OCI Consistent Client Cache

Oracle 11g introduces a number of features to expand the use of memory caching in order to improve query performance.  These features also decrease the overall memory requirements of queries and applications. 

In addition to the features that support server-side caching, which is discussed in Chapter 6, there is also a new feature to support a client-side result cache that allows OCI clients to fetch result sets stored in process memory instead of requiring access to the server's memory.  The Client Result Cache is enabled and configured at the database level and available to all OCI-based clients. 

Also see how to activate Oracle Client Caching for OCI Applications.

Using the client's memory to cache query result sets significantly improves the performance of repetitive queries by OCI-based clients that would otherwise require a cache hit on the server-side.  The client cache utilizes per-process memory on the OCI client and its contents can be shared across multiple sessions and threads. 

When executing repetitive queries, the clients fetch results directly from the client cache rather than having the server execute the query.  In certain situations, this will greatly reduce the number of round-trips between the client and the database server.  This also reduces CPU consumption on the server and greatly increases the query response times.  Situations that will benefit most from the client cache are queries that use small lookup tables or mostly read-only tables. 

The consistency between the client cache and the database server is managed internally by the database, and is a feature available to all OCI clients. 

Configuring the Client Cache from the Database

The current client cache initialization parameters can be viewed by using:

SQL> show parameter client_result

NAME                                 TYPE        VALUE
------------------------------------ ----------- -----
client_result_cache_lag              big integer 3000
client_result_cache_size             big integer 0

SQL>

To enable the client cache, the initialization parameter client_result_cache_size must be set to a value greater than zero since the value of zero disables the client cache.  This parameter is the maximum size of the client-per-process result set cache.  All OCI client processes get this maximum size.

The client_result_cache_lag parameter specifies the maximum amount of time, in milliseconds, that the client result cache can lag behind any changes in the database that affect the result sets.  By default, the value is 3000ms (or three seconds).  Since ideally this feature is used for read-only tables, this setting should not have a significant impact on the consistency of the cached results. 

Both of these initialization parameters are static parameters. This being, changing their values should be done with SCOPE=SPFILE and any new values will not become effective until the database is restarted.  For example, to enable the client cache with a value of 2M for the client cache, issue the following:

SQL> alter system set client_result_cache_size=2M scope=SPFILE;
System altered.

After restarting the database, the new settings have taken effect:

SQL> show parameter client_result

NAME                                 TYPE        VALUE
------------------------------------ ----------- -----
client_result_cache_lag              big integer 3000
client_result_cache_size             big integer 2M

SQL>