针对非唯一聚簇索引的搜索的逻辑读取

Question

对于表定义

CREATE  TABLE Accounts
(
AccountID INT ,
Filler CHAR(1000)
)

包含21行(每个AccountId值为7行4,6,7).

它有1个根页面和4个叶子页面

index_depth page_count           index_level
----------- -------------------- -----------
2           4                    0
2           1                    1

根页看起来像

FileId      PageId      ROW         LEVEL       ChildFieldId ChildPageId AccountId (KEY) UNIQUIFIER (KEY) KeyHashValue
----------- ----------- ----------- ----------- ------------ ----------- --------------- ---------------- ------------------------------
1           121         0           1           1            119         NULL            NULL             NULL
1           121         1           1           1            151         6               0                NULL
1           121         2           1           1            175         6               3                NULL
1           121         3           1           1            215         7               1                NULL

AccountId这些页面上的记录的实际分布是

AccountID   page_id     Num
----------- ----------- -----------
4           119         7
6           151         3
6           175         4
7           175         1
7           215         6

查询

SELECT AccountID 
FROM Accounts 
WHERE AccountID IN (4,6,7) 

提供以下IO统计信息

Table 'Accounts'. Scan count 3, logical reads 13

为什么？

我想为每次搜索它会寻找可能包含该值的第一页,然后(如果需要)继续沿链表继续,直到它发现第一行不等于搜索值.

但是,这最多只能增加10次页面访问

4)  Root Page -> Page 119 -> Page 151             (Page 151 Contains a 6 so should stop)
6)  Root Page -> Page 119 -> Page 151 -> Page 175 (Page 175 Contains a 7 so should stop)
7)  Root Page -> Page 175 -> Page 215             (No more pages)      

那么附加3的原因是什么呢？

完整的脚本来重现

USE tempdb

SET NOCOUNT ON;

CREATE  TABLE Accounts
(
AccountID INT ,
Filler CHAR(1000)
)

CREATE CLUSTERED INDEX ix ON Accounts(AccountID)


INSERT INTO Accounts(AccountID)
SELECT C
FROM (SELECT 4 UNION ALL SELECT 6 UNION ALL SELECT 7) Vals(C)
CROSS JOIN (SELECT TOP (7) 1 FROM master..spt_values) T(X)

DECLARE @AccountID INT

SET STATISTICS IO ON
SELECT @AccountID=AccountID FROM Accounts WHERE AccountID IN (4,6,7) 
SET STATISTICS IO OFF

SELECT index_depth,page_count,index_level
FROM
sys.dm_db_index_physical_stats (2,OBJECT_ID('Accounts'), DEFAULT,DEFAULT, 'DETAILED')

SELECT AccountID, P.page_id, COUNT(*) AS Num
FROM Accounts
CROSS APPLY sys.fn_PhysLocCracker(%%physloc%%) P
GROUP BY AccountID, P.page_id
ORDER BY AccountID, P.page_id

DECLARE @index_info  TABLE
(PageFID  VARCHAR(10), 
  PagePID VARCHAR(10),   
  IAMFID   TINYINT, 
  IAMPID  INT, 
  ObjectID  INT,
  IndexID  TINYINT,
  PartitionNumber TINYINT,
  PartitionID BIGINT,
  iam_chain_type  VARCHAR(30),    
  PageType  TINYINT, 
  IndexLevel  TINYINT,
  NextPageFID  TINYINT,
  NextPagePID  INT,
  PrevPageFID  TINYINT,
  PrevPagePID INT, 
  PRIMARY KEY (PageFID, PagePID));

INSERT INTO @index_info 
    EXEC ('DBCC IND ( tempdb, Accounts, -1)'  ); 

DECLARE @DynSQL NVARCHAR(MAX) = 'DBCC TRACEON (3604);'
SELECT @DynSQL = @DynSQL + '
DBCC PAGE(tempdb, ' + PageFID + ', ' + PagePID + ', 3); '
FROM @index_info     
WHERE IndexLevel = 1

SET @DynSQL = @DynSQL + '
DBCC TRACEOFF(3604); '

CREATE TABLE #index_l1_info  
(FileId  INT, 
  PageId INT,   
  ROW   INT, 
  LEVEL  INT, 
  ChildFieldId  INT,
  ChildPageId INT,
  [AccountId (KEY)] INT,
  [UNIQUIFIER (KEY)] INT,
  KeyHashValue  VARCHAR(30));

INSERT INTO #index_l1_info  
EXEC(@DynSQL)


SELECT *
FROM #index_l1_info

DROP TABLE #index_l1_info
DROP TABLE Accounts

Answer 1

只是为了以答案形式提供答案，而不是作为评论中的讨论...

由于预读机制，会出现额外的读取。这会扫描叶级的父页面，以防它需要发出异步 IO 将叶级页面带入缓冲区缓存，以便在范围查找到达它们时准备就绪。

可以使用跟踪标志 652 来禁用该机制（服务器范围内）并验证读取次数现在是否如预期的那样正好为 10。