如何在实体框架中指定索引提示？

Posted 2023-02-15

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【中文标题】如何在实体框架中指定索引提示？【英文标题】：How can I specify an index hint in Entity Framework? 【发布时间】：2011-12-23 06:47:40 【问题描述】：

sql

select * from table1 with(index=IX_table1_1)

Linq to sql 使用 ado.net 实体想写上面的代码。我找不到实体，特别是索引提示的使用。

linq

var querysample = from a in db.table1
select a;

【问题讨论】：

你不能在 EF LINQ 中指定查询提示...你必须使用 ExecuteStoreQuery @Sam 是 ExecuteStoreQuery L2S 的 EF 版本 ExecuteQuery<T>? 是的......除了它慢了 2* @MarcGravell 我更改了标题 - 这是否准确地代表了问题？ @Sam 如果有人会编写一个工具来执行这些基于 SQL 的查询，而无需通常与 ORM 相关的开销！ 【参考方案1】：

除了 wh1sp3r 的回答，请参见下面的另一个拦截器，它依赖于 EF query tags 而不是线程静态变量：

public class QueryHintInterceptor : DbCommandInterceptor

    private static readonly Regex _tableAliasRegex = new Regex("( AS [^ ]+)",
        RegexOptions.Multiline | RegexOptions.IgnoreCase);
    private readonly string _hintPrefix;

    public QueryHintInterceptor(string hintPrefix)
    
        _hintPrefix = "-- " + hintPrefix;
    

    public override InterceptionResult<DbDataReader> ReaderExecuting(DbCommand command,
        CommandEventData eventData, InterceptionResult<DbDataReader> result)
    
        PatchCommandText(command);
        return base.ReaderExecuting(command, eventData, result);
    

    public override ValueTask<InterceptionResult<DbDataReader>> ReaderExecutingAsync(
        DbCommand command, CommandEventData eventData,
        InterceptionResult<DbDataReader> result,
        CancellationToken cancellationToken = default)
    
        PatchCommandText(command);
        return base.ReaderExecutingAsync(command, eventData, result, cancellationToken);
    

    public override InterceptionResult<object> ScalarExecuting(DbCommand command,
        CommandEventData eventData, InterceptionResult<object> result)
    
        PatchCommandText(command);
        return base.ScalarExecuting(command, eventData, result);
    

    public override ValueTask<InterceptionResult<object>> ScalarExecutingAsync(
        DbCommand command, CommandEventData eventData, InterceptionResult<object> result,
        CancellationToken cancellationToken = default)
    
        PatchCommandText(command);
        return base.ScalarExecutingAsync(command, eventData, result, cancellationToken);
    

    private void PatchCommandText(DbCommand command)
    
        if (command.CommandText.StartsWith(_hintPrefix, StringComparison.Ordinal))
        
            int index = command.CommandText.IndexOfAny(Environment.NewLine.ToCharArray(),
                _hintPrefix.Length);
            command.CommandText = _tableAliasRegex
                .Replace(command.CommandText, "$0 WITH (" +  command.CommandText
                    .Substring(_hintPrefix.Length, index - _hintPrefix.Length) + ")")
                .Substring(index);

扩展方法：

public static class QueryHintsDbContextOptionsBuilderExtensions

    private const string HintTag = "Use hint: ";
    public static IQueryable<T> WithHint<T>(this IQueryable<T> source,
        TableHint hint) =>
        source.TagWith(HintTag + hint);

    public static DbContextOptionsBuilder<TContext> AddQueryHints<TContext>(
        this DbContextOptionsBuilder<TContext> builder)
        where TContext : DbContext =>
        builder.AddInterceptors(new QueryHintInterceptor(HintTag));



public enum TableHint

    KeepIdentity,
    KeepDefaults,
    HoldLock,
    Ignore_Constraints,
    Ignore_Triggers,
    Nolock,
    NoWait,
    PagLock,
    ReadCommitted,
    ReadCommittedLock,
    ReadPast,
    RepeatableRead,
    RowLock,
    Serializable,
    Snapshot,
    TabLock,
    TabLockX,
    UpdLock,
    Xlock

示例用法：

await using var context = new TestDbContext(
    new DbContextOptionsBuilder<TestDbContext>()
        .UseSqlServer("<connection string>")
        .AddQueryHints()
        .LogTo(message => Console.WriteLine("EF: 0", message))
        .Options);

var result = await context.SomeEntities
    .WithHint(TableHint.TabLock)
    .ToListAsync();

