Question

我在datacontext中映射了一个表。这是感兴趣的列上的属性和属性：

[Column(Storage="_CustomerNumber", DbType="VarChar(25)")]
public string CustomerNumber
{

这个列实际上是一个varchar（25）并且有一个索引。

我有一些简单的代码：

DataClasses1DataContext myDC = new DataClasses1DataContext();
myDC.Log = Console.Out;

List<string> myList = new List<string>() { "111", "222", "333" };
myDC.Customers
    .Where(c => myList.Contains(c.CustomerNumber))
    .ToList();

生成此SQL文本：

SELECT [t0].[CustomerNumber], [t0].[CustomerName]
FROM [dbo].[Customers] AS [t0]
WHERE [t0].[CustomerNumber] IN (@p0, @p1, @p2)
-- @p0: Input NVarChar (Size = 3; Prec = 0; Scale = 0) [111]
-- @p1: Input NVarChar (Size = 3; Prec = 0; Scale = 0) [222]
-- @p2: Input NVarChar (Size = 3; Prec = 0; Scale = 0) [333]
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.21022.8

请注意，参数是nvarchar！

当此查询到达数据库时，它会生成一个可怕的计划，其中涉及将CustomerNumber上的数百万行索引转换为nvarchar ，然后再在其中搜索。

我不允许更改表，但我可以更改查询和dbml。如果不进行此索引转换，我该怎么做才能获取数据？

Answer 1

以下是我现在解决这个问题的方法。这会将参数转换为所需类型，然后运行查询。它生成与最初生成的sql相同的sql，只是使用不同的参数类型。

DbCommand myCommand = myDataContext.GetCommand(query);

foreach (DbParameter dbParameter in myCommand.Parameters)
{
  if (dbParameter.DbType == System.Data.DbType.String)
  {
    dbParameter.DbType = System.Data.DbType.AnsiString;
  }
}    

myDataContext.Connection.Open();

System.Data.Common.DbDataReader reader = myCommand.ExecuteReader();
List<RecordType> result = myDataContext.Translate<RecordType>(reader).ToList();

myDataContext.Connection.Close();

Answer 2

这就是我可以解决的问题。我也有兴趣看到其他解决方案：

List<IQueryable<Customer>> myQueries = 
    myList.Select(s => myDC.Customers.Where(c => c.CustomerNumber == s)).ToList();
IQueryable<Customers> myQuery = myQueries.First();
foreach(IQueryable<Customer> someQuery in myQueries.Skip(1))
{
    myQuery = myQuery.Concat(someQuery);
}
myQuery.ToList();

这将生成以下SQL：

SELECT [t4].[CustomerNumber], [t4].[CustomerName]
FROM (
    SELECT [t2].[CustomerNumber], [t2].[CustomerName]
    FROM (
        SELECT [t0].[CustomerNumber], [t0].[CustomerName]
        FROM [dbo].[Customer] AS [t0]
        WHERE [t0].[CustomerNumber] = @p0
        UNION ALL
        SELECT [t1].[CustomerNumber], [t1].[CustomerName]
        FROM [dbo].[Customer] AS [t1]
        WHERE [t1].[CustomerNumber] = @p1
        ) AS [t2]
    UNION ALL
    SELECT [t3].[CustomerNumber], [t3].[CustomerName]
    FROM [dbo].[Customer] AS [t3]
    WHERE [t3].[CustomerNumber] = @p2
    ) AS [t4]
-- @p0: Input VarChar (Size = 3; Prec = 0; Scale = 0) [111]
-- @p1: Input VarChar (Size = 3; Prec = 0; Scale = 0) [222]
-- @p2: Input VarChar (Size = 3; Prec = 0; Scale = 0) [333]
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.21022.8

Answer 3

对于我正在处理的项目，我们必须使用ObjectContext而不是DbContext，因此我无权访问GetCommand()或DbParameters 。我所做的是将查询转换为字符串，删除查询中的NVARCHAR指标，然后针对ObjectContext执行查询。这是给出的示例的详细实现：

List<string> myList = new List<string>() { "111", "222", "333" };

IQueryable<Customers> badQuery =  myDC.Customers
    .Where(c => myList.Contains(c.CustomerNumber));

string query = ((System.Data.Objects.ObjectQuery)badQuery).ToTraceString();
query = query.Replace(",N'", ",'").Replace("(N'","('");

List<Customers> customers = myDC.ExecuteStoreQuery<Customers>(query).ToList();

LinqToSql .Contains和nvarchar vs varchar参数 - ＆gt;指数转换计划

3 个答案: