Data definition changes can have dramatic effects on the design of a database. The Schema Viewer tries to be smart about what queries you make, invalidating its cache of schema information for any tables that it believes have been altered.
However, if the information displayed might be out of date (because someone else made a change or MacSQL didn't notice that you changed the structure of the schema -- say by using a stored procedure), you can manually clear the Schema Viewer's cache.
If you would like to purge all the schema information MacSQL has cached for your current document, select Refresh All Schema from the Schema menu. You can also invalidate the data for a specific table by selecting that table in schema browser and then choosing Refresh Table Schema from the Schema menu.