How to ADD COLUMN in Redshift

How do I add a column to a table in Amazon Redshift?

ALTER TABLE ... ADD COLUMN adds one or more new columns to an existing Amazon Redshift table without rewriting existing data.

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Description

What does ALTER TABLE ADD COLUMN do in Redshift?

ALTER TABLE ADD COLUMN updates a table’s metadata so you can store new attributes without recreating or copying data. The command is quick because Redshift appends the column definition rather than rewriting blocks.

What is the basic syntax?

The shortest form is:

ALTER TABLE table_name ADD [ COLUMN ] column_name data_type [ DEFAULT expr ] [ ENCODE encoding ];

Can I add multiple columns at once?

Yes, separate definitions with commas:

ALTER TABLE table_name ADD COLUMN col1 type1, col2 type2 DEFAULT val2;

How to add a nullable column?

Columns are NULL-able by default, so omit NOT NULL:

ALTER TABLE Orders ADD COLUMN shipping_method VARCHAR(20);

How to add a column with a default value?

Add DEFAULT to back-fill future inserts. Existing rows store NULL until you run an UPDATE:

ALTER TABLE Products ADD COLUMN last_restocked DATE DEFAULT CURRENT_DATE;

How do I back-fill existing rows?

Run an UPDATE immediately after adding the column:

UPDATE Products SET last_restocked = order_date FROM OrderItems oi JOIN Orders o ON o.id = oi.order_id WHERE Products.id = oi.product_id;

Best practices for ADD COLUMN

1) Batch multiple columns in one statement to reduce commits. 2) Always specify ENCODE when adding large text or numeric columns to keep compression consistent. 3) Use DEFAULT only when every future row needs the same value—otherwise leave NULL and handle in queries.

Common mistakes

Omitting DEFAULT on NOT NULL. Redshift does not allow NOT NULL without DEFAULT. Fix by adding DEFAULT or leave the column nullable.

Expecting data rewrite time. ADD COLUMN is metadata-only; plan a separate UPDATE if you need historical values.

Why can’t I drop the column right after adding it?

You can, but VACUUM may be required to reclaim space if you inserted data. Use: ALTER TABLE table_name DROP COLUMN col;

Frequently asked questions

Does ADD COLUMN lock the table?

It takes a brief metadata lock, usually milliseconds, so concurrent reads continue unaffected.

Can I add identity or generated columns?

Yes, but only one identity per table and it must be BIGINT. Syntax: ADD COLUMN new_id BIGINT IDENTITY(1,1);

How do I specify compression encoding?

Apply ENCODE: ADD COLUMN details VARCHAR(100) ENCODE ZSTD; Use ANALYZE COMPRESSION to pick the best codec.

Why How to ADD COLUMN in Redshift is important

How to ADD COLUMN in Redshift Example Usage


-- Add a "discount_code" column to Orders
ALTER TABLE Orders
ADD COLUMN discount_code VARCHAR(20);

-- Update current orders with a placeholder
UPDATE Orders SET discount_code = 'NONE' WHERE discount_code IS NULL;

How to ADD COLUMN in Redshift Syntax


ALTER TABLE table_name
ADD [ COLUMN ] column_name data_type
    [ DEFAULT default_expr ]
    [ IDENTITY(seed,step) ]
    [ ENCODE encoding ]
[, ADD [ COLUMN ] column_name2 ... ];
-- Example with ecommerce tables
ALTER TABLE Customers
ADD COLUMN phone VARCHAR(15) ENCODE BYTEDICT,
ADD COLUMN opted_in BOOLEAN DEFAULT FALSE;

Common Mistakes

Attempting to define NOT NULL without a DEFAULT. Redshift rejects this because existing rows would violate the constraint. Fix by adding a DEFAULT or by removing NOT NULL.
Forgetting to specify ENCODE for large columns, which leads to suboptimal compression and slower scans. Run ANALYZE COMPRESSION first or explicitly set ENCODE ZSTD or LZO.