TN045: MFC/Database Support for Long Varchar/Varbinary

This note describes how to retrieve and send the ODBC SQL_LONGVARCHAR and SQL_LONGVARBINARY data types using the MFC database classes.

Overview of Long Varchar/Varbinary Support

The ODBC SQL_LONG_VARCHAR and SQL_LONGBINARY data types (referred to here as long data columns) can hold huge amounts of data. There are 3 ways you can handle this data:

• Bind it to a CString/CByteArray.

• Bind it to a CLongBinary.

• Do not bind it at all and retrieve and send the long data value manually, independent of the database classes.

Each of the three methods has advantages and disadvantages.

Long data columns are not supported for parameters to a query. They are only supported for outputColumns.

Binding a Long Data Column to a CString/CByteArray

Advantages:

This approach is simple to understand, and you work with familiar classes. The framework provides CFormView support for CString with DDX_Text. You have lots of general string or collection functionality with the CString and CByteArray classes, and you can control the amount of memory allocated locally to hold the data value. The framework maintains an old copy of field data during Edit or AddNew function calls, and the framework can automatically detect changes to the data for you.

The RFX functions for CString and CByteArray have an additional argument which lets you override the default size of allocated memory to hold the retrieved value for the data column. Note the nMaxLength argument in the following function declarations:

void AFXAPI RFX_Text(CFieldExchange* pFX, const char *szName,
    CString& value, int nMaxLength = 255, int nColumnType =
    SQL_VARCHAR);

void AFXAPI RFX_Binary(CFieldExchange* pFX, const char *szName, 
    CByteArray& value,int nMaxLength = 255);

If you retrieve a long data column into a CString or CByteArray, the maximum returned amount of data is, by default, 255 bytes. Anything beyond this is ignored. In this case, the framework will throw the exception AFX_SQL_ERROR_DATA_TRUNCATED. Fortunately, you can explicitly increase nMaxLength to greater values, up to MAXINT.

ClassWizard will bind a SQL_LONGVARCHAR to a CString, or a SQL_LONGVARBINARY to a CByteArray for you. If you want to allocate more than 255 bytes into which you retrieve your long data column, you can then supply an explicit value for nMaxLength.

When a long data column is bound to a CString or CByteArray, updating the field works just the same as when it is bound to a SQL_VARCHAR or SQL_VARBINARY. During Edit, the data value is cached away and later compared when Update is called to detect changes to the data value and set the Dirty and Null values for the column appropriately.

Binding a Long Data Column to a CLongBinary

If your long data column may contain more MAXINT bytes of data, you should probably consider retrieving it into a CLongBinary.

Advantages:

This retrieves an entire long data column, up to available memory.

Disadvantages:

The data is held in memory. This approach is also prohibitively expensive for very large amounts of data. You must call SetFieldDirty for the bound data member to ensure the field is included in an Update operation.

If you retrieve long data columns into a CLongBinary, the database classes will check the total size of the long data column, then allocate an HGLOBAL memory segment large enough to hold it the entire data value. The database classes then retrieve the entire data value into the allocated HGLOBAL.

If the data source cannot return the expected size of the long data column, the framework will throw the exception AFX_SQL_ERROR_SQL_NO_TOTAL. If the attempt to allocate the HGLOBAL fails, a standard memory exception is thrown.

ClassWizard will bind an SQL_LONGVARCHAR or SQL_LONGVARBINARY to a CLongBinary for you. Select CLongBinary as the Variable Type in the Add Member Variable dialog. ClassWizard will then add an RFX_LongBinary call to your DoFieldExchange call and increment the total number of bound fields.

To update long data column values, first make sure the allocated HGLOBAL is large enough to hold your new data by calling ::GlobalSize on the m_hData member of the CLongBinary. If it is too small, release the HGLOBAL and allocate one the appropriate size. Then set m_dwDataLength to reflect the new size.

Otherwise, if m_dwDataLength is larger than the size of the data you're replacing, you can either free and reallocate the HGLOBAL, or leave it allocated. Make sure to indicate the number of bytes actually used in m_dwDataLength.

How Updating a CLongBinary Works

It is not necessary to understand how updating a CLongBinary works, but it may be useful as an example on how to send long data values to a data source, if you choose this third method, described below.

When updating a CLongBinary field, the database classes use ODBC's DATA_AT_EXEC mechanism (see ODBC documentation on SQLSetPos's rgbValue argument). When the framework prepares the insert or update statement, instead of pointing to the HGLOBAL containing the data, the address of the CLongBinary is set as the value of the column instead, and the length indicator set to SQL_DATA_AT_EXEC. Later, when the update statement is sent to the data source, SQLExecDirect will return SQL_NEED_DATA. This alerts the framework that the value of the param for this column is actually the address of a CLongBinary. The framework calls SQLGetData once with a small buffer, expecting the driver to return the actual length of the data. If the driver returns the actual length of the binary large object (the BLOB), MFC reallocates as much space as necessary to fetch the BLOB. If the data source returns SQL_NO_TOTAL, indicating that it can't determine the size of the BLOB, MFC will create smaller blocks. The default initial size is 64K, and subsequent blocks will be double the size; for example, the second will be 128K, the third is 256K, and so on. The initial size is configurable.

Not Binding: Retrieving/Sending Data Directly from ODBC with SQLGetData

With this method you completely bypass the database classes, and deal with the long data column yourself.

Advantages:

You can cache data to disk if necessary, or decide dynamically how much data to retrieve.

Disadvantages:

You don't get the framework's Edit or AddNew support, and you must write code yourself to perform basic functionality (Delete does work though, since it is not a column level operation).

In this case, the long data column must be in the select list of the recordset, but should not be bound to by the framework. One way to do this is to supply your own SQL statement via GetDefaultSQL or as the lpszSQL argument to CRecordset's Open function, and not bind the extra column with an RFX_ function call. ODBC requires that unbound fields appear to the right of bound fields, so add your unbound column or columns to the end of the select list.

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