更新

上述自定义 SQL 生成的方式不会影响.ToQueryString()，因为命令拦截器在实际 SQL 生成之后才起作用。它会导致测试和日志记录出现问题，例如在 ASP.Net 核心中。所以另一种解决方案是使用自定义QuerySqlGenerator：

public static class TableHintsDbContextOptionsBuilderExtensions

    public static IQueryable<T> WithHint<T>(this IQueryable<T> source,
        TableHint hint) =>
        source.TagWith(hint.ToString());

    public static DbContextOptionsBuilder UseTableHints(
        this DbContextOptionsBuilder builder) =>
        builder.ReplaceService<IQuerySqlGeneratorFactory,
            HintTagSqlServerQuerySqlGeneratorFactory>();


class HintTagSqlServerQuerySqlGeneratorFactory : IQuerySqlGeneratorFactory

    private readonly QuerySqlGeneratorDependencies dependencies;

    public HintTagSqlServerQuerySqlGeneratorFactory(
        QuerySqlGeneratorDependencies dependencies)
    
        this.dependencies = dependencies;
    

    public QuerySqlGenerator Create()
    
        return new HintTagSqlServerQuerySqlGenerator(dependencies);
    


class HintTagSqlServerQuerySqlGenerator : SqlServerQuerySqlGenerator

    private readonly HashSet<TableHint> tableHints = new();

    public HintTagSqlServerQuerySqlGenerator(
        QuerySqlGeneratorDependencies dependencies) :
        base(dependencies)
    
    

    protected override void GenerateTagsHeaderComment(
        SelectExpression selectExpression)
    
        foreach (var tag in selectExpression.Tags)
            if (Enum.TryParse(typeof(TableHint), tag, out var hint))
            
                tableHints.Add((TableHint)hint!);
                selectExpression.Tags.Remove(tag);
            

        base.GenerateTagsHeaderComment(selectExpression);
    

    protected override Expression VisitTable(
        TableExpression tableExpression)
    
        var result = base.VisitTable(tableExpression);

        if (tableHints.Count > 0)
            Sql.Append($" WITH (string.Join(", ", tableHints).ToUpperInvariant())");

        return result;

但是，警告表明

Microsoft.EntityFrameworkCore.SqlServer.Query.Internal.SqlServerQuerySqlGenerator 是一个内部 API，它支持 Entity Framework Core 基础结构，并且不受与公共 API 相同的兼容性标准的约束。它可能会在任何版本中更改或删除，恕不另行通知。

【讨论】：

【参考方案2】：

添加到 stop-crans 答案，这适用于（我在）EF Core 5.0 并支持WITH(INDEX(param)) 的参数。

public class QueryHintInterceptor : DbCommandInterceptor

    private static readonly Regex _tableAliasRegex = new Regex(@"(FROM[\s\r\n]+\S+(?:[\s\r\n]+AS[\s\r\n]+[^\s\r\n]+)?)", RegexOptions.Multiline | RegexOptions.IgnoreCase);
    private readonly string _hintPrefix;

    public QueryHintInterceptor(string hintPrefix)
    
        _hintPrefix = "-- " + hintPrefix;
    

    public override InterceptionResult<DbDataReader> ReaderExecuting(DbCommand command, CommandEventData eventData, InterceptionResult<DbDataReader> result)
    
        PatchCommandtext(command);
        return base.ReaderExecuting(command, eventData, result);
    

    public override ValueTask<InterceptionResult<DbDataReader>> ReaderExecutingAsync(DbCommand command, CommandEventData eventData, InterceptionResult<DbDataReader> result, CancellationToken cancellationToken = default)
    
        PatchCommandtext(command);
        return base.ReaderExecutingAsync(command, eventData, result, cancellationToken);
    

    public override InterceptionResult<object> ScalarExecuting(DbCommand command, CommandEventData eventData, InterceptionResult<object> result)
    
        PatchCommandtext(command);
        return base.ScalarExecuting(command, eventData, result);
    

    public override ValueTask<InterceptionResult<object>> ScalarExecutingAsync(DbCommand command, CommandEventData eventData, InterceptionResult<object> result, CancellationToken cancellationToken = default)
    
