Usar tipos de valor grande

Proveedor OLE DB de SQL Server Native Client

El proveedor OLE DB de SQL Server Native Client expone los tipos varchar(max), varbinary(max) y nvarchar(max) como DBTYPE_STR, DBTYPE_BYTES y DBTYPE_WSTR, respectivamente.

Cuando los tipos de datos varchar(max), varbinary(max) y nvarchar(max) aparecen en columnas con el tamaño max establecido en ilimitado, se representan como ISLONG en las interfaces y en los conjuntos de filas de esquema de OLE DB básicos que devuelven tipos de datos de columna.

Se ha modificado la implementación de IAccessor del objeto de comando para permitir el enlace como DBTYPE_IUNKNOWN. Si el consumidor especifica DBTYPE_IUNKNOWN y establece pObject en NULL, el proveedor devolverá la interfaz ISequentialStream al consumidor para que éste pueda transmitir los datos varchar(max), nvarchar(max) o varbinary(max) fuera de las variables de salida.

Los valores de los parámetros de salida transmitidos se devuelven después de las filas de resultados. Si la aplicación intenta pasar al siguiente conjunto de resultados mediante una llamada a IMultipleResults::GetResult sin consumir todos los valores de parámetros de salida devueltos, se devolverá DB_E_OBJECTOPEN.

El proveedor OLE DB de SQL Server Native Client exige que se obtenga acceso a los parámetros de longitud variable en orden secuencial para permitir la transmisión en secuencias. Esto significa que DBPROP_ACCESSORDER debe establecerse en DBPROPVAL_AO_SEQUENTIALSTORAGEOBJECTS o DBPROPVAL_AO_SEQUENTIAL cada vez que las columnas o los parámetros de salida varchar(max), nvarchchar(max) o varbinary(max) se enlazan a DBTYPE_IUNKNOWN. Si no se cumple esta restricción de orden de acceso, se producirá un error en las llamadas a IRowset::GetData con DBSTATUS_E_UNAVAILABLE. Esta restricción no se aplica cuando no se realizan enlaces de salida mediante DBTYPE_IUNKNOWN.

El proveedor OLE DB de SQL Server Native Client también permite enlazar parámetros de salida, como DBTYPE_IUNKNOWN, en tipos de datos de valor grande para facilitar escenarios en los que un procedimiento almacenado devuelve tipos de valor grande, como valores devueltos expuestos como DBTYPE_IUNKNOWN al cliente.

Para trabajar con estos tipos, una aplicación dispone de las opciones siguientes:

• Enlace como un tipo que incluye enlaces compatibles con el tipo base de la columna; por ejemplo, en nvarchar(max), el enlace como un tipo que puede enlazarse a nvarchar. Si el búfer no es suficientemente grande, se producirá un truncamiento, exactamente igual que en el tipo base, aunque ahora estén disponibles estos valores mayores.

• Enlace como un tipo que incluye conversiones compatibles con el tipo base de la columna y también especificación de DBTYPE_BYREF.

• Enlace como DBTYPE_IUNKNOWN y uso de la transmisión en secuencias.

Al notificar el tamaño máximo de una columna, el proveedor OLE DB de SQL Server Native Client notificará:

• El tamaño máximo definido que, por ejemplo, es 2000 para una columna varchar(2000), o bien

• El valor "ilimitado", que en el caso de una columna varchar(max) es igual a ~0. Este valor se establece para la propiedad de metadatos DBCOLUMN_COLUMNSIZE.

En una columna varchar(max) se aplicarán las reglas de conversión estándar, es decir, cualquier conversión que sea válida para una columna varchar(2000) también será válida para una columna varchar(max). Lo mismo sucede en las columnas nvarchar(max) y varbinary(max).

A la hora de recuperar tipos de valor grande, el enfoque más eficaz consiste en enlazar como DBTYPE_IUNKNOWN y establecer la propiedad DBPROP_ACCESSORDER del conjunto de filas en DBPROPVAL_AO_SEQUENTIALSTORAGEOBJECTS. Esto hará que el valor se transmita directamente desde la red sin almacenarse en el búfer intermedio, como en el ejemplo siguiente:

#define UNICODE
#define _UNICODE
#define DBINITCONSTANTS
#define INITGUID
#define OLEDBVER 0x0250  // To include the correct interfaces.

