将 BLOB 值写入 SQL Server 时保留资源

将 BLOB 值写入 SQL Server 时保留资源

通过插入或更新带有字符串值或字节数组（取决于数据库中的字段类型）的字段，可以将二进制大对象 (BLOB) 写入数据库（请参见将 BLOB 值写入数据库）。但是，BLOB 可能相当大，因此在作为单个值写入时可能要使用大量的系统内存，从而降低应用程序的性能。

为减少写入 BLOB 值时使用的内存量，通常是将 BLOB 以“块区”的形式写入数据库。以此方法将 BLOB 写入数据库的过程依赖于数据库的功能。

以下示例演示如何将 BLOB 以块区形式写入 SQL Server。该示例向 Northwind 数据库的 Employees 表添加了一个包含员工图像的新记录，该图像就是一个 BLOB。该示例使用 SQL Server 的 UPDATETEXT 函数将新添加的员工的图像以指定大小的块区写入 Photo 字段。

UPDATETEXT 函数要求一个指向所更新的 BLOB 字段的指针。在此示例中，在添加新员工的记录后，将调用 SQL Server TEXTPTR 函数以返回一个指向新记录的 Photo 字段的指针。返回的指针值将作为输出参数传递回去。示例中的代码保留此指针，并在追加数据块区时将其传递到 UPDATETEXT。

用于插入新员工记录和保留指向 Photo 字段的指针的 Transact-SQL 将在下例中显示（其中 @Identity 和 @Pointer 被标识为 SqlCommand 的输出参数）。

INSERT INTO Employees (LastName, FirstName, Title, HireDate, ReportsTo, Photo)
Values(@LastName, @FirstName, @Title, @HireDate, @ReportsTo, 0x0)
SELECT @Identity = SCOPE_IDENTITY()
SELECT @Pointer = TEXTPTR(Photo) FROM Employees WHERE EmployeeID = @Identity

请注意，在 Photo 字段中插入了初始值 0x0（空）。这确保可以检索到新插入记录的 Photo 字段的指针值。但是，空值不会影响追加的数据块区。

在保留指向新插入记录中的 Photo 字段的指针后，示例可以接着使用 SQL Server 的 UPDATETEXT 函数向 BLOB 字段追加数据块区。UPDATETEXT 函数接受以下对象作为输入：字段标识符 (Employees.Photo)、指向 BLOB 字段的指针、表示 BLOB 中写入当前块区的位置的偏移量值，以及要追加的数据块区。以下代码示例显示 UPDATETEXT 函数的语法（其中 @Pointer、@Offset 和 @Bytes 被标识为 SqlCommand 的输入参数）。

UPDATETEXT Employees.Photo @Pointer @Offset 0 @Bytes

偏移量值由内存缓冲区的大小确定，而该大小取决于应用程序的需要。大的缓冲区写入 BLOB 的速度较快，但会使用更多的系统内存。此示例使用的缓冲区相当小，只有 128 字节。为第一个数据块区分配的偏移量值为 0，然后偏移量值按每个连续块区的缓冲区大小递增。

该示例按块区从提供的文件路径中检索员工相片。它根据指定的缓冲区大小，将每个块区读入一个字节数组。然后，将字节数组设置为 SqlCommand 的 @Bytes 输入参数的值。更新 @Offset 参数值并执行 SqlCommand 后，当前的字节块区追加到员工记录的 Photo 字段中。

