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程序員在無處尋求幫助時，就得想辦法自救，努力讓自己變成 "偽 DBA"。

索引

1. 找出哪些表的 Index 需要改進
2. 在指定數據庫中查找哪些表的 Index 需要改進
3. 根據緩存的查詢計劃判斷 SP 是否需要優化
4. 發現那些 Index 的寫遠多於讀的表
5. 查看 Index 的 Statistics 最後更新時間
6. 查看哪些 Index 被修改的最頻繁
7. 查看 Index 碎片化指數
8. 哪個 Index 上的讀操作最活躍
9. 哪個 Index 上的寫操作最活躍
10. 查看 Index 所使用的 Buffer 數量
11. 按照 IO Latch 等待請求對索引進行排行

找出哪些表的 Index 需要改進

SELECT CONVERT(DECIMAL(18, 2), user_seeks * avg_total_user_cost * (avg_user_impact * 0.01)) AS [index_advantage]
    ,migs.last_user_seek
    ,mid.[statement] AS [Database.Schema.Table]
    ,mid.equality_columns
    ,mid.inequality_columns
    ,mid.included_columns
    ,migs.unique_compiles
    ,migs.user_seeks
    ,migs.avg_total_user_cost
    ,migs.avg_user_impact
FROM sys.dm_db_missing_index_group_stats AS migs WITH (NOLOCK)
INNER JOIN sys.dm_db_missing_index_groups AS mig WITH (NOLOCK) ON migs.group_handle = mig.index_group_handle
INNER JOIN sys.dm_db_missing_index_details AS mid WITH (NOLOCK) ON mig.index_handle = mid.index_handle
ORDER BY index_advantage DESC
OPTION (RECOMPILE);

這裏查詢出的數據，只是說明數據尋址時間有點兒長，不一定就是缺少索引所引起的。

在指定數據庫中查找哪些表的 Index 需要改進

SELECT DISTINCT CONVERT(DECIMAL(18, 2), user_seeks * avg_total_user_cost * (avg_user_impact * 0.01)) AS [index_advantage]
    ,migs.last_user_seek
    ,mid.[statement] AS [Database.Schema.Table]
    ,mid.equality_columns
    ,mid.inequality_columns
    ,mid.included_columns
    ,migs.unique_compiles
    ,migs.user_seeks
    ,migs.avg_total_user_cost
    ,migs.avg_user_impact
    ,OBJECT_NAME(mid.[object_id]) AS [Table Name]
    ,p.rows AS [Table Rows]
FROM sys.dm_db_missing_index_group_stats AS migs WITH (NOLOCK)
INNER JOIN sys.dm_db_missing_index_groups AS mig WITH (NOLOCK) ON migs.group_handle = mig.index_group_handle
INNER JOIN sys.dm_db_missing_index_details AS mid WITH (NOLOCK) ON mig.index_handle = mid.index_handle
INNER JOIN sys.partitions AS p WITH (NOLOCK) ON p.[object_id] = mid.[object_id]
WHERE mid.database_id = DB_ID()
ORDER BY index_advantage DESC
OPTION (RECOMPILE);

根據緩存的查詢計劃判斷 SP 是否需要優化

SELECT TOP (25) OBJECT_NAME(objectid) AS [ObjectName]
    ,query_plan
    ,cp.objtype
    ,cp.usecounts
FROM sys.dm_exec_cached_plans AS cp WITH (NOLOCK)
CROSS APPLY sys.dm_exec_query_plan(cp.plan_handle) AS qp
WHERE CAST(query_plan AS NVARCHAR(MAX)) LIKE N‘%MissingIndex%‘
    AND dbid = DB_ID()
ORDER BY cp.usecounts DESC
OPTION (RECOMPILE);

發現那些 Index 的寫遠多於讀的表

SELECT OBJECT_NAME(s.[object_id]) AS [Table Name]
    ,i.[name] AS [Index Name]
    ,i.index_id
    ,i.is_disabled
    ,i.is_hypothetical
    ,i.has_filter
    ,i.fill_factor
    ,user_updates AS [Total Writes]
    ,user_seeks + user_scans + user_lookups AS [Total Reads]
    ,user_updates - (user_seeks + user_scans + user_lookups) AS [Difference]
FROM sys.dm_db_index_usage_stats AS s WITH (NOLOCK)
INNER JOIN sys.indexes AS i WITH (NOLOCK) ON s.[object_id] = i.[object_id]
    AND i.index_id = s.index_id
WHERE OBJECTPROPERTY(s.[object_id], ‘IsUserTable‘) = 1
    AND s.database_id = DB_ID()
    AND user_updates > (user_seeks + user_scans + user_lookups)
    AND i.index_id > 1
ORDER BY [Difference] DESC
    ,[Total Writes] DESC
    ,[Total Reads] ASC
OPTION (RECOMPILE);

由於對索引的寫操作遠多於讀操作，看起來 Index 的幫助不大，但需要根據業務需求來判斷是否能夠 Drop 掉該索引。

查看 Index 的 Statistics 最後更新時間

SELECT SCHEMA_NAME(o.[schema_id]) + N‘.‘ + o.[name] AS [Object Name]
    ,o.type_desc AS [Object Type]
    ,i.[name] AS [Index Name]
    ,STATS_DATE(i.[object_id], i.index_id) AS [Statistics Date]
    ,s.auto_created
    ,s.no_recompute
    ,s.user_created
    ,st.row_count
    ,st.used_page_count
FROM sys.objects AS o WITH (NOLOCK)
INNER JOIN sys.indexes AS i WITH (NOLOCK) ON o.[object_id] = i.[object_id]
INNER JOIN sys.stats AS s WITH (NOLOCK) ON i.[object_id] = s.[object_id]
    AND i.index_id = s.stats_id
INNER JOIN sys.dm_db_partition_stats AS st WITH (NOLOCK) ON o.[object_id] = st.[object_id]
    AND i.[index_id] = st.[index_id]
WHERE o.[type] IN (
        ‘U‘
        ,‘V‘
        )
    AND st.row_count > 0
ORDER BY STATS_DATE(i.[object_id], i.index_id) DESC
OPTION (RECOMPILE);

參考資料：

• Statistics
• UPDATE STATISTICS (Transact-SQL)
• sp_updatestats (Transact-SQL)
• Rebuilding Indexes vs. Updating Statistics
• Does a re-index update statistics?
• SQL Server Index and Statistics Maintenance

查看哪些 Index 被修改的最頻繁

SQL Server 2008 R2

SELECT TableName = OBJECT_NAME(s.[object_id])
    ,SchemaName = SCHEMA_NAME(o.[schema_id])
    ,IndexName = i.[name]
    ,user_updates
    ,i.is_primary_key
FROM sys.dm_db_index_usage_stats s
JOIN sys.objects O ON s.[object_id] = O.[object_id]
JOIN sys.indexes i ON s.[object_id] = i.[object_id]
    AND s.index_id = i.index_id
WHERE OBJECTPROPERTY(s.[object_id], ‘IsMsShipped‘) = 0
    AND user_seeks = 0
    AND user_scans = 0
    AND user_lookups = 0
    AND i.NAME IS NOT NULL -- Ignore HEAP indexes.
ORDER BY user_updates DESC

The user_updates counter indicates the level of maintenance on the index caused by insert, update, or delete operations on the underlying table or view.

SQL Server 2012

SELECT o.[name] AS [Object Name]
    ,o.[object_id]
    ,o.type_desc
    ,s.[name] AS [Statistics Name]
    ,s.stats_id
    ,s.no_recompute
    ,s.auto_created
    ,sp.modification_counter
    ,sp.rows
    ,sp.rows_sampled
    ,sp.last_updated
FROM sys.objects AS o WITH (NOLOCK)
INNER JOIN sys.stats AS s WITH (NOLOCK) ON s.object_id = o.object_id
CROSS APPLY sys.dm_db_stats_properties(s.object_id, s.stats_id) AS sp
WHERE o.type_desc NOT IN (
        N‘SYSTEM_TABLE‘
        ,N‘INTERNAL_TABLE‘
        )
    AND sp.modification_counter > 0
ORDER BY sp.modification_counter DESC
    ,o.[name]
OPTION (RECOMPILE);

查看 Index 碎片化指數

SELECT DB_NAME(ps.database_id) AS [Database Name]
    ,OBJECT_NAME(ps.[object_id]) AS [Object Name]
    ,i.[name] AS [Index Name]
    ,ps.index_id
    ,ps.index_type_desc
    ,ps.avg_fragmentation_in_percent
    ,ps.fragment_count
    ,ps.page_count
    ,i.fill_factor
    ,i.has_filter
    ,i.filter_definition
FROM sys.dm_db_index_physical_stats(DB_ID(), NULL, NULL, NULL, N‘LIMITED‘) AS ps
INNER JOIN sys.indexes AS i WITH (NOLOCK) ON ps.[object_id] = i.[object_id]
    AND ps.index_id = i.index_id
WHERE ps.database_id = DB_ID()
    AND ps.page_count > 2500
ORDER BY ps.avg_fragmentation_in_percent DESC
OPTION (RECOMPILE);

參考資料：

• Stop Worrying About SQL Server Fragmentation
• Importance of index maintenance
• Reorganize and Rebuild Indexes
• Fragmentation and Index Maintenance Tips
• Index Fragmentation–"If it isn’t broken, don’t fix it"