        PatchCommandtext(command);
        return base.ScalarExecutingAsync(command, eventData, result, cancellationToken);
    

    private void PatchCommandtext(DbCommand command)
    
        if (command.CommandText.StartsWith(_hintPrefix, StringComparison.Ordinal))
        
            int index = command.CommandText.IndexOfAny(Environment.NewLine.ToCharArray(), _hintPrefix.Length);
            command.CommandText = _tableAliasRegex
                .Replace(command.CommandText, "$0 WITH (" + command.CommandText
                .Substring(_hintPrefix.Length, index - _hintPrefix.Length) + ")")
                .Substring(index);
        
    


public static class QueryHintsDbContextOptionsBuilderExtensions

    private const string HintTag = "Use hint: ";
    public static IQueryable<T> WithHint<T>(this IQueryable<T> source, TableHint hint) =>
        source.TagWith(HintTag + hint);
    public static IQueryable<T> WithHint<T>(this IQueryable<T> source, TableHint hint, string param) =>
        source.TagWith(HintTag + hint + " (" + param+")");

    public static DbContextOptionsBuilder AddQueryHints(this DbContextOptionsBuilder builder) =>
        builder.AddInterceptors(new QueryHintInterceptor(HintTag));


public enum TableHint

    Index,
    KeepIdentity,
    KeepDefaults,
    HoldLock,
    Ignore_Constraints,
    Ignore_Triggers,
    Nolock,
    NoWait,
    PagLock,
    ReadCommitted,
    ReadCommittedLock,
    ReadPast,
    RepeatableRead,
    RowLock,
    Serializable,
    Snapshot,
    TabLock,
    TabLockX,
    UpdLock,
    Xlock

【讨论】：

【参考方案3】：

解决方案很简单。让我们添加一个拦截器！！！

    public class HintInterceptor : DbCommandInterceptor

    private static readonly Regex _tableAliasRegex = new Regex(@"(?<tableAlias>AS \[Extent\d+\](?! WITH \(*HINT*\)))", RegexOptions.Multiline | RegexOptions.IgnoreCase);

    [ThreadStatic] public static string HintValue;

    public override void ScalarExecuting(DbCommand command, DbCommandInterceptionContext<object> interceptionContext)
    
        if (!String.IsNullOrWhiteSpace(HintValue))
        
            command.CommandText = _tableAliasRegex.Replace(command.CommandText, "$tableAlias WITH (*HINT*)");
            command.CommandText = command.CommandText.Replace("*HINT*", HintValue);
        

        HintValue = String.Empty;
    

    public override void ReaderExecuting(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
    
        if (!String.IsNullOrWhiteSpace(HintValue))
        
            command.CommandText = _tableAliasRegex.Replace(command.CommandText, "$tableAlias WITH (*HINT*)");
            command.CommandText = command.CommandText.Replace("*HINT*", HintValue);
        

        HintValue = String.Empty;

我知道，正则表达式可能会更好。让我们在 Config 类中注册我们的拦截器

public class PbsContextConfig : DbConfiguration

    public PbsContextConfig()
    
        this.AddInterceptor(new HintInterceptor());

让我们为 DbSet 制作漂亮的提示扩展

public static class HintExtension

    public static DbSet<T> WithHint<T>(this DbSet<T> set, string hint) where T : class
    
        HintInterceptor.HintValue = hint;
        return set;

如何使用？

context.Persons.WithHint("INDEX(XI_DOWNTIME_LOCK)").Where( x => x.ID == ....

欢迎修改！

【讨论】：

有趣的想法，但是当我尝试它时，我发现在我的查询之前执行了许多上下文设置查询，并且在第一个上下文设置查询中它会清除 HintValue 并且永远不会将其应用于我的查询。此外，由于它正在执行通过 ReaderExecuting 和 ScalarExecuting 传递的上下文设置查询，因此很有可能将“提示”应用于错误的查询。不知道如何更改代码以使其特定于我的查询，如果我找到方法，我会发布更新。【参考方案4】：

L2S 和 EF 都不会像定制的那样提供对 SQL 的直接支持（索引提示等），尽管使用 L2S 您可以通过ExecuteQuery<T>(...) 实现它（它采用原始 TSQL）。如果您需要这种级别的控制，请考虑存储过程或替代 ORM。

这里特别的一个问题是查询提示非常特定于平台，但 EF 试图与平台无关。

【讨论】：

"如果您需要这种级别的控制，请考虑存储过程或替代 ORM" => 当然，但不实用。当你知道事实是这样的时候，已经太晚了

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