Implementing bulk record fetching - c++

Question

At the start of my program, I need to read data from a MS Access database (.mdb) into a drop down control. This is done so that whenever the user types in that control, the application can auto-complete.
Anyway, the reading from database took forever so I thought I'd implement bulk row fetching.
This is the code I have:
CString sDsn;
CString sField;
sDsn.Format("ODBC;DRIVER={%s};DSN='';DBQ=%s",sDriver,sFile);
TRY
{
// Open the database
database.Open(NULL,false,false,sDsn);
// Allocate the rowset
CMultiRowset recset( &database );
// Build the SQL statement
SqlString = "SELECT NAME "
"FROM INFOTABLE";
// Set the rowset size. These many rows will be fetched in one bulk operation
recset.SetRowsetSize(25);
// Open the rowset
recset.Open(CRecordset::forwardOnly, SqlString, CRecordset::readOnly | CRecordset::useMultiRowFetch);
// Loop through each rowset
while( !recset.IsEOF() )
{
int rowsFetched = (int)recset.GetRowsFetched(); // This value is always 1 somehow
for( int rowCount = 1; rowCount <= rowsFetched; rowCount++ )
{
recset.SetRowsetCursorPosition(rowCount);
recset.GetFieldValue("NAME",sField);
m_nameDropDown.AddString(sField);
}
// Go to next rowset
recset.MoveNext();
}
// Close the database
database.Close();
}
CATCH(CDBException, e)
{
// If a database exception occured, show error msg
AfxMessageBox("Database error: "+e->m_strError);
}
END_CATCH;
MultiRowset.cpp looks like:
#include "stdafx.h"
#include "afxdb.h"
#include "MultiRowset.h"
// Constructor
CMultiRowset::CMultiRowset(CDatabase *pDB)
: CRecordset(pDB)
{
m_NameData = NULL;
m_NameDataLengths = NULL;
m_nFields = 1;
CRecordset::CRecordset(pDB);
}
void CMultiRowset::DoBulkFieldExchange(CFieldExchange *pFX)
{
pFX->SetFieldType(CFieldExchange::outputColumn);
RFX_Text_Bulk(pFX, _T("[NAME]"), &m_NameData, &m_NameDataLengths, 30);
}
MultiRowset.h looks like:
#if !defined(__MULTIROWSET_H_AD12FD1F_0566_4cb2_AE11_057227A594B8__)
#define __MULTIROWSET_H_AD12FD1F_0566_4cb2_AE11_057227A594B8__
class CMultiRowset : public CRecordset
{
public:
// Field data members
LPSTR m_NameData;
// Pointers for the lengths of the field data
long* m_NameDataLengths;
// Constructor
CMultiRowset(CDatabase *);
// Methods
void DoBulkFieldExchange(CFieldExchange *);
};
#endif
And in my database, the INFOTABLE looks like:
NAME AGE
---- ---
Name1 Age1
Name2 Age2
.
.
.
.
All I need to do is only read the data from the database. Can someone please tell me what I'm doing wrong? My code right now behaves exactly like a normal fetch. There's no bulk fetching happening.
EDIT:
I just poked around in DBRFX.cpp and found out that RFX_Text_Bulk() initializes my passed m_NameData as new char[nRowsetSize * nMaxLength]!
This means m_NameData is only a character array! I need to fetch multiple names, so wouldn't I need a 2D character array? The strangest thing is, the same RFX_Text_Bulk() initializes my passed m_NDCDataLengths as new long[nRowsetSize]. Why in the world would a character array need an array of lengths?!

Answer 1

According to http://msdn.microsoft.com/en-us/library/77dcbckz.aspx#_core_how_crecordset_supports_bulk_row_fetching you have to open CRecordset with CRecordset::useMultiRowFetch flag before call SetRowsetSize:
To implement bulk row fetching, you must specify the
CRecordset::useMultiRowFetch option in the dwOptions parameter of the
Open member function. To change the setting for the rowset size, call
SetRowsetSize.

Answer 2

You almost got it right. To fetch the values,
I would change your
for( int rowCount = 1; rowCount <= rowsFetched; rowCount++ )
{
recset.SetRowsetCursorPosition(rowCount);
recset.GetFieldValue("NAME",sField);
m_nameDropDown.AddString(sField);
}
by something like this
for( int nPosInRowset = 0; nPosInRowset < rowsFetched; nPosInRowset++ )
{
//Check if value is null
if (*(recset.m_NameDataLengths + nPosInRowset) == SQL_NULL_DATA)
continue;
CString csComboString;
csComboString = (recset.m_NameData + (nPosInRowset * 30)); //Where 30 is the size specified in RFX_Text_Bulk
m_nameDropDown.AddString(csComboString);
}
EDIT: To fetch more than one row, remove the CRecordset::forwardOnly option
EDIT 2 : You can also keep CRecordset::forwardonly, but add the CRecordset::useExtendedFetch option

Answer 3

Just faced the same problem.
You should use in recset.Open() call for dwOptions parameter only CRecordset::useMultiRowFetch, and not CRecordset::readOnly | CRecordset::useMultiRowFetch.
Hope this helps someone...
EDIT:- After re-check here is the situation - when using bulk recordset and opening with CRecordset::forwardOnly and CRecordset::readOnly, you must also specify CRecordset::useExtendedFetch in dwOptions. For other types of scrolling, using CRecordset::readOnly | CRecordset::useMultiRowFetch is just fine.