Notifying the User About Access-Method Constraints

The database server cannot detect unsupported or restricted features for which the sysams system catalog table has no setting.

Data Integrity Limitations

Specify any precautions that an application might require for isolation levels, lock types, and logging.

Advise users whether the access method handles logging and data recovery. Notify users about parameters that they might set to turn logging on. For an example, refer to Figure 7.

Provide the precise wording for the isolation levels that the access method supports. It is recommended that you use standard wording for isolation level. The following example shows the language to define the ways in which the qualifying data set might change in the transaction.

The access method fully supports the ANSI Repeatable Read level of
isolation. The user need not account for dirty reads or
nonrepeatable reads. It is recommended that you take precautions
against phantom reads.

WHERE Clause Limitations

The sysams system catalog table has no indicator to inform the database server that a secondary access method cannot process complex qualifications. If the access method does not process the Boolean operators in a WHERE clause, perform the following actions:

• Provide examples in the user guide of UNION and subqueries that replace AND or OR operators in a WHERE clause, as the following example demonstrates.

  Query Using Boolean Operator	Query using UNION
  SELECT * FROM videos WHERE title = 'Hamlet' OR year > 1980;	SELECT * FROM videos WHERE title = 'Hamlet UNION SELECT * FROM videos WHERE year > 1980;

• In the am_scancost purpose function, call the mi_qual_issimple() or mi_qual_boolop() accessor function to detect a Boolean operator.

  If mi_qual_issimple() returns MI_FALSE, for example, return a value that forces the optimizer to ignore this access method for the particular query. For an example, refer to Figure 31.

• Raise an error if mi_qual_issimple() returns MI_FALSE to the am_getnext purpose function.