SAP ASE guarantees repeatable reads on materialized computed columns, regardless of their deterministic property, because they are not reevaluated when you reference them in a query. Instead, SAP ASE uses the preevaluated values.
Deterministic materialized computed columns always have the same values, no matter how often they are reevaluated.
Nondeterministic materialized computed columns must adhere to these rules:
Each evaluation of the same computed column may return a different result, even using the same set of inputs.
References to nondeterministic preevaluated computed columns use the preevaluated results, which may differ from current evaluation results. In other words, historical rather than current data is used in nondeterministic preevaluated computed columns.
In the first example from Examples of Nondeterministic Computed Columns, Start_Date is a nondeterministic materialized computed column. Its results differ, depending on what day you insert the row. For instance, if the rental period begins on “02/05/04,” “02/05/04” is inserted into the column, and later references to Start_Date use this value. If you reference this value later, on 06/05/04, the query continues to return “02/05/04,” not “06/05/04,” as you would expect if the expression was evaluated every time you query the column.