SqlServer索引碎片

Posted 2020-10-03 石

1.产生碎片的操作

通过sys.dm_index_physical_stats来查看，索引上的页不在具有连续性时就会产生碎片，碎片是索引上页拆分的物理结果。

（1）.插入操作：

INSERT操作在聚集索引和非聚集索引上都可以引起碎片

使用业务键或者GUID等类型 做聚集索引，很容易产生碎片

代码如下：

 IF OBJECT_ID(\'dbo.Table_GUID\') IS NOT NULL
DROP TABLE dbo.Table_GUID;
CREATE TABLE Table_GUID
(
RowID UNIQUEIDENTIFIER CONSTRAINT DF_GUIDValue DEFAULT NEWID(),--使用GUID作为默认值
Name sysname,
Value VARCHAR(2000)
);

--插入数据,注意此时还没有创建聚集索引
INSERT INTO dbo.Table_GUID(  Name, Value )
		SELECT name,REPLICATE(\'X\',2000)
		 FROM sys.columns


		 SELECT * FROM dbo.Table_GUID
--在列上创建聚集索引
CREATE CLUSTERED INDEX CLUS_UsingUniqueidentifer ON dbo.Table_GUID(RowID);

--查看平均碎片
SELECT index_type_desc,
index_depth,
index_level,
page_count,
record_count,
CAST(avg_fragmentation_in_percent AS DECIMAL(6,2)) AS avg_fragmentation_in_percent,
fragment_count,
avg_fragment_size_in_pages,
CAST(avg_page_space_used_in_percent AS DECIMAL(6,2)) AS avg_page_space_used_in_percent
 FROM sys.dm_db_index_physical_stats(DB_ID(),OBJECT_ID(N\'dbo.Table_GUID\'),NULL,NULL,\'DETAILED\')

 

平均碎片为0，插入后才建索引。

插入数据：

 --插入新数据
 INSERT INTO dbo.Table_GUID
         (  Name, Value )
SELECT name, REPLICATE(\'X\',2000) FROM sys.objects

 查看索引碎片：

--查看平均碎片
SELECT index_type_desc,
index_depth,
index_level,
page_count,
record_count,
CAST(avg_fragmentation_in_percent AS DECIMAL(6,2)) AS avg_fragmentation_in_percent,
fragment_count,
avg_fragment_size_in_pages,
CAST(avg_page_space_used_in_percent AS DECIMAL(6,2)) AS avg_page_space_used_in_percent
 FROM sys.dm_db_index_physical_stats(DB_ID(),OBJECT_ID(N\'dbo.Table_GUID\'),NULL,NULL,\'DETAILED\')

 

非聚集索引：

 CREATE NONCLUSTERED INDEX IX_Name ON dbo.Table_GUID(Name) INCLUDE(Value)

 执行以上两次操作。

由此可见：当INSERT操作发生时，产生碎片再所难免，唯一要做的是尽可能降低碎片的产生速率。

（2）：更新操作

 --跟新操作
  IF OBJECT_ID(\'dbo.Update_Fr\') IS NOT NULL
DROP TABLE dbo.Update_Fr;
CREATE TABLE Update_Fr
(
RowID INT IDENTITY(1,1),--使用GUID作为默认值
Name sysname,
Value VARCHAR(2000)
);

INSERT INTO dbo.Update_Fr
        (  Name, Value )
		SELECT name,REPLICATE(\'X\',1000)
 FROM sys.columns

 CREATE CLUSTERED INDEX CLUS_UsingUniqueidentifier ON dbo.Update_Fr(RowID);

 --检查一下空间
SELECT index_type_desc,
index_depth,
index_level,
page_count,
record_count,
CAST(avg_fragmentation_in_percent AS DECIMAL(6,2)) AS avg_fragmentation_in_percent,
fragment_count,
avg_fragment_size_in_pages,
CAST(avg_page_space_used_in_percent AS DECIMAL(6,2)) AS avg_page_space_used_in_percent
 FROM sys.dm_db_index_physical_stats(DB_ID(),OBJECT_ID(N\'dbo.Update_Fr\'),NULL,NULL,\'DETAILED\')

 

 

--更新数据让长度变长
 UPDATE dbo.Update_Fr SET Value=REPLICATE(\'X\',2000)
 WHERE RowID % 5=1

 

键值的改变导致碎片的产生：

 --创建一个非聚集索引
 CREATE NONCLUSTERED INDEX IX_Name ON dbo.Update_Fr(Name) INCLUDE (Value);

