Microsoft KB Archive/178942

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

Article ID: 178942

Article Last Modified on 11/21/2006

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APPLIES TO


 * Microsoft Foundation Class Library 4.2
 * Microsoft Visual C++ 5.0 Enterprise Edition
 * Microsoft Visual C++ 6.0 Enterprise Edition
 * Microsoft Visual C++ 5.0 Professional Edition
 * Microsoft Visual C++ 6.0 Professional Edition
 * Microsoft Visual C++ 6.0 Standard Edition

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This article was previously published under Q178942



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: