Package cache is memory that temporarily stores package and section information required for the execution of static and dynamic SQL statements in the database. Caching packages allows the database manager to reduce its internal overhead by eliminating the need to access the system catalog when reloading a package. Likewise, caching dynamic SQL information eliminates the need to compile the statement. In general, caching the section for a static or dynamic SQL statement can improve performance, especially when the same statement is used multiple times by the application.
Sections are held in package cache until one of the following occurs:
The database is shut down
The package or dynamic SQL statement is invalidated
The cache runs out of space
The Package cache graph contains two series:
Hit Rate—Plots the percentage of package cache lookups that were satisfied by data already residing in the package cache. If this hit rate is high, the cache is performing effectively. A smaller ratio might indicate the need to increase package cache.
Overflow—Plots, as a percentage of total package cache activity, the number of times the cache overflowed the limits of the memory allocated for it.
The graph title area shows the number of package cache inserts and lookups that occurred during the last monitoring interval.
The Statistics tab contains entries for all the package cache statistical counts used to generate this graph. The names for these statistics begin with Package Cache. These statistics are derived from the PKG_CACHE data elements that the DB2 snapshot monitor captures.