 --通过REVERST函数把名称反转监控跟新前后的碎片情况
SELECT index_type_desc,
index_depth,
index_level,
page_count,
record_count,
CAST(avg_fragmentation_in_percent AS DECIMAL(6,2)) AS avg_fragmentation_in_percent,
fragment_count,
avg_fragment_size_in_pages,
CAST(avg_page_space_used_in_percent AS DECIMAL(6,2)) AS avg_page_space_used_in_percent
 FROM sys.dm_db_index_physical_stats(DB_ID(),OBJECT_ID(N\'dbo.Update_Fr\'),NULL,NULL,\'DETAILED\')

 

 

 --通过REVERST函数把名称反转监控跟新前后的碎片情况

UPDATE dbo.Update_Fr
 SET Name=REVERSE(Name)
 WHERE RowID%9=1

 SELECT index_type_desc,
index_depth,
index_level,
page_count,
record_count,
CAST(avg_fragmentation_in_percent AS DECIMAL(6,2)) AS avg_fragmentation_in_percent,
fragment_count,
avg_fragment_size_in_pages,
CAST(avg_page_space_used_in_percent AS DECIMAL(6,2)) AS avg_page_space_used_in_percent
 FROM sys.dm_db_index_physical_stats(DB_ID(),OBJECT_ID(N\'dbo.Update_Fr\'),NULL,NULL,\'DETAILED\')

 

 

（3）.收缩操作：

 

 IF DB_ID(N\'Fragmentation\') IS NOT NULL
 DROP DATABASE Fragmentation
 CREATE DATABASE Fragmentation
  USE Fragmentation

  IF OBJECT_ID(\'dbo.FirstTable\') IS NOT NULL
DROP TABLE dbo.FirstTable;

CREATE TABLE dbo.FirstTable
(
RowID INT IDENTITY(1,1),
Name sysname,
Value VARCHAR(2000),
CONSTRAINT PK_FirstTable PRIMARY KEY CLUSTERED(RowID)
);

INSERT INTO dbo.FirstTable
        (  Name, Value )
SELECT name,REPLICATE(\'X\',2000) FROM sys.columns



 IF OBJECT_ID(\'dbo.SecondTable\') IS NOT NULL
DROP TABLE dbo.SecondTable;

CREATE TABLE dbo.SecondTable
(
RowID INT IDENTITY(1,1),
Name sysname,
Value VARCHAR(2000),
CONSTRAINT PK_SecondTable PRIMARY KEY CLUSTERED(RowID)
);

INSERT INTO dbo.SecondTable
        (  Name, Value )
SELECT name,REPLICATE(\'X\',2000) FROM sys.columns


INSERT INTO dbo.FirstTable
        (  Name, Value )
SELECT name,REPLICATE(\'X\',2000) FROM sys.columns


INSERT INTO dbo.SecondTable
        (  Name, Value )
SELECT name,REPLICATE(\'X\',2000) FROM sys.columns


INSERT INTO dbo.FirstTable
        (  Name, Value )
SELECT name,REPLICATE(\'X\',2000) FROM sys.columns


 SELECT index_type_desc,
index_depth,
index_level,
page_count,
record_count,
CAST(avg_fragmentation_in_percent AS DECIMAL(6,2)) AS avg_fragmentation_in_percent,
fragment_count,
avg_fragment_size_in_pages,
CAST(avg_page_space_used_in_percent AS DECIMAL(6,2)) AS avg_page_space_used_in_percent
 FROM sys.dm_db_index_physical_stats(DB_ID(),OBJECT_ID(N\'dbo.FirstTable\'),NULL,NULL,\'DETAILED\')


 
 IF OBJECT_ID(\'dbo.SecondTable\') IS NOT NULL
DROP TABLE dbo.SecondTable;

 SELECT index_type_desc,
index_depth,
index_level,
page_count,
record_count,
CAST(avg_fragmentation_in_percent AS DECIMAL(6,2)) AS avg_fragmentation_in_percent,
fragment_count,
avg_fragment_size_in_pages,
CAST(avg_page_space_used_in_percent AS DECIMAL(6,2)) AS avg_page_space_used_in_percent
 FROM sys.dm_db_index_physical_stats(DB_ID(),OBJECT_ID(N\'dbo.FirstTable\'),NULL,NULL,\'DETAILED\')

删除前后的索引碎片一样。

由于SqlServer不会自动回收，调用DBCC SHRINKDATABASE来收缩数据库，再次查看碎片情况。

 DBCC SHRINKDATABASE(Fragmentation)

 

再次查看碎片：

所以自动收缩数据不建议使用。