索引过多的表怎么办？

Question

我是这家公司的新 DBA。我看到他们的一些表有很多索引；例如，有些超过 50 或 60。

这是一件好事吗？从我的研究来看，这似乎对性能不利。

你会如何处理这个？

我想删除所有索引并从 0 开始。我认为开发人员一直在使用 Tuning Advisor 并没有真正了解索引。

Answer 1

可能使用了其中一些索引，因此将它们全部删除并不是一个好主意。

您可以使用此查询（取自Glenn Berry 的诊断查询）查看索引使用统计数据并查找错误的 NC 索引：

-- Possible Bad NC Indexes (writes > reads)  (Query 47) (Bad NC Indexes)
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);

您还可以使用此查询找到推荐的缺失索引（同样来自 Glenn Berry 的诊断查询）：

-- Missing Indexes for current database by Index Advantage  (Query 48) (Missing Indexes)
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);

不要像建议的那样创建推荐的缺失索引，而是尝试将它们与现有索引结合起来，并找到平衡以保持索引数量较少。