Disclosed herein are system, method, and computer program product embodiments for altering the data type of a column in a database. An embodiment operates by converting an original dictionary associated with a column into a new dictionary. The new dictionary stores the values of the original dictionary using a different data type. An index vector containing the keys of the original dictionary is then updated to contain the associated keys of the new dictionary. Because the size of the original dictionary is often substantially smaller than the number of rows in the associated column, this dictionary conversion decreases the computation cost to the database system of altering the data type of the column and reduces or even minimizes database downtime for users.