[Visual Basic]
Imports System
Imports System.Data
Imports System.Data.SqlClient
Imports System.IO
Public Class EmployeeData
Public Shared Sub Main()
Dim hireDate As DateTime = DateTime.Parse("4/27/98")
Dim newID As Integer = _
AddEmployee("Smith", "John", "Sales Representative", hireDate, 5, "smith.bmp")
Console.WriteLine("New Employee added. EmployeeID = " & newID)
End Sub
Public Shared Function AddEmployee(lastName As String, firstName As String, title As String, hireDate As DateTime, _
reportsTo As Integer, photoFilePath As String) As Integer
Dim nwindConn As SqlConnection = New SqlConnection("Data Source=localhost;Integrated Security=SSPI;Initial Catalog=Northwind;")
Dim addEmp As SqlCommand = New SqlCommand("INSERT INTO Employees (LastName, FirstName, Title, HireDate, ReportsTo, Photo) " & _
"Values(@LastName, @FirstName, @Title, @HireDate, @ReportsTo, 0x0);" & _
"SELECT @Identity = SCOPE_IDENTITY();" & _
"SELECT @Pointer = TEXTPTR(Photo) FROM Employees WHERE EmployeeID = @Identity", nwindConn)
addEmp.Parameters.Add("@LastName",  SqlDbType.NVarChar, 20).Value = lastName
addEmp.Parameters.Add("@FirstName", SqlDbType.NVarChar, 10).Value = firstName
addEmp.Parameters.Add("@Title",     SqlDbType.NVarChar, 30).Value = title
addEmp.Parameters.Add("@HireDate",  SqlDbType.DateTime).Value     = hireDate
addEmp.Parameters.Add("@ReportsTo", SqlDbType.Int).Value          = reportsTo
Dim idParm As SqlParameter = addEmp.Parameters.Add("@Identity", SqlDbType.Int)
idParm.Direction = ParameterDirection.Output
Dim ptrParm As SqlParameter = addEmp.Parameters.Add("@Pointer", SqlDbType.Binary, 16)
ptrParm.Direction = ParameterDirection.Output
nwindConn.Open()
addEmp.ExecuteNonQuery()
Dim newEmpID As Integer = CType(idParm.Value, Integer)
StorePhoto(photoFilePath, ptrParm.Value, nwindConn)
nwindConn.Close()
Return newEmpID
End Function
Public Shared Sub StorePhoto(fileName As String, pointer As Byte(), nwindConn As SqlConnection)
Dim bufferLen As Integer = 128   ' The size of the "chunks" of the image.
Dim appendToPhoto As SqlCommand = New SqlCommand("UPDATETEXT Employees.Photo @Pointer @Offset 0 @Bytes", nwindConn)
Dim ptrParm As SqlParameter = appendToPhoto.Parameters.Add("@Pointer", SqlDbType.Binary, 16)
ptrParm.Value = pointer
Dim photoParm As SqlParameter = appendToPhoto.Parameters.Add("@Bytes", SqlDbType.Image, bufferLen)
Dim offsetParm As SqlParameter = appendToPhoto.Parameters.Add("@Offset", SqlDbType.Int)
offsetParm.Value = 0
''''''''''''''''''''''''''''''''''''
'' Read the image in and write it to the database 128 (bufferLen) bytes at a time.
'' Tune bufferLen for best performance. Larger values write faster, but
'' use more system resources.
Dim fs As FileStream = New FileStream(fileName, FileMode.Open, FileAccess.Read)
Dim br As BinaryReader = New BinaryReader(fs)
Dim buffer() As Byte = br.ReadBytes(bufferLen)
Dim offset_ctr As Integer = 0
Do While buffer.Length > 0
photoParm.Value = buffer
appendToPhoto.ExecuteNonQuery()
offset_ctr += bufferLen
offsetParm.Value = offset_ctr
buffer = br.ReadBytes(bufferLen)
Loop
br.Close()
fs.Close()
End Sub
End Class
[C#]
using System;
using System.Data;
using System.Data.SqlClient;
using System.IO;
public class EmployeeData
{
public static void Main()
{
DateTime hireDate = DateTime.Parse("4/27/98");
int newID  = AddEmployee("Smith", "John", "Sales Representative", hireDate, 5, "smith.bmp");
Console.WriteLine("New Employee added. EmployeeID = " + newID);
}
public static int AddEmployee(string lastName, string firstName, string title, DateTime hireDate , int reportsTo, string photoFilePath)
{
SqlConnection nwindConn = new SqlConnection("Data Source=localhost;Integrated Security=SSPI;Initial Catalog=Northwind;");
SqlCommand addEmp  = new SqlCommand("INSERT INTO Employees (LastName, FirstName, Title, HireDate, ReportsTo, Photo) " +
"Values(@LastName, @FirstName, @Title, @HireDate, @ReportsTo, 0x0);" +
"SELECT @Identity = SCOPE_IDENTITY();" +
"SELECT @Pointer = TEXTPTR(Photo) FROM Employees WHERE EmployeeID = @Identity", nwindConn);
addEmp.Parameters.Add("@LastName",  SqlDbType.NVarChar, 20).Value = lastName;
addEmp.Parameters.Add("@FirstName", SqlDbType.NVarChar, 10).Value = firstName;
addEmp.Parameters.Add("@Title",     SqlDbType.NVarChar, 30).Value = title;
addEmp.Parameters.Add("@HireDate",  SqlDbType.DateTime).Value     = hireDate;
addEmp.Parameters.Add("@ReportsTo", SqlDbType.Int).Value          = reportsTo;
SqlParameter idParm = addEmp.Parameters.Add("@Identity", SqlDbType.Int);
idParm.Direction = ParameterDirection.Output;
SqlParameter ptrParm = addEmp.Parameters.Add("@Pointer", SqlDbType.Binary, 16);
ptrParm.Direction = ParameterDirection.Output;
nwindConn.Open();
addEmp.ExecuteNonQuery();
int newEmpID = (int)idParm.Value;
StorePhoto(photoFilePath, (byte[])ptrParm.Value, nwindConn);
nwindConn.Close();
return newEmpID;
}
public static void StorePhoto(string fileName, byte[] pointer,  SqlConnection nwindConn)
{
int bufferLen = 128;  // The size of the "chunks" of the image.
SqlCommand appendToPhoto = new SqlCommand("UPDATETEXT Employees.Photo @Pointer @Offset 0 @Bytes", nwindConn);
SqlParameter ptrParm  = appendToPhoto.Parameters.Add("@Pointer", SqlDbType.Binary, 16);
ptrParm.Value = pointer;
SqlParameter photoParm = appendToPhoto.Parameters.Add("@Bytes", SqlDbType.Image, bufferLen);
SqlParameter offsetParm = appendToPhoto.Parameters.Add("@Offset", SqlDbType.Int);
offsetParm.Value = 0;
//''''''''''''''''''''''''''''''''''
// Read the image in and write it to the database 128 (bufferLen) bytes at a time.
// Tune bufferLen for best performance. Larger values write faster, but
// use more system resources.
FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
BinaryReader br = new BinaryReader(fs);
byte[] buffer = br.ReadBytes(bufferLen);
int offset_ctr = 0;
while (buffer.Length > 0)
{
photoParm.Value = buffer;
appendToPhoto.ExecuteNonQuery();
offset_ctr += bufferLen;
offsetParm.Value = offset_ctr;
buffer = br.ReadBytes(bufferLen);
}
br.Close();
fs.Close();
}
}