实现批量记录获取

2022-01-12 00:00:00 database fetch c++ mfc bulk

在我的程序开始时，我需要将 MS Access 数据库 (.mdb) 中的数据读取到下拉控件中.这样做是为了在用户输入该控件时，应用程序可以自动完成.

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.

无论如何，从数据库中读取数据需要很长时间，所以我想我应该实现批量行获取.

Anyway, the reading from database took forever so I thought I'd implement bulk row fetching.

这是我的代码:

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 看起来像:

#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 看起来像:

#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

在我的数据库中，INFOTABLE 看起来像:

And in my database, the INFOTABLE looks like:

NAME    AGE
----    ---
Name1   Age1
Name2   Age2
      .
      .
      .
      .

我需要做的只是读取数据库中的数据.有人可以告诉我我做错了什么吗?我的代码现在的行为与正常提取完全一样.不会发生批量提取.

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.

我刚刚在 DBRFX.cpp 中闲逛，发现 RFX_Text_Bulk() 将我传递的 m_NameData 初始化为 new char[nRowsetSize * nMaxLength]!

I just poked around in DBRFX.cpp and found out that RFX_Text_Bulk() initializes my passed m_NameData as new char[nRowsetSize * nMaxLength]!

这意味着 m_NameData 只是一个字符数组！我需要获取多个名称，所以我不需要二维字符数组吗?最奇怪的是，同样的 RFX_Text_Bulk() 将我传递的 m_NDCDataLengths 初始化为 new long[nRowsetSize].为什么一个字符数组需要一个长度数组?！

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?!

推荐答案

你几乎做对了.要获取值，我会改变你的

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);
        }

通过这样的方式

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);
}

要获取多于一行，请删除 CRecordset::forwardOnly 选项

To fetch more than one row, remove the CRecordset::forwardOnly option

编辑 2:您也可以保留 CRecordset::forwardonly，但添加 CRecordset::useExtendedFetch 选项

EDIT 2 : You can also keep CRecordset::forwardonly, but add the CRecordset::useExtendedFetch option

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