#include <stdio.h>
#include <tchar.h>
#include <stddef.h>
#include <iostream>

using std::cout;
using std::endl;

#include <windows.h>

#include <oledb.h>
#include "sqlncli.h"
#include <oledberr.h>

#define CHKHR_GOTO(hr, errMsg, Label) \
   if (FAILED(hr)) \
   { \
      cout << errMsg << endl; \
      goto Label; \
   }

#define MAX_COL_SIZE 8000

// ROUNDUP on all platforms pointers must be aligned properly.
#define ROUNDUP_AMOUNT 8
#define ROUNDUP_(size,amount) (((ULONG)(size)+((amount)-1))&~((amount)-1))
#define ROUNDUP(size) ROUNDUP_(size, ROUNDUP_AMOUNT)

HRESULT InitializeAndEstablishConnection(IDBInitialize** ppIDBInitialize);
void UnInitializeConnection(IDBInitialize* pIDBInitialize);
HRESULT CreateAndSetCommand(IDBInitialize* pIDBInitialize, ICommandText** ppICommandText);
HRESULT ProcessResultSet(IRowset* pIRowset);

void DisplayTime()
{
   SYSTEMTIME st;
   GetSystemTime(&st);
   cout<< st.wHour << ":" << st.wMinute << ":" << st.wSecond << "." << st.wMilliseconds << endl;
}

void main()
{
   HRESULT hr;
   IDBInitialize* pIDBInitialize = NULL;
   ICommandText* pICommandText = NULL;
   IMultipleResults* pIMultipleResults = NULL;
   IRowset* pIRowset = NULL;

   hr = InitializeAndEstablishConnection(&pIDBInitialize);
   CHKHR_GOTO(hr, L"Failed to establish connection.", _ExitMain);

   hr = CreateAndSetCommand(pIDBInitialize, &pICommandText);
   CHKHR_GOTO(hr, L"Failed to set up command object.", _ExitMain);

   DisplayTime();

   hr = pICommandText->Execute(NULL, 
      IID_IMultipleResults, 
      NULL, 
      NULL, 
     (IUnknown **) &pIMultipleResults);

   CHKHR_GOTO(hr, L"Failed to execute command.", _ExitMain);

   while (1)
   {
      hr = pIMultipleResults->GetResult(
         NULL, 
         DBRESULTFLAG_DEFAULT, 
         IID_IRowset, 
         NULL, 
         (IUnknown**)&pIRowset);

   CHKHR_GOTO(hr, L"Failed to obtain a results from MR object.", _ExitMain);

   if (hr == DB_S_NORESULT)
      break;

      if (pIRowset)
      {
         hr = ProcessResultSet(pIRowset); 
         CHKHR_GOTO(hr, L"Failed to process the current Rowset.", _ExitMain);

         pIRowset->Release();
         pIRowset = NULL;
      }
   }

   DisplayTime();

_ExitMain:

   if (pIRowset)
   {
      pIRowset->Release();
      pIRowset = NULL;
   }

   if (pIMultipleResults)
   {
      pIMultipleResults->Release();
      pIMultipleResults = NULL;
   }

   if (pICommandText)
   {
      pICommandText->Release();
      pICommandText = NULL;
   }

   UnInitializeConnection(pIDBInitialize);
   return;
};

HRESULT InitializeAndEstablishConnection(IDBInitialize** ppIDBInitialize)
{
   HRESULT hr;
   IDBInitialize* pIDBInitialize = NULL;
   IDBProperties* pIDBProperties = NULL;

   const int NUM_DBINIT_PROPS = 3;
   const wchar_t* const g_wszServer = L".";
   const wchar_t* const g_wszCatalog = L"AdventureWorks2008R2";
   const wchar_t* const g_wszSecurity = L"SSPI";

