A section is a stored representation of an access path chosen by the
optimizer. Sections are created by SQLCore when you do the following:
Preprocess an application using the ALLBASE/SQL preprocessors.
Issue a statement in ISQL.
Use dynamic operations in an application. Such operations
are built using the PREPARE and EXECUTE statements.
Issue a CREATE PROCEDURE statement.
Sections are one of three types, depending on how you create them:
Permanent sections, stored in the system catalog.
Semi-permanent sections, deleted from memory after your session ends.
Temporary sections, deleted from memory after your transaction ends.
Permanent sections are stored for non-dynamic statements you include in
preprocessed applications. These sections are located in a system
table called HPRDBSS.STOREDSECT, and their names are listed in the
SYSTEM.SECTION view. You cannot access HPRDBSS.STOREDSECT, since
it is an ALLBASE/SQL internal table.
Section Caching |
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Reading sections into memory from the system catalog at run time
can cause a lot of I/O activity. Section caching allows sections
to remain in memory between transactions. By default, up to 12 sections can be cached.
Refer to the "System
Administration" chapter for information about changing
these defaults.
Validation |
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A section becomes invalid when something it depends on is no longer
as it was when the section was originally marked valid. For example,
a section containing a query on a particular table will be marked
invalid if an index on that table is dropped, or when the UPDATE
STATISTICS statement is used on the table.
A status of VALID means the section contains an appropriate access
path for getting to data; a status of INVALID means the access
path may no longer be appropriate.
When a section becomes invalid, it must be revalidated before it
can be used again. SQLCore attempts to validate application program
sections at run time. If a necessary section cannot be revalidated,
an error results. During revalidation, exclusive locks are obtained
on the system catalog base tables HPRDBSS.SECTION,
HPRDBSS.STOREDSECT, and HPRDBSS.DEPENDENCY.
You can use the VALIDATE statement to revalidate sections singly or
in groups at any time, such as after issuing the UPDATE STATISTICS
statement. For best performance, issue all the UPDATE STATISTICS statements
first, then issue all the VALIDATE statements. This ensures that you
only invalidate and revalidate a section once, even if it is dependent
on several tables.
