Linq缓慢实现复杂查询

Question

我经常发现如果我在Linq查询中有太多连接(无论是使用Entity Framework还是NHibernate)和/或生成的匿名类的形状太复杂,Linq需要很长时间才能将结果集具体化为对象.

这是一个通用的问题,但这是使用NHibernate的一个具体示例:

var libraryBookIdsWithShelfAndBookTagQuery = (from shelf in session.Query<Shelf>()
    join sbttref in session.Query<ShelfBookTagTypeCrossReference>() on
         shelf.ShelfId equals sbttref.ShelfId
    join bookTag in session.Query<BookTag>() on
         sbttref.BookTagTypeId equals (byte)bookTag.BookTagType
    join btbref in session.Query<BookTagBookCrossReference>() on
         bookTag.BookTagId equals btbref.BookTagId
    join book in session.Query<Book>() on
         btbref.BookId equals book.BookId
    join libraryBook in session.Query<LibraryBook>() on
         book.BookId equals libraryBook.BookId
    join library in session.Query<LibraryCredential>() on
         libraryBook.LibraryCredentialId equals library.LibraryCredentialId
    join lcsg in session
         .Query<LibraryCredentialSalesforceGroupCrossReference>()
          on library.LibraryCredentialId equals lcsg.LibraryCredentialId
    join userGroup in session.Query<UserGroup>() on
         lcsg.UserGroupOrganizationId equals userGroup.UserGroupOrganizationId
    where
         shelf.ShelfId == shelfId &&
         userGroup.UserGroupId == userGroupId &&
         !book.IsDeleted &&
         book.IsDrm != null &&
         book.BookFormatTypeId != null
    select new
    {
        Book = book,
        LibraryBook = libraryBook,
        BookTag = bookTag
    });

// add a couple of where clauses, then...
var result = libraryBookIdsWithShelfAndBookTagQuery.ToList();

我知道这不是查询执行,因为我在数据库上放了一个嗅探器,我可以看到查询占用了0ms,但代码大约花了一秒钟来执行该查询并带回所有11条记录.

所以,是的,这是一个过于复杂的查询,在9个表之间有8个连接,我可能会将它重组为几个较小的查询.或者我可以把它变成一个存储过程 - 但这会有帮助吗？

我想要理解的是,在一个高性能的查询和一个开始与物化的斗争之间交叉的红线在哪里？引擎盖下发生了什么？如果这是一个SP的平坦结果我随后在记忆中操纵到正确的形状会有帮助吗？

编辑:响应评论中的请求,这是SQL发出的:

SELECT DISTINCT book4_.bookid                 AS BookId12_0_, 
                libraryboo5_.librarybookid    AS LibraryB1_35_1_, 
                booktag2_.booktagid           AS BookTagId15_2_, 
                book4_.title                  AS Title12_0_, 
                book4_.isbn                   AS ISBN12_0_, 
                book4_.publicationdate        AS Publicat4_12_0_, 
                book4_.classificationtypeid   AS Classifi5_12_0_, 
                book4_.synopsis               AS Synopsis12_0_, 
                book4_.thumbnailurl           AS Thumbnai7_12_0_, 
                book4_.retinathumbnailurl     AS RetinaTh8_12_0_, 
                book4_.totalpages             AS TotalPages12_0_, 
                book4_.lastpage               AS LastPage12_0_, 
                book4_.lastpagelocation       AS LastPag11_12_0_, 
                book4_.lexilerating           AS LexileR12_12_0_, 
                book4_.lastpageposition       AS LastPag13_12_0_, 
                book4_.hidden                 AS Hidden12_0_, 
                book4_.teacherhidden          AS Teacher15_12_0_, 
                book4_.modifieddatetime       AS Modifie16_12_0_, 
                book4_.isdeleted              AS IsDeleted12_0_, 
                book4_.importedwithlexile     AS Importe18_12_0_, 
                book4_.bookformattypeid       AS BookFor19_12_0_, 
                book4_.isdrm                  AS IsDrm12_0_, 
                book4_.lightsailready         AS LightSa21_12_0_, 
                libraryboo5_.bookid           AS BookId35_1_, 
                libraryboo5_.libraryid        AS LibraryId35_1_, 
                libraryboo5_.externalid       AS ExternalId35_1_, 
                libraryboo5_.totalcopies      AS TotalCop5_35_1_, 
                libraryboo5_.availablecopies  AS Availabl6_35_1_, 
                libraryboo5_.statuschangedate AS StatusCh7_35_1_, 
                booktag2_.booktagtypeid       AS BookTagT2_15_2_, 
                booktag2_.booktagvalue        AS BookTagV3_15_2_ 
FROM   shelf shelf0_, 
       shelfbooktagtypecrossreference shelfbookt1_, 
       booktag booktag2_, 
       booktagbookcrossreference booktagboo3_, 
       book book4_, 
       librarybook libraryboo5_, 
       library librarycre6_, 
       librarycredentialsalesforcegroupcrossreference librarycre7_, 
       usergroup usergroup8_ 
WHERE  shelfbookt1_.shelfid = shelf0_.shelfid 
       AND booktag2_.booktagtypeid = shelfbookt1_.booktagtypeid 
       AND booktagboo3_.booktagid = booktag2_.booktagid 
       AND book4_.bookid = booktagboo3_.bookid 
       AND libraryboo5_.bookid = book4_.bookid 
       AND librarycre6_.libraryid = libraryboo5_.libraryid 
       AND librarycre7_.librarycredentialid = librarycre6_.libraryid 
       AND usergroup8_.usergrouporganizationid = 
           librarycre7_.usergrouporganizationid 
       AND shelf0_.shelfid = @p0 
       AND usergroup8_.usergroupid = @p1 
       AND NOT ( book4_.isdeleted = 1 ) 
       AND ( book4_.isdrm IS NOT NULL ) 
       AND ( book4_.bookformattypeid IS NOT NULL ) 
       AND book4_.lightsailready = 1 

编辑2:这是ANTS Performance Profiler的性能分析:

Answer 1

在视图中放置大量连接或超级公共连接通常是数据库"好"的做法.ORM不会让你忽略这些事实,也不会补充数十年来微调数据库以有效地完成这些事情的时间.如果在应用程序的更大视角中更有意义,那么将这些连接重构为单个视图或几个视图.

NHibernate应该优化查询并减少数据,以便.Net只需要弄乱重要的部分.但是,如果这些域对象自然很大,那仍然是很多数据.此外,如果它是返回的行的真正大的结果集,那么即使数据库能够快速返回集合,也会对很多对象进行实例化.将此查询重构为仅返回实际需要的数据的视图也会减少对象实例化开销.

另一种想法是不做一个.ToList().返回枚举,让你的代码懒惰地使用数据.