Custom Sitecore SqlDataProvider and QueryToSqlTranslator which fixes field querying in Sitecore Fast queries.

SqlServerDataProvider.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Sitecore.Data.DataProviders.Sql.FastQuery;
using Sitecore.Data.Query;

namespace Kraftvaerk
{
    public class KvSqlServerDataProvider : Sitecore.Data.SqlServer.SqlServerDataProvider
    {
        public KvSqlServerDataProvider(string connectionString)
            : base(connectionString)
        {

        }

        protected override Sitecore.Data.DataProviders.Sql.FastQuery.QueryToSqlTranslator CreateSqlTranslator()
        {
            return new KvQueryToSqlTranslator(this.Api);
        }
    }

    public class KvQueryToSqlTranslator : Sitecore.Data.DataProviders.Sql.FastQuery.QueryToSqlTranslator
    {
        public KvQueryToSqlTranslator(Sitecore.Data.DataProviders.Sql.SqlDataApi api)
            : base(api)
        {
            ((Sitecore.Data.DataProviders.Sql.FastQuery.BasicTranslatorFactory)this._factory).Register(typeof(FieldElement), new KvFieldTranslator());
        }
    }

    public class KvFieldTranslator : FieldTranslator
    {
        protected override string RenderFieldByGuid(Sitecore.Data.ID fieldID, ITranslationContext context)
        {
            context.Data["complex-fields"] = true;
            var info = context.Fields[fieldID] as IFieldInfo;
            if (info == null)
            {
                info = new KvIDFieldInfo(fieldID, context);
                context.Fields[fieldID] = info;
            }

            return this.RenderField(info, context);
        }
    }

    public class KvIDFieldInfo : IFieldInfo
    {
        public string Alias
        {
            get;
            private set;
        }

        public KvIDFieldInfo(Sitecore.Data.ID fieldId, ITranslationContext context)
        {
            this.Alias = context.AddSubquery(context.SqlApi.Format("(SELECT {0}ItemId{1}, {0}Value{1} FROM {0}Fields{1} WHERE {0}FieldID{1}='") + context.SqlApi.Safe(fieldId.ToGuid().ToString()) + "')", "Fields");
        }
    }
}

Usage.cs

var fieldId = ID.Parse("{252F9152-D23E-4203-8A21-88AF7007FE06}");

// Field querying syntax: @f252F9152D23E42038A2188AF7007FE06='Some value'
// Notice the f before field id.
var item = Context.Database.SelectSingleItem(
    string.Format(@"fast://*[@@templateid='{{C29492D2-6F79-463C-A7FB-B318FAB90639}}'
    and @f{0}='Some value']", fieldId.ToShortID()));

Web.config

<dataProviders>
    <main type="Kraftvaerk.KvSqlServerDataProvider, Kraftvaerk">
      <param connectionStringName="$(1)" />      
      <Name>$(1)</Name>
    </main>
</dataProviders>