哪個 Index 上的讀操作最活躍

SELECT OBJECT_NAME(s.[object_id]) AS [ObjectName]
    ,i.[name] AS [IndexName]
    ,i.index_id
    ,user_seeks + user_scans + user_lookups AS [Reads]
    ,s.user_updates AS [Writes]
    ,i.type_desc AS [IndexType]
    ,i.fill_factor AS [FillFactor]
    ,i.has_filter
    ,i.filter_definition
    ,s.last_user_scan
    ,s.last_user_lookup
    ,s.last_user_seek
FROM sys.dm_db_index_usage_stats AS s WITH (NOLOCK)
INNER JOIN sys.indexes AS i WITH (NOLOCK) ON s.[object_id] = i.[object_id]
WHERE OBJECTPROPERTY(s.[object_id], ‘IsUserTable‘) = 1
    AND i.index_id = s.index_id
    AND s.database_id = DB_ID()
ORDER BY user_seeks + user_scans + user_lookups DESC
OPTION (RECOMPILE);

哪個 Index 上的寫操作最活躍

SELECT OBJECT_NAME(s.[object_id]) AS [ObjectName]
    ,i.[name] AS [IndexName]
    ,i.index_id
    ,s.user_updates AS [Writes]
    ,user_seeks + user_scans + user_lookups AS [Reads]
    ,i.type_desc AS [IndexType]
    ,i.fill_factor AS [FillFactor]
    ,i.has_filter
    ,i.filter_definition
    ,s.last_system_update
    ,s.last_user_update
FROM sys.dm_db_index_usage_stats AS s WITH (NOLOCK)
INNER JOIN sys.indexes AS i WITH (NOLOCK) ON s.[object_id] = i.[object_id]
WHERE OBJECTPROPERTY(s.[object_id], ‘IsUserTable‘) = 1
    AND i.index_id = s.index_id
    AND s.database_id = DB_ID()
ORDER BY s.user_updates DESC
OPTION (RECOMPILE);

查看 Index 所使用的 Buffer 數量

SELECT TOP 25 obj.[name] AS TableName
    ,i.[name] AS IndexName
    ,i.[type_desc] AS IndexType
    ,count(*) AS Buffered_Page_Count
    ,count(*) * 8192 / (1024 * 1024) AS Buffer_MB
    ,obj.index_id
FROM sys.dm_os_buffer_descriptors AS bd
INNER JOIN (
    SELECT object_name(object_id) AS NAME
        ,index_id
        ,allocation_unit_id
        ,object_id
    FROM sys.allocation_units AS au
    INNER JOIN sys.partitions AS p ON au.container_id = p.hobt_id
        AND (
            au.type = 1
            OR au.type = 3
            )
    
    UNION ALL
    
    SELECT object_name(object_id) AS NAME
        ,index_id
        ,allocation_unit_id
        ,object_id
    FROM sys.allocation_units AS au
    INNER JOIN sys.partitions AS p ON au.container_id = p.hobt_id
        AND au.type = 2
    ) AS obj ON bd.allocation_unit_id = obj.allocation_unit_id
LEFT JOIN sys.indexes i ON i.object_id = obj.object_id
    AND i.index_id = obj.index_id
WHERE database_id = db_id()
GROUP BY obj.NAME
    ,obj.index_id
    ,i.[name]
    ,i.[type_desc]
ORDER BY Buffered_Page_Count DESC

按照 IO Latch 等待請求對索引進行排行

SELECT OBJECT_SCHEMA_NAME(ios.object_id) + ‘.‘ + OBJECT_NAME(ios.object_id) AS table_name
    ,i.[name] AS index_name
    ,page_io_latch_wait_count
    ,page_io_latch_wait_in_ms
    ,CAST(1. * page_io_latch_wait_in_ms / NULLIF(page_io_latch_wait_count, 0) AS DECIMAL(12, 2)) AS page_io_avg_lock_wait_ms
    ,page_latch_wait_count
    ,page_latch_wait_in_ms
    ,CAST(1. * page_latch_wait_in_ms / NULLIF(page_latch_wait_count, 0) AS DECIMAL(12, 2)) AS page_avg_lock_wait_ms
FROM sys.dm_db_index_operational_stats(DB_ID(), NULL, NULL, NULL) ios
INNER JOIN sys.indexes i ON i.object_id = ios.object_id
    AND i.index_id = ios.index_id
WHERE OBJECTPROPERTY(ios.object_id, ‘IsUserTable‘) = 1
ORDER BY 3 DESC

《人人都是 DBA》系列文章索引：

人人都是 DBA（XIII）索引信息收集腳本匯編