"Adding a column with a DEFAULT clause or changing the type of an existing column will require the entire table and its indexes to be rewritten. As an exception when changing the type of an existing column, if the USING clause does not change the column contents and the old type is either binary coercible to the new type or an unconstrained domain over the new type, a table rewrite is not needed; but any indexes on the affected columns must still be rebuilt. Adding or removing a system oid column also requires rewriting the entire table. Table and/or index rebuilds may take a significant amount of time for a large table; and will temporarily require as much as double the disk space."