   DBPROPSET rgdbPropSetInit[1];
   DBPROP rgdbPropInit [NUM_DBINIT_PROPS];

   *ppIDBInitialize = NULL;
   hr = CoInitialize(NULL);
   CHKHR_GOTO(hr, L"Failed to initialize COM.", _ExitInitialize);

   hr = CoCreateInstance(CLSID_SQLNCLI10, 
      NULL, 
      CLSCTX_INPROC_SERVER,
      IID_IDBInitialize, 
      (void**)&pIDBInitialize);

   CHKHR_GOTO(hr, L"Failed to create SQLNCLI10 DataSource object.", _ExitInitialize);

   for(int idxProp = 0; idxProp < NUM_DBINIT_PROPS; idxProp++) 
   {
      VariantInit(&rgdbPropInit[idxProp].vValue);
   }

   rgdbPropInit[0].dwPropertyID = DBPROP_INIT_DATASOURCE;
   rgdbPropInit[0].vValue.vt = VT_BSTR;
   rgdbPropInit[0].vValue.bstrVal= SysAllocString(g_wszServer);
   rgdbPropInit[0].dwOptions = DBPROPOPTIONS_REQUIRED;
   rgdbPropInit[0].colid = DB_NULLID;

   if (rgdbPropInit[0].vValue.bstrVal == NULL)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitInitialize;
   }

   rgdbPropInit[1].dwPropertyID = DBPROP_INIT_CATALOG;
   rgdbPropInit[1].vValue.vt = VT_BSTR;
   rgdbPropInit[1].vValue.bstrVal= SysAllocString(g_wszCatalog);
   rgdbPropInit[1].dwOptions = DBPROPOPTIONS_REQUIRED;
   rgdbPropInit[1].colid = DB_NULLID;

   if (rgdbPropInit[1].vValue.bstrVal == NULL)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitInitialize;
   }

   rgdbPropInit[2].dwPropertyID = DBPROP_AUTH_INTEGRATED;
   rgdbPropInit[2].vValue.vt = VT_BSTR;
   rgdbPropInit[2].vValue.bstrVal= SysAllocString(g_wszSecurity);
   rgdbPropInit[2].dwOptions = DBPROPOPTIONS_REQUIRED;
   rgdbPropInit[2].colid = DB_NULLID;

   if (rgdbPropInit[2].vValue.bstrVal == NULL)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitInitialize;
   }

   rgdbPropSetInit[0].guidPropertySet = DBPROPSET_DBINIT;
   rgdbPropSetInit[0].cProperties = NUM_DBINIT_PROPS;
   rgdbPropSetInit[0].rgProperties = rgdbPropInit;

   hr = pIDBInitialize->QueryInterface(IID_IDBProperties, (void **)&pIDBProperties);
   CHKHR_GOTO(hr, L"Failed to QI DataSource object for IDBProperties.", _ExitInitialize);

   hr = pIDBProperties->SetProperties(1, rgdbPropSetInit); 
   CHKHR_GOTO(hr, L"Failed to set DataSource object Properties.", _ExitInitialize);

   pIDBProperties->Release();
   pIDBProperties = NULL;

   hr = pIDBInitialize->Initialize();
   CHKHR_GOTO(hr, L"Failed to establish connection with the server.", _ExitInitialize);

_ExitInitialize:

   if (pIDBProperties)
   {
      pIDBProperties->Release();
      pIDBProperties = NULL;
   }

   if (FAILED(hr))
   {
      if (pIDBInitialize)
      {
         pIDBInitialize->Release();
         pIDBInitialize = NULL;
      }
   }

   *ppIDBInitialize = pIDBInitialize;
   return hr;
}

void UnInitializeConnection(IDBInitialize* pIDBInitialize)
{
   if (pIDBInitialize)
   {
      pIDBInitialize->Uninitialize();
      pIDBInitialize->Release();
      pIDBInitialize = NULL;
   }
   CoUninitialize();
}

