There are two types of computed columns in Adaptive Server:
Virtual computed columns
Materialized computed columns
The main difference between these two types of computed columns is that when a virtual computed column is referenced by a query, it is evaluated each time a query accesses it.
A materialized computed column’s result is stored in the table when a data row is inserted, or when any base columns are updated. When a materialized computed column is referenced in a query, it is not reevaluated. Its preevaluated result is used.
A nonmaterialized, or virtual, computed column becomes a materialized computed column once it is used as an index key.
A materialized computed column is re-evaluated only when one of its base columns is updated.
