Under certain circumstances the processing of retrieval statements can be very timeconsuming and can block other requests. In order to avoid slowing performance to too great a degree, the DBH modifies its search strategy to suit the situation in such cases.
Processing via secondary indexes
The DBH interrupts the processing of retrieval statements via secondary indexes and continues it sequentially if it sees no advantage in performing searches by means of secondary indexes.
Sequential processing
The DBH stops the sequential processing of retrieval statements as soon as the number of logical accesses to the user data exceeds a defined limit (see the “Database Operation” manual).
Statements interrupted as a result of this are queued until processing can continue. The request remains assigned to the thread. The thread is not available for use by other requests.
If user data is accessed too frequently from the DBH's point of view (in other words, a set limit is exceeded), the DBH terminates processing completely.
Controlling the processing of retrieval statements
Using the DBH option RETRIEVAL-CONTROL, system administrators can adapt the criteria by which the DBH chooses its search strategy to suit the requirements of the session. For example, they can define a limit for the number of logical accesses to user data which, when reached, should cause the DBH to abort sequential processing. Administration statements are also available which they can use to adjust the DBH's search strategy to current requirements during the current DBH session (see section “Administration statements and commands”).