HRESULT CreateAndSetCommand(IDBInitialize* pIDBInitialize, ICommandText** ppICommandText)
{
   HRESULT hr;
   IDBCreateSession* pIDBCreateSession = NULL;
   IDBCreateCommand* pIDBCreateCommand = NULL;
   ICommandText* pICommandText = NULL;
   ICommandProperties* pICommandProperties = NULL;
   DBPROPSET rgCmdPropSet[1];
   DBPROP rgCmdProperties[1];

const wchar_t* const g_wCmdString = L"declare @x xml, @y nvarchar(max); select @x = (SELECT * FROM Sales.SalesOrderHeader FOR XML AUTO); select @x;";

   *ppICommandText = NULL;

   if (!pIDBInitialize)
   {
      hr = E_FAIL;
      goto _ExitCreateAndSetCommand;
   }

   hr = pIDBInitialize->QueryInterface(IID_IDBCreateSession, (void**) &pIDBCreateSession);
   CHKHR_GOTO(hr, L"Failed to obtain IDBCreateSession interface from DSO.", _ExitCreateAndSetCommand);

   hr = pIDBCreateSession->CreateSession(
      NULL, 
      IID_IDBCreateCommand, 
      (IUnknown**) &pIDBCreateCommand);

   CHKHR_GOTO(hr, L"Failed to Create a Session for command execution.", _ExitCreateAndSetCommand);

   hr = pIDBCreateCommand->CreateCommand(
      NULL, 
      IID_ICommandText, 
      (IUnknown**)&pICommandText);

   CHKHR_GOTO(hr, L"Failed to Create a Command object.", _ExitCreateAndSetCommand);

   hr = pICommandText->SetCommandText(DBGUID_DBSQL, g_wCmdString);
   CHKHR_GOTO(hr, L"Failed to Set Command Text.", _ExitCreateAndSetCommand);

   hr = pICommandText->QueryInterface(IID_ICommandProperties, (void**) &pICommandProperties);
   CHKHR_GOTO(hr, L"Failed to obtain ICommandProperties interface from the command object.", _ExitCreateAndSetCommand);

   rgCmdProperties[0].dwPropertyID = DBPROP_ACCESSORDER;
   rgCmdProperties[0].vValue.vt = VT_I4;
   rgCmdProperties[0].vValue.lVal = DBPROPVAL_AO_SEQUENTIAL;
   rgCmdProperties[0].dwOptions = DBPROPOPTIONS_REQUIRED;
   rgCmdProperties[0].colid = DB_NULLID;

   rgCmdPropSet[0].guidPropertySet = DBPROPSET_ROWSET;
   rgCmdPropSet[0].cProperties = 1;
   rgCmdPropSet[0].rgProperties = rgCmdProperties;

   hr = pICommandProperties->SetProperties(1, rgCmdPropSet); 
   CHKHR_GOTO(hr, L"Failed to Set Command object Properties.", _ExitCreateAndSetCommand);

_ExitCreateAndSetCommand:

   if (pICommandProperties)
   {
      pICommandProperties->Release();
      pICommandProperties = NULL;
   }

   if (pIDBCreateCommand)
   {
      pIDBCreateCommand->Release();
      pIDBCreateCommand = NULL;
   }

   if (pIDBCreateSession)
   {
      pIDBCreateSession->Release();
      pIDBCreateSession = NULL;
   }

   if (FAILED(hr))
   {
      if (pICommandText)
      {
         pICommandText->Release();
         pICommandText = NULL;
      }
   }

   *ppICommandText = pICommandText;
   return hr;
}

HRESULT ProcessResultSet(IRowset* pIRowset)
{
   HRESULT hr;

   IColumnsInfo* pIColumnsInfo = NULL;
   DBCOLUMNINFO* pDBColumnInfo = NULL;
   ULONG lNumCols = 0;
   wchar_t* pStringsBuffer = NULL;

