How to Change a Column Type in Redshift
Last updated July 25, 2026 · By the SaturnSQL team
ALTER TABLE ... ALTER COLUMN ... TYPE only works for widening VARCHAR columns in place. For any other type change, add a new column, backfill it with UPDATE, drop the old column, and rename the new one.
VARCHAR widening (the only in-place change)
ALTER TABLE customers ALTER COLUMN email TYPE VARCHAR(512);Add-backfill-rename for everything else
ALTER TABLE orders ADD COLUMN amount_new NUMERIC(12,2);
UPDATE orders SET amount_new = amount::NUMERIC(12,2);
ALTER TABLE orders DROP COLUMN amount;
ALTER TABLE orders RENAME COLUMN amount_new TO amount;Run the pattern inside one transaction if readers must never see the half-migrated state, and VACUUM afterwards since the UPDATE leaves deleted row versions behind.
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