How to modify the performance characteristics for the instance of the Jet engine in an MFC application in Visual C++ (178942)

SUMMARY

MFC does not document or directly expose any methods to manipulate the performance parameters of the Jet Database (DB) engine. However, if you are using DAO 3.5, you can gain access to the DB engine from within an MFC application and alter the performance characteristics just for the instance of the Jet engine your application is using. This method allows the user to programmatically set performance characteristics, which will apply only to the specific application. For example, the MaxBufferSize could be set during program startup.

MORE INFORMATION

To gain access to the functions that allow manipulation of Jet, you must first get access to the DB engine. The undocumented function AfxDaoGetEngine() returns a pointer to the current instance of the DB engine. The function is found in Daocore.cpp:

   DAODBEngine* AFXAPI AfxDaoGetEngine()
   {
     // Return DAODBEngine object for direct access
     return AfxGetDaoState()->m_pDAODBEngine;
   }

The following line of code returns a pointer to the DB engine. No special header files are required:

   DAODBEngine* m_pDAODBEngine = AfxDaoGetEngine();

Once you have a pointer to the DB engine, the DAO SDK function SetOption() can be used to set performance parameters. These parameters only affect the specific instance of the engine for your application. They do not affect the default Jet performance parameters stored in the registry. The following code examples show how to change some of the specific parameters:

   // Change the MaxBufferSize to 128. When doing thousands of
   // inserts, this will restrict memory growth.
   COleVariant  vBufferSize((long)128, VT_I4);
   m_pDAODBEngine->SetOption( dbMaxBufferSize, vBufferSize );

   // Reduce the time to defer asynchronous flushes on a
   // database opened for exclusive use.
   COleVariant  vExclusiveAsyncDelay((long)10, VT_I4);
   m_pDAODBEngine->SetOption(dbExclusiveAsyncDelay,vExclusiveAsyncDelay);

   // Reduce the time to defer asynchronous flushes on a
   // database opened for shared use.
   COleVariant vSharedAsyncDelay((long)15, VT_I4);
   m_pDAODBEngine->SetOption(dbSharedAsyncDelay, vSharedAsyncDelay);

Note It is necessary to cast the first parameter in the constructor for the COleVariant in order to avoid the "ambiguous call to overloaded function" error at compile time.

The following values and their defaults can be changed:

PageTimeout	5000
SharedAsyncDelay	100
ExclusiveAsyncDelay	2000
LockRetry	20
UserCommitSync	Yes
ImplicitCommitSync	NO
MaxBufferSize	0
MaxLocksPerFile	9500
LockDelay	100
RecycleLVs	0
FlushTransactionTimeout	500

References

Note The best source for additional information on these settings is the Access 97 Help Files. Search for the topic:

Initializing the Microsoft Jet 3.5 Database Engine Driver