SQL Server 索引基礎知識(2)

由於需要給同事培訓數據庫的索引知識，就收集整理了這個系列的博客。發表在這裏，也是對索引知識的一個總結回顧吧。通過總結，我發現自己以前很多很模糊的概念都清晰了很多。

　　不論是 聚集索引，還是非聚集索引，都是用B+樹來實現的。我們在瞭解這兩種索引之前，需要先了解B+樹。如果你對B樹不瞭解的話，建議參看以下幾篇文章：

　　BTree,B-Tree,B+Tree,B*Tree都是什麼

　　http://blog.csdn.net/manesking/archive/2007/02/09/1505979.aspx

　　B+ 樹的結構圖:

　　B+ 樹的特點:

　　所有關鍵字都出現在葉子結點的鏈表中（稠密索引），且鏈表中的關鍵字恰好是有序的；

　　不可能在非葉子結點命中；

　　非葉子結點相當於是葉子結點的索引（稀疏索引），葉子結點相當於是存儲（關鍵字）數據的數據層；

　　B+ 樹中增加一個數據，或者刪除一個數據，需要分多種情況處理，比較複雜，這裏就不詳述這個內容了。　

　　聚集索引（Clustered Index）

　　

　　聚集索引的葉節點就是實際的數據頁

　　在數據頁中數據按照索引順序存儲

　　行的物理位置和行在索引中的位置是相同的

　　每個表只能有一個聚集索引

　　聚集索引的平均大小大約爲表大小的5%左右

　　下面是兩副簡單描述聚集索引的示意圖：　

　　在聚集索引中執行下面語句的的過程：

select * from table where firstName = 'Ota'

　　一個比較抽象點的聚集索引圖示：

　　非聚集索引 （Unclustered Index）　

　　非聚集索引的頁，不是數據，而是指向數據頁的頁。

　　若未指定索引類型，則默認爲非聚集索引

　　葉節點頁的次序和表的物理存儲次序不同

　　每個表最多可以有249個非聚集索引

　　在非聚集索引創建之前創建聚集索引（否則會引發索引重建）

　　在非聚集索引中執行下面語句的的過程：

select * from employee where lname = 'Green'

　　一個比較抽象點的非聚集索引圖示：

　　什麼是 Bookmark Lookup

　　雖然SQL 2005 中已經不在提　Bookmark Lookup 了(換湯不換藥)，但是我們的很多搜索都是用的這樣的搜索過程，如下：

　　先在非聚集中找，然後再在聚集索引中找。

　　在 http://www.sqlskills.com/ 提供的一個例子中，就給我們演示了 Bookmark Lookup　比 Table Scan 慢的情況，例子的腳本如下：

USE CREDITgo-- These samples use the Credit database. You can download and restore the-- credit database from here:-- http://www.sqlskills.com/resources/conferences/CreditBackup80.zip-- NOTE: This is a SQL Server 2000 backup and MANY examples will work on -- SQL Server 2000 in addition to SQL Server 2005.--------------------------------------------------------------------------------- (1) Create two tables which are copies of charge:--------------------------------------------------------------------------------- Create the HEAPSELECT * INTO ChargeHeap FROM Chargego-- Create the CL TableSELECT * INTO ChargeCL FROM ChargegoCREATE CLUSTERED INDEX ChargeCL_CLInd ON ChargeCL (member_no, charge_no)go--------------------------------------------------------------------------------- (2) Add the same non-clustered indexes to BOTH of these tables:--------------------------------------------------------------------------------- Create the NC index on the HEAPCREATE INDEX ChargeHeap_NCInd ON ChargeHeap (Charge_no)go-- Create the NC index on the CL TableCREATE INDEX ChargeCL_NCInd ON ChargeCL (Charge_no)go--------------------------------------------------------------------------------- (3) Begin to query these tables and see what kind of access and I/O returns--------------------------------------------------------------------------------- Get ready for a bit of analysis:SET STATISTICS IO ON-- Turn Graphical Showplan ON (Ctrl+K)-- First, a point query (also, see how a bookmark lookup looks in 2005)SELECT * FROM ChargeHeap WHERE Charge_no = 12345goSELECT * FROM ChargeCL WHERE Charge_no = 12345go-- What if our query is less selective?-- 1000 is .0625% of our data... (1,600,000 million rows)SELECT * FROM ChargeHeap WHERE Charge_no < 1000goSELECT * FROMChargeCL WHERE Charge_no < 1000go-- What if our query is less selective?-- 16000 is 1% of our data... (1,600,000 million rows)SELECT * FROM ChargeHeap WHERE Charge_no < 16000goSELECT * FROM ChargeCL WHERE Charge_no < 16000go--------------------------------------------------------------------------------- (4) What's the EXACT percentage where the bookmark lookup isn't worth it?--------------------------------------------------------------------------------- What happens here: Table Scan or Bookmark lookup?SELECT * FROM ChargeHeap WHERE Charge_no < 4000goSELECT * FROM ChargeCL WHERE Charge_no < 4000go-- What happens here: Table Scan or Bookmark lookup?SELECT * FROM ChargeHeap WHERE Charge_no < 3000goSELECT * FROM ChargeCL WHERE Charge_no < 3000go-- And - you can narrow it down by trying the middle ground:-- What happens here: Table Scan or Bookmark lookup?SELECT * FROM ChargeHeap WHERE Charge_no < 3500goSELECT * FROM ChargeCL WHERE Charge_no < 3500go-- And again:SELECT * FROM ChargeHeap WHERE Charge_no < 3250goSELECT * FROM ChargeCL WHERE Charge_no < 3250go-- And again:SELECT * FROM ChargeHeap WHERE Charge_no < 3375goSELECT * FROM ChargeCL WHERE Charge_no < 3375go-- Don't worry, I won't make you go through it all :)-- For the Heap Table (in THIS case), the cutoff is: 0.21%SELECT * FROM ChargeHeap　WHERE Charge_no < 3383goSELECT * FROM ChargeHeap WHERE Charge_no < 3384go-- For the Clustered Table (in THIS case), the cut-off is: 0.21%SELECT * FROM ChargeCL WHERE Charge_no < 3438SELECT * FROM ChargeCL WHERE Charge_no < 3439go

　　這個例子也就是 吳家震 在Teched 2007 上的那個演示例子。

　　小結：

　　這篇博客只是簡單的用幾個圖表來介紹索引的實現方法：B+數， 聚集索引，非聚集索引，Bookmark Lookup 的信息而已。

　　參考資料：

　　表組織和索引組織

　　http://technet.microsoft.com/zh-cn/library/ms189051.aspx

　　http://technet.microsoft.com/en-us/library/ms189051.aspx

　　How Indexes Work

　　http://manuals.sybase.com/onlinebooks/group-asarc/asg1200e/aseperf/@Generic__BookTextView/3358

　　Bookmark Lookup

　　http://blogs.msdn.com/craigfr/archive/2006/06/30/652639.aspx　

　　Logical and Physical Operators Reference

　　http://msdn2.microsoft.com/en-us/library/ms191158.aspx