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Asynchronous Query Execution

, 8 Dec 1999
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Two classes that provide support for asynchronous SQL execution.
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    Introduction

    Because of multithreading support is available, the MFC ODBC classes no longer use asynchronous processing. By default, drivers execute ODBC functions synchronously; that is, the application calls a function and the driver does not return control to the application until it has finished executing the function. However, some functions can be executed asynchronously; that is, the application calls the function, and the driver, after minimal processing, returns control to the application. The application can then call other functions while the first function is still executing.

    Asynchronous execution is supported for most functions that are largely executed on the data source, such as the functions to prepare and execute SQL statements, retrieve metadata, and fetch data. It is most useful when the task being executed on the data source takes a long time, such as a complex query against a large database.

    In MFC version 4.2 and higher, CDatabase::SetSynchronousMode() function which allows asynchronous execution has become obsolete. The MFC ODBC classes now use only synchronous processing.

    Two classes CAsyncDatabase and CAsyncRecordset provide support for asynchronous execution. This allows an application perform other tasks while a complex query is being executed.

    Usage is very simple. Copy CAsyncDatabase.* and CAsyncRecordset.* files into the project. Do not forget

    #include "AsyncDatabase.h"
    #include "AsyncRecordset.h"
    

    Examine the following samples:

    For CAsyncDatabase:

    • Open database as usual;
    • Call ExecuteSQLAsync(<sql_query_text_here>);
    • Call CAsyncDatabase's SQLStillExecuting() to determine whether the query is still executing;
    • Call CDatabase's Cancel() function with the HSTMT returned by ExecuteSQLAsync() to cancel the query execution.

    CAsyncDatabase Sample

    // asynchronous query execution
    void ExecuteLongQuery(strQuery)
    {
    	CAsyncDatabase DB;
    	DB.OpenEx("");
    
    	HSTMT hstmt = DB.ExecuteSQLAsync(strQuery);	// complex query
    	while(DB.SQLStillExecuting(hstmt))
    	{
    		// check for Cancel command, for exmpl
    		_GET_MESSAGE_
    		if(Cancelled()) 	// by Cancel button, for example
    		{
    			DB.Cancel(hstmt);
    			return;
    		}
    	}
    }
    

    For CAsyncRecordset:

    • Open recordset using OpenAsync(...). (parameters are the same as for Open());
    • Call CAsyncRecordset's StillExecuting() to determine whether the query is still executing;
    • Call CRecordset's Cancel() function to cancel the recordset opening (i.e. SELECT query execution).

    CAsyncRecordset Sample

    // Open Asynchronous Recordset
    {
    	// DB is a CDatabase
    	CAsyncRecordset rs(&DB);
    
    	rs.OpenAsync(CRecordset::snapshot, strQuery, CRecordset::executeDirect);
    	
    	while(rs.StillExecuting())
    	{
    		_GET_MESSAGE_
    		if(Cancelled())
    		{
    			rs.Cancel();
    			rs.Close();
    			return;
    		}
    	}
    	
    	if(!rs.IsOpen())
    		return;
    	else
    		ContinueTask();	// perform some task
    	rs.Close();
    }
    

    A Few Words About OnSetOptions() Virtual

    The framework calls this member function to set initial options for the recordset or database. CRecordset and CDatabase's OnSetOptions() determines the data source's support for scrollable cursors and for cursor concurrency and sets the recordset's options accordingly.

    You can override OnSetOptions() to set additional options specific to the driver or the data source. For example, if your data source supports opening for exclusive access, you might override OnSetOptions() to take advantage of that ability.

    CAsyncDatabase and CAsyncRecordset classes use OnSetOptions() to determine the data source's support for asynchronous function execution and set options accordingly. (If data source does not support asynchronous execution both CAsyncDatabase and CAsyncRecordset execute functions synchronously without warning user.) So when You override OnSetOptions() be sure you call CAsyncDatabase::OnSetOptions() or CAsyncRecordset::OnSetOptions() from derived class OnSetOptions().

    Note:According to MFC documentation asynchronous processing slows performance.

    License

    This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

    A list of licenses authors might use can be found here

    About the Author

    Vachik Hovhannisyan

    United States United States
    No Biography provided

    Comments and Discussions

     
    GeneralAsynchronous Query execution in .Net Pingroupelizas8-Feb-10 2:43 
    GeneralDynamicRecordset.h PinmemberDo Manh Hung1-Apr-03 17:19 
    Dir sir,
    I can not find DynamicRecordset.h attached with zip file that I downloaded. Where can I get it?
    Is it faster than to use ORACE library support for database ( ODatabase ..)
     
    Thank you,
     
    Do Manh Hung
    GeneralRe: DynamicRecordset.h PinmemberRolando E. Cruz-Marshall23-Dec-08 8:38 
    GeneralAsynchronous query execution vs SQLCancel PinmemberTom Tempelaere, aka TiTi3-Sep-01 2:00 

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