这篇文章将为大家详细讲解有关Sql Server中非聚集索引的作用是什么，文章内容质量较高，因此小编分享给大家做个参考，希望大家阅读完这篇文章后对相关知识有一定的了解。

一：现象

先让我们一睹非聚集索引的真容，看看到底在执行计划看来是个什么玩意。。。我这里有个product表，里面灌了8w多数据，然后在Name列上建立

一个非聚集索引，就像下图一样：

从上图中看到了两个好玩的东西，一个就是我想看到的“索引查找[nonclustered]”，这个大家很熟悉，也是这篇要说的，然后我们还看到了一个“RID查找”，乍一看这是什么鸡巴玩意。。。非聚集索引跟它扯上什么关系了？？？

二：什么是RID

　　通过前面几篇，我想大家都知道了数据页中的记录是如何寻找的？秘密就是通过slot槽位中的偏移量决定的，那问题来了，如果上升到数据页层面，我只需要（pageID：slotID）就可以找到记录了，对不对？那如果我上升了文件层面，那是不是只需要知道（fileID:pageID:slotID)就可以找到数据页中的记录了？其实这里的RID就是站在文件的高度通过(fileID:pageID:slotID)找到表记录的。。。既RID=RowID=(fileID:pageID:slotID)，如果你非要眼见实的话，在sq中l还真提供了这么个函数（sys.fn_PhysLocFormatter(%%physloc%%)），我们看下图：

看了上面的图，是不是很兴奋，一目了然，比如productID=18088这条记录，然来是在1号文件，34941号数据页，0号槽位上，productID=18089是在1号槽位上，好了，当你知道RID是个什么东西的时候，我想你已经离彻底理解非聚集索引不远啦。。。

三：非聚集索引

　　有一点我们肯定知道，就是非聚集索引是可以加速查找的，要是跟表扫描那样的龟速，那也就失去了索引的目的，既然能加速，是因为它和聚集索引一样，在底层都玩起了B树，首先我们插入一些样例数据。

DROPTABLEdbo.PersonCREATETABLEPerson(IDINTIDENTITY,NAMECHAR(900))CREATEINDEXidx_Person_NameONdbo.Person(Name)DECLARE@chASINT=97WHILE@ch<=122BEGININSERTINTOdbo.PersonVALUES(REPLICATE(CHAR(@ch),5))SET@ch=@ch+1END

上面的sql，我故意在Name列设置为900个char，这也是索引的上限值，这样的话，我DBCC就可以导出很多数据页和索引页了。

可以看到，当我dbcc ind 的时候，发现Person表中已经有4个数据页，5个索引页，其中151号数据页是表跟踪页，174号为索引跟踪页，这也就说明当我建立索引后，引擎给我们分配了专门的索引页来存放我们建立的Name索引，那下一步就是我们来看看这些索引中都存放着什么，这也是我非常关心的，接下来我导出173号索引页。

复制代码 代码如下:DBCC PAGE(Ctrip,1,173,1)

Slot0,Offset0x60,Length912,DumpStyleBYTERecordType=INDEX_RECORDRecordAttributes=NULL_BITMAPRecordSize=912MemoryDump@0x000000000EF1C0600000000000000000:16616161616120202020202020202020†.aaaaa0000000000000010:20202020202020202020202020202020†0000000000000020:20202020202020202020202020202020†0000000000000030:20202020202020202020202020202020†0000000000000040:20202020202020202020202020202020†0000000000000050:20202020202020202020202020202020†0000000000000060:20202020202020202020202020202020†0000000000000070:20202020202020202020202020202020†0000000000000080:20202020202020202020202020202020†0000000000000090:20202020202020202020202020202020†00000000000000A0:20202020202020202020202020202020†00000000000000B0:20202020202020202020202020202020†00000000000000C0:20202020202020202020202020202020†00000000000000D0:20202020202020202020202020202020†00000000000000E0:20202020202020202020202020202020†00000000000000F0:20202020202020202020202020202020†0000000000000100:20202020202020202020202020202020†0000000000000110:20202020202020202020202020202020†0000000000000120:20202020202020202020202020202020†0000000000000130:20202020202020202020202020202020†0000000000000140:20202020202020202020202020202020†0000000000000150:20202020202020202020202020202020†0000000000000160:20202020202020202020202020202020†0000000000000170:20202020202020202020202020202020†0000000000000180:20202020202020202020202020202020†0000000000000190:20202020202020202020202020202020†00000000000001A0:20202020202020202020202020202020†00000000000001B0:20202020202020202020202020202020†00000000000001C0:20202020202020202020202020202020†00000000000001D0:20202020202020202020202020202020†00000000000001E0:20202020202020202020202020202020†00000000000001F0:20202020202020202020202020202020†0000000000000200:20202020202020202020202020202020†0000000000000210:20202020202020202020202020202020†0000000000000220:20202020202020202020202020202020†0000000000000230:20202020202020202020202020202020†0000000000000240:20202020202020202020202020202020†0000000000000250:20202020202020202020202020202020†0000000000000260:20202020202020202020202020202020†0000000000000270:20202020202020202020202020202020†0000000000000280:20202020202020202020202020202020†0000000000000290:20202020202020202020202020202020†00000000000002A0:20202020202020202020202020202020†00000000000002B0:20202020202020202020202020202020†00000000000002C0:20202020202020202020202020202020†00000000000002D0:20202020202020202020202020202020†00000000000002E0:20202020202020202020202020202020†00000000000002F0:20202020202020202020202020202020†0000000000000300:20202020202020202020202020202020†0000000000000310:20202020202020202020202020202020†0000000000000320:20202020202020202020202020202020†0000000000000330:20202020202020202020202020202020†0000000000000340:20202020202020202020202020202020†0000000000000350:20202020202020202020202020202020†0000000000000360:20202020202020202020202020202020†0000000000000370:20202020202020202020202020202020†0000000000000380:20202020209400000001000000020000†...........

复制代码 代码如下:Row - Offset 7 (0x7) - 6480 (0x1950) 6 (0x6) - 5568 (0x15c0) 5 (0x5) - 4656 (0x1230) 4 (0x4) - 3744 (0xea0) 3 (0x3) - 2832 (0xb10) 2 (0x2) - 1920 (0x780) 1 (0x1) - 1008 (0x3f0) 0 (0x0) - 96 (0x60)

从上面至少可以发现三个有趣的现象：

<1>:173号索引页中slot0和slot1槽位指向记录的内容已经有序了，比如：aaaaa，bbbbb。。。。这样。。。。原来非聚集索引也是有序呀。。。

<2>:6161616161就是16进制的aaaaa。

　　 9400000001000000 ：这几个数字非常重要，因为是16进制表示，所以2位16进制表示一个字节，所以可以这么解释，前面4个字节表示

　　 pageID，中间2个字节表示fileID，后面2个字节表示slot，看到这里你是不是想起了RID。。。因为RID就是这三样的组合。。。原来非聚集索

　　引的记录存放的就是“key+RowID”呀。。。。

<3>:通过最后的槽位列表，可以得知173号索引页上存放着8条索引记录。

关于Sql Server中非聚集索引的作用是什么就分享到这里了，希望以上内容可以对大家有一定的帮助，可以学到更多知识。如果觉得文章不错，可以把它分享出去让更多的人看到。