PostgreSQL 7 doesn't allow you to alter an existing column's size attributes.
You'll need to do several steps.
Alter the table to add a new temporary varchar column with size 100.
copy the data from existing varchar column to the new temp column.
Alter the table to drop the existing 70 character column.
Alter the table to add the old column name with a size of 100 characters.
Copy the data from the temp column to the new column
Drop the temp column.
Beginning with pg version 8, you can directly alter an existing column.
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