   DBBINDING* pBindings = NULL;
   DBOBJECT dbobj;
   ULONG idxBinding;
   IAccessor* pIAccessor = NULL;
   HACCESSOR hAccessor = DB_NULL_HACCESSOR;
   HROW hRows[1] = {DB_NULL_HROW};
   HROW* pRow = &hRows[0];
   BYTE* pBuffer = NULL;

   ULONG lNumRowsRetrieved;
   DBLENGTH dwOffset = 0;

   hr = pIRowset->QueryInterface(IID_IColumnsInfo, (void **)&pIColumnsInfo);
   CHKHR_GOTO(hr, L"Failed to QI Rowset for IColumnsInfo.", _ExitProcessResultSet);

   hr = pIColumnsInfo->GetColumnInfo(&lNumCols, &pDBColumnInfo, &pStringsBuffer);
   CHKHR_GOTO(hr, L"Failed to obtain Column Information.", _ExitProcessResultSet);

   pBindings = new DBBINDING[lNumCols];

   if (!pBindings)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitProcessResultSet;
   }

   memset(pBindings, 0, sizeof(DBBINDING) * lNumCols);

   dbobj.dwFlags = STGM_READ;
   dbobj.iid = IID_ISequentialStream;

   for (idxBinding = 0; idxBinding < lNumCols; idxBinding++) 
   {
      pBindings[idxBinding].iOrdinal = idxBinding + 1;
      pBindings[idxBinding].obStatus = dwOffset;
      pBindings[idxBinding].obLength = dwOffset + sizeof(DBSTATUS);
      pBindings[idxBinding].obValue = dwOffset + sizeof(DBSTATUS) + sizeof(DBLENGTH);

      pBindings[idxBinding].pTypeInfo = NULL;
      pBindings[idxBinding].pBindExt = NULL;
      pBindings[idxBinding].dwPart = DBPART_VALUE | DBPART_LENGTH | DBPART_STATUS;
      pBindings[idxBinding].dwMemOwner = DBMEMOWNER_CLIENTOWNED;
      pBindings[idxBinding].eParamIO = DBPARAMIO_NOTPARAM;
      pBindings[idxBinding].bPrecision = pDBColumnInfo[idxBinding].bPrecision;
      pBindings[idxBinding].bScale = pDBColumnInfo[idxBinding].bScale;

      pBindings[idxBinding].cbMaxLen = 0;
      pBindings[idxBinding].wType = DBTYPE_WSTR;

   // Determine the maximum number of bytes required in our buffer to
   // contain the Unicode string representation of the provider's native
   // data type, including room for the NULL-termination character
   switch( pDBColumnInfo[idxBinding].wType )
   {
      case DBTYPE_NULL:
      case DBTYPE_EMPTY:
      case DBTYPE_I1:
      case DBTYPE_I2:
      case DBTYPE_I4:
      case DBTYPE_UI1:
      case DBTYPE_UI2:
      case DBTYPE_UI4:
      case DBTYPE_R4:
      case DBTYPE_BOOL:
      case DBTYPE_I8:
      case DBTYPE_UI8:
      case DBTYPE_R8:
      case DBTYPE_CY:
      case DBTYPE_ERROR:
      // When the above types are converted to a string, they
      // will all fit into 25 characters, so use that plus space
      // for the NULL-terminator.

      pBindings[idxBinding].cbMaxLen = (25 + 1) * sizeof(WCHAR);
      break;

      case DBTYPE_DECIMAL:
      case DBTYPE_NUMERIC:
      case DBTYPE_DATE:
      case DBTYPE_DBDATE:
      case DBTYPE_DBTIMESTAMP:
      case DBTYPE_GUID:
      // Converted to a string, the above types will all fit into
      // 50 characters, so use that plus space for the terminator.

      pBindings[idxBinding].cbMaxLen = (50 + 1) * sizeof(WCHAR);
      break;

      case DBTYPE_BYTES:
      // In converting DBTYPE_BYTES to a string, each byte
      // becomes two characters (e.g. 0xFF -> "FF"), so we
      // will use double the maximum size of the column plus
      // include space for the NULL-terminator.

