution of a table, table data is automatically redistributed.
Changing the distribution policy to a random distribution does not cause the data to be redistributed. For example:
ALTER TABLE sales SET DISTRIBUTED RANDOMLY;
Redistributing Table Data
To redistribute table data for tables with a random distribution policy (or when the hash distribution policy has not changed) use REORGANIZE=TRUE. Reorganizing data may be necessary to correct a data skew problem, or when segment resources are added to the system. For example, the following command redistributes table data across all segments using the current distribution policy, including random distribution.
ALTER TABLE sales SET WITH (REORGANIZE=TRUE);
Altering the Table Storage Model
Table storage, compression, and orientation can be declared only at creation. To change the storage model, you must create a table with the correct storage options,
load the original table data into the new table, drop the original table, and rename the new table with the original table’s name. You must also re-grant any table
permissions. For example:
CREATE TABLE sales2 (LIKE sales) WITH (appendonly=true, compresstype=quicklz, compresslevel=1, orientation=column);
INSERT INTO sales2 SELECT * FROM sales;
DROP TABLE sales;
ALTER TABLE sales2 RENAME TO sales;
GRANT ALL PRIVILEGES ON sales TO admin;
GRANT SELECT ON sales TO guest;
Use ALTER TABLE command to add a compressed column to a table.
ALTER TABLE sachi ADD COLUMN newcompresscol1 int DEFAULT 0 ENCODING (COMPRESSTYPE=zlib);
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