SQLServer2005动态管理视图(DMVs)返回会话、事务、请求的服务器状态信息。它可用于诊断、内存和过程调优、监控(SQLServer2000不支持)。SQLServer引擎跟踪详细的资源使用情况，用select语句从DMVs中可查到，但是这些信息不会长期驻留在磁盘上。
由于索引提供了代替表扫描的一个选择，且DMVs返回索引使用计数，所以可以比较索引的成本和其性能。这个比较包括保持索引最新的成本，与使用索引而不是表扫描读数据的性能之比。谨记一个更新或删除操作先要读数据从而定位数据，然后对定位的数据进行写操作。一个插入操作在所有的索引上只是写操作。因此，一个大量的插入将使写操作次数超过读操作次数。一个大量的更改操作(包括更新和删除)，读和写的次数通常很接近(假定没有'记录找不到'的情况发生)。一个大量的读操作，读的次数将超过写。引用约束如外键还要求额外的读操作(对于插入、更新、删除而言)去确保引用完整性得到维护
哪些表和索引是没用或者很少用的?
---1. 未使用的表和索引。表都有一个索引ID,如果是0则为堆表，1则为聚集索引
Declare @dbid int
Select @dbid = db_id('Northwind')
Select objectname=object_name(i.object_id)
, indexname=i.name
, i.index_id
from sys.indexes i, sys.objects o
where objectproperty(o.object_id,'IsUserTable') = 1
and i.index_id NOT IN (select s.index_id
fromsys.dm_db_index_usage_stats s
where s.object_id=i.object_idand
i.index_id=s.index_id and
database_id = @dbid )
and o.object_id = i.object_id
order by objectname,i.index_id,indexname asc
--2.缺失的索引
SELECT TOP 50
[Total Cost] = ROUND(avg_total_user_cost * avg_user_impact * (user_seeks + user_scans)，0)
, avg_user_impact
, TableName = statement
, [EqualityUsage] = equality_columns
, [InequalityUsage] = inequality_columns
, [Include Cloumns] = included_columns
,user_seeks , user_scans
FROM sys.dm_db_missing_index_groups g
INNER JOIN sys.dm_db_missing_index_group_stats s
ON s.group_handle = g.index_group_handle
INNER JOIN sys.dm_db_missing_index_details d ON d.index_handle = g.index_handle
WHERE statement LIKE '%tablename%'
ORDER BY [Total Cost] DESC;
--3.使用很少的索引和频繁使用的索引一样，都会记录在sys.dm_db_index_usage_stats中。为了找出这些索引，需要查看 诸如user_seeks、 user_scans、user_lookups和user_updates的列。
;WITH IXC AS(
SELECT IXC.object_id,
IXC.index_id,
IXC.index_column_id,
IXC.is_descending_key,
IXC.is_included_column,
column_name = C.name
FROM sys.index_columns IXC
INNER JOIN sys.columns C ON IXC.object_id = C.object_id AND IXC.column_id = C.column_id
)
SELECT TOP 50
o.name AS 表名
, i.name AS 索引名
, i.index_id AS 索引id
, dm_ius.user_seeks AS 搜索次数
, dm_ius.user_scans AS 扫描次数
, dm_ius.user_lookups AS 查找次数
, dm_ius.user_updates AS 更新次数
, p.TableRows as 表行数
,index_columns = Stuff(IXC_COL.index_columns,1,2,N'')
,index_columns_include = Stuff(IXC_COL_INCLUDE.index_columns_include,1,2,N'')
,'DROP INDEX ' + QUOTENAME(s.name) + '.' + QUOTENAME(OBJECT_NAME(i.OBJECT_ID))
+ '.' + QUOTENAME(i.name) AS '删除语句'
FROM sys.dm_db_index_usage_stats dm_ius
INNER JOIN sys.indexes i ON i.index_id = dm_ius.index_id AND dm_ius.OBJECT_ID = i.OBJECT_ID
INNER JOIN sys.objects o ON dm_ius.OBJECT_ID = o.OBJECT_ID
INNER JOIN sys.schemas s ON o.schema_id = s.schema_id
INNER JOIN (SELECT SUM(p.rows) TableRows, p.index_id, p.OBJECT_ID
FROM sys.partitions p GROUP BY p.index_id, p.OBJECT_ID) p
ON p.index_id = dm_ius.index_id AND dm_ius.OBJECT_ID = p.OBJECT_ID
CROSS APPLY(
SELECT index_columns = (
SELECT N', ' + quotename(column_name) + CASE is_descending_key
WHEN 1 THEN N' DESC '
ELSE N''
END
FROM IXC
WHERE object_id = I.object_id
AND index_id = I.index_id
AND is_included_column = 0
ORDER BY index_column_id
FOR xml path('')，root('r')，TYPE
)。value('/r[1]','nvarchar(max)')
) IXC_COL
OUTER APPLY(
SELECT index_columns_include = (
SELECT N', ' + quotename(column_name)
FROM IXC
WHERE object_id = I.object_id
AND index_id = I.index_id
AND is_included_column = 1
ORDER BY index_column_id
FOR xml path('')，root('r')，TYPE
)。value('/r[1]','nvarchar(max)')
) IXC_COL_INCLUDE
WHERE OBJECTPROPERTY(dm_ius.OBJECT_ID,'IsUserTable') = 1
AND dm_ius.database_id = DB_ID()
--AND i.type_desc = 'nonclustered'
--AND i.is_primary_key = 0
--AND i.is_unique_constraint = 0
and o.name='tablename' --根据实际修改表名
ORDER BY (dm_ius.user_seeks + dm_ius.user_scans + dm_ius.user_lookups) ASC
结论：user_updates很大，而发现user_seeks和user_scans很少或者就是0,那就说明该索引一直在更新，
但是从来不被使用，仅仅创建和修改，没有为查询提供任何帮助，就可以考虑删除了
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