      pBindings[idxBinding].cbMaxLen = (pDBColumnInfo[idxBinding].ulColumnSize * 2 + 1) * sizeof(WCHAR);
      break;

      case DBTYPE_STR:
      case DBTYPE_WSTR:
      case DBTYPE_BSTR:
      // Going from a string to our string representation,
      // we can just take the maximum size of the column,
      // a count of characters, and include space for the
      // terminator, which is not included in the column size.

      pBindings[idxBinding].cbMaxLen = (pDBColumnInfo[idxBinding].ulColumnSize + 1) * sizeof(WCHAR);
      break;

      default:
      // For any other type, we will simply use our maximum
      // column buffer size, since the display size of these
      // columns may be variable (e.g. DBTYPE_VARIANT) or
      // unknown (e.g. provider-specific types).
      pBindings[idxBinding].cbMaxLen = MAX_COL_SIZE;
      break;
   }

   // If the provider's native data type for this column is
   // DBTYPE_IUNKNOWN or this is a BLOB column and the user
   // has requested that we bind BLOB columns as ISequentialStream
   // objects, bind this column as an ISequentialStream object if
   // the provider supports our creating another ISequentialStream
   // binding.
   if(pDBColumnInfo[idxBinding].dwFlags & DBCOLUMNFLAGS_ISLONG)
   {
      pBindings[idxBinding].wType = DBTYPE_IUNKNOWN;

      pBindings[idxBinding].cbMaxLen = sizeof(ISequentialStream*);

      pBindings[idxBinding].pObject = (DBOBJECT *)CoTaskMemAlloc(sizeof(DBOBJECT));

      if (!pBindings[idxBinding].pObject)
      {
         hr = E_OUTOFMEMORY;
         goto _ExitProcessResultSet;
      }

      // Direct the provider to create an ISequentialStream
      // object over the data for this column.
      pBindings[idxBinding].pObject->iid = IID_ISequentialStream;

      // We want read access on the ISequentialStream
      // object that the provider will create for us
      pBindings[idxBinding].pObject->dwFlags = STGM_READ;
      }

      // Ensure that the bound maximum length is no more than the
      // maximum column size in bytes that we've defined.
      pBindings[idxBinding].cbMaxLen = min(pBindings[idxBinding].cbMaxLen, MAX_COL_SIZE);

      // Update the offset past the end of this column's data, so
      // that the next column will begin in the correct place in
      // the buffer.
      dwOffset = pBindings[idxBinding].cbMaxLen + pBindings[idxBinding].obValue;

      // Ensure that the data for the next column will be correctly
      // aligned for all platforms, or, if we're done with columns,
      // that if we allocate space for multiple rows that the data
      // for every row is correctly aligned.
      dwOffset = ROUNDUP(dwOffset);
   }

   hr = pIRowset->QueryInterface(IID_IAccessor, (void **) &pIAccessor);
   CHKHR_GOTO(hr, L"Failed to obtain Accessor interface", _ExitProcessResultSet);

   hr = pIAccessor->CreateAccessor(DBACCESSOR_ROWDATA,
      lNumCols,
      pBindings,
      0,
      &hAccessor,
      NULL);

   CHKHR_GOTO(hr, L"Failed to create Accessor", _ExitProcessResultSet);
   for (idxBinding = 0; idxBinding < lNumCols; idxBinding++) 
   {
      cout << pDBColumnInfo[idxBinding].pwszName << endl;
   }

   lNumRowsRetrieved = 0;

   hr = pIRowset->GetNextRows(
      NULL,
      0,
      1,
      &lNumRowsRetrieved,
      &pRow);

