PRB: RFX_LongBinary Not Optimized for SQL Server

• The Microsoft Foundation Classes (MFC), included with: - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2

SYMPTOMS

Performing inserts of image fields into a SQL Server table using the MFC database classes may result in less than optimum performance. Other operations involving image fields, such as updates, do not suffer from this reduced performance.

CAUSE

MFC's RFX_LongBinary function uses an obsolete method to specify the size of the image field when binding the CLongBinary buffer for the insertion.

Due to limitations of the SQL Server driver, MFC will always use SQL statements instead of SQLSetPos functionality to perform inserts of image fields to a recordset opened on a SQL Server database. MFC also uses parameterized insert statements. As a result, SQLBindParameter() is called to bind the CLongBinary variable's buffer to the parameter in the insert statement. MFC specifies that the length of the long binary field is SQL_DATA_AT_EXEC, a symbol that signifies that data will be transferred via SQLPutData() without providing any information about the field's size. The SQL Server driver generates an internal plan for the insert that contains a subquery of the form "SELECT ... LIKE" that results in a table scan that can be quite time- consuming.

RESOLUTION

To avoid unnecessary table scans when inserting data into image fields, you can modify MFC to use the SQL_LEN_DATA_AT_EXEC() macro that provides information about the size of the image data and thereby minimizes the need for table scans.

NOTE: Visual C++ version 5.0 now uses the SQL_LEN_DATA_AT_EXEC() macro.

STATUS

This behavior is by design.

MORE INFORMATION

The following steps detail one method of making the needed changes in RFX_LongBinary:

1. Copy the implementation of RFX_LongBinary() (MSDEV\MFC\SRC\DBRFX.CPP) into a new .cpp file, and rename the function to something like RFX_LongBinary2(). Create a header file for the .cpp file and include that in your recordset .cpp file. Be sure to #include "stdafx.h" at the top of your new .cpp file, and include the .cpp file in your project.

2. Change all RFX_LongBinary() calls in your recordset's DoFieldExchange() to calls to RFX_LongBinary2().

3. In your new RFX_LongBinary2() implementation, copy the following code that appears under "case CFieldExchange::SetFieldNull":

         // Length is signed value, it's limited by LONG_MAX
         if (value.m_dwDataLength >
             (ULONG)(LONG_MAX - labs(SQL_LEN_DATA_AT_EXEC_OFFSET)))
         {
            ASSERT(FALSE);
            *plLength = LONG_MAX - labs(SQL_LEN_DATA_AT_EXEC_OFFSET);
         }
         else
            *plLength = value.m_dwDataLength;
   
         *plLength = SQL_LEN_DATA_AT_EXEC(*plLength);
   
      and paste this code over the line that contains SQL_DATA_AT_EXEC
      in the following code that appears under "case CFieldExchange::Value":
   
         {
            // Indicate data will be sent after SQLExecute
            *plLength = SQL_DATA_AT_EXEC;       << REPLACE THIS LINE
         }
   
      This will cause the macro to be used instead of the constant.
   
   

4. In the CFieldExchange::SetFieldNull case, you can simply comment out the conditional that currently causes the macro to be used only in those cases where m_bUseUpdateSQL is FALSE. You will comment out a total of five lines as noted below:

        if (pFX->m_prs->m_bUseUpdateSQL)      // COMMENT OUT THIS LINE
           *plLength = SQL_DATA_AT_EXEC;      // COMMENT OUT THIS LINE
        else                                  // COMMENT OUT THIS LINE
        {                                     // COMMENT OUT THIS LINE
           // Length is signed value, it's limited by LONG_MAX
           if (value.m_dwDataLength >
               (ULONG)(LONG_MAX - labs(SQL_LEN_DATA_AT_EXEC_OFFSET)))
           {
              ASSERT(FALSE);
              *plLength = LONG_MAX - labs(SQL_LEN_DATA_AT_EXEC_OFFSET);
           }
           else
              *plLength = value.m_dwDataLength;
   
           *plLength = SQL_LEN_DATA_AT_EXEC(*plLength);
        }                                     // COMMENT OUT THIS LINE
   
   

REFERENCES