HP 3000 Manuals HP 3000 Manuals
Accessing Mapped Tables
ALLBASE/Turbo CONNECT provides transparent read access to TurboIMAGE/XL
data. However, Turbo CONNECT users should keep some considerations in
mind when reading TurboIMAGE/XL data. This section discusses these
considerations.
Performance Considerations
The following factors can affect Turbo CONNECT performance:
Up-to-Date Information. When data from two or more tables is combined
(joined), it is important to have accurate statistics on these tables.
Use the ALLBASE/SQL UPDATE STATISTICS command to ensure that this
information is up-to-date. Mapped table statistics should be updated
immediately after the database is attached and again when the number of
data entries in mapped tables has changed significantly. Refer to the
ALLBASE/SQL Database Administration Guide for information about how to
use this command.
Search Items in Queries. When a mapped column represents a TurboIMAGE/XL
search item, Turbo CONNECT uses the search item to retrieve the data more
efficiently whenever possible. (An exception: If the search item is of
type P or Z, it is not used, a serial read is always done.) Because of
this, using a mapped column that represents a search item in the WHERE
clause of an ALLBASE/SQL SELECT statement can improve performance in
certain circumstances. Task 14, "Selecting ALLBASE/SQL Data," describes
these circumstances and gives examples of their use.
Guidelines for ALLBASE/SQL Programmers
ALLBASE/SQL programmers should keep the following in mind when accessing
mapped tables:
* The EXECUTE command can only be used to invoke commands that do not
alter data content or structure.
* OPEN with KEEP CURSOR, TRANSFER OWNERSHIP, and LOCK TABLE commands
cannot be used with mapped tables.
* All mapped tables are accessed at the ALLBASE/SQL Read Uncommitted
isolation level, regardless of the isolation level requested in a
BEGIN WORK command.
* DECLARE CURSOR FOR UPDATE is not allowed. Use DECLARE CURSOR FOR NO
UPDATE instead.