   CHKHR_GOTO(hr, L"Failed to fetch a row from the rowset", _ExitProcessResultSet);

   pBuffer = new BYTE[sizeof(DBSTATUS) + sizeof(DBLENGTH) + sizeof(IUnknown*)];

   if (!pBuffer)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitProcessResultSet;
   }

   while(lNumRowsRetrieved && hr != DB_S_ENDOFROWSET) 
   {
      memset(pBuffer, 0, sizeof(DBSTATUS) + sizeof(DBLENGTH) + sizeof(IUnknown*));

      hr = pIRowset->GetData(hRows[0], hAccessor, pBuffer);
      CHKHR_GOTO(hr, L"Failed to obtain row data", _ExitProcessResultSet);

      for (idxBinding = 0; idxBinding < lNumCols; idxBinding++)
      {
         if (pBindings[idxBinding].wType == DBTYPE_IUNKNOWN)
         {
            BYTE pbBuff[3000];
            ULONG cbNeeded = sizeof(pbBuff)/sizeof(BYTE);
            ULONG cbRead;
            ULONG cbReadTotal = 0;
            ISequentialStream* pISequentialStream = NULL;

            IUnknown* pIUnknown = *((IUnknown**)(pBuffer + pBindings[idxBinding].obValue));
            pIUnknown->QueryInterface(IID_ISequentialStream, (void**)&pISequentialStream);

            do
            {
               hr = pISequentialStream->Read(pbBuff, cbNeeded, &cbRead);
               cbReadTotal += cbRead;
            }
            while (SUCCEEDED(hr) && hr != S_FALSE && cbRead == cbNeeded);

               cout << "Total Bytes Read: " << cbReadTotal << endl;

               pISequentialStream->Release();
               pISequentialStream = NULL;
               pIUnknown->Release();
               pIUnknown = NULL;
            }
         }

         pIRowset->ReleaseRows(1, pRow, NULL, NULL, NULL);

         hr = pIRowset->GetNextRows(NULL,
            0,
            1,
            &lNumRowsRetrieved,
            &pRow);

         CHKHR_GOTO(hr, L"Failed to fetch a row from the rowset.", _ExitProcessResultSet);
   }

_ExitProcessResultSet:

   pIRowset->ReleaseRows(1, pRow, NULL, NULL, NULL);
   delete [] pBuffer;

   if (pIAccessor)
   {
      if (hAccessor != DB_NULL_HACCESSOR)
      {
         pIAccessor->ReleaseAccessor(hAccessor, NULL);
      }

      pIAccessor->Release();
      pIAccessor = NULL;
   }

   if (pBindings)
   {
      for (idxBinding = 0; idxBinding < lNumCols; idxBinding++)
      {
         if (pBindings[idxBinding].pObject)
         CoTaskMemFree(pBindings[idxBinding].pObject);
      }
   }

   delete [] pBindings;

   CoTaskMemFree(pDBColumnInfo);
   CoTaskMemFree(pStringsBuffer);

   if (pIColumnsInfo)
   {
      pIColumnsInfo->Release();
      pIColumnsInfo = NULL;
   }

   return hr;
}

Para obtener más información sobre la forma en que el proveedor OLE DB de SQL Server Native Client expone los tipos de datos de valor grande, vea BLOB y objetos OLE.

Controlador ODBC de SQL Server Native Client

El controlador ODBC de SQL Server Native Client expone los tipos varchar(max), varbinary(max) y nvarchar(max) como SQL_VARCHAR, SQL_VARBINARY y SQL_WVARCHAR en funciones de la API de ODBC que aceptan o devuelven tipos de datos de ODBC SQL.

Al notificar el tamaño máximo de una columna, el controlador notificará:

• El tamaño máximo definido que, por ejemplo, es 2000 para una columna varchar(2000), o bien

• El valor "ilimitado", que en el caso de una columna varchar(max) es igual a 0.

En una columna varchar(max) se aplican las reglas de conversión estándar, es decir, cualquier conversión que sea válida para una columna varchar(2000) también será válida para una columna varchar(max). Lo mismo sucede en las columnas nvarchar(max) y varbinary(max).

A continuación se incluye una lista de las funciones de la API de ODBC en las que se han realizado mejoras para trabajar con tipos de datos de valor grande:

• SQLBindCol

• SQLBindParameter

• SQLColAttribute

• SQLColumns

• SQLDescribeCol

• SQLDescribeParam

• SQLGetData

• SQLGetTypeInfo

Vea también

Conceptos

Otros recursos