You’ve probably heard: “Indexes are not free”. And this is right!
Indexes can greatly improve query performance in the SQL Server, but they may hamper the performance too.
There are costs in storage (disk space) as well as resources for the maintenance required on Data Manipulation Language (DML) operations.
So, it’s very important to understand your overall workload and find a good balance between making your queries efficient and not paying too much for that.
There are many approaches, concepts, and techniques about the index, below you will see some of them.
Missing indexes
SQL Server will sometimes suggest an index that it believes will help that query run faster.
These are known as missing index suggestions.
SQL Server is very good at determining when an index is needed but sometimes it fails.
The script below identifies missing indexes that have more than 70,000 unique compiles, user seeks, or user scans, and an average usage impact higher than 80%.
Also, take into account the following:
- You should not create all the missing indexes this script suggests. They are just for your guidance.
- If the server was rebooted earlier than 2 weeks ago, the stats are useless.
- You also probably shouldn’t have more than 5 indexes per table (over-indexing).
SELECT sd.name
,substring(d.statement, charindex('.', d.statement) + 1, len(d.statement)) AS tablename
,s.unique_compiles
,s.user_seeks
,s.user_scans
,s.avg_user_impact
,'use [' + sd.name + ']; create index IX_' + replace(reverse(substring(reverse(d.statement), 0, charindex('[', reverse(d.statement)))), ']', '') + '_nn ON ' + substring(d.statement, charindex('.', d.statement) + 1, len(d.statement)) + ' (' + CASE
WHEN d.equality_columns IS NOT NULL
AND d.inequality_columns IS NOT NULL
THEN d.equality_columns + ',' + d.inequality_columns
WHEN d.equality_columns IS NOT NULL
AND d.inequality_columns IS NULL
THEN d.equality_columns
WHEN d.equality_columns IS NULL
AND d.inequality_columns IS NOT NULL
THEN d.inequality_columns
END + ')' + CASE
WHEN d.included_columns IS NOT NULL
THEN ' include (' + d.included_columns + ')'
ELSE ''
END AS create_cmd
FROM master.sys.dm_db_missing_index_details d
INNER JOIN master.sys.dm_db_missing_index_groups g ON d.index_handle = g.index_handle
INNER JOIN master.sys.dm_db_missing_index_group_stats s ON g.index_group_handle = s.group_handle
INNER JOIN master.dbo.sysdatabases sd ON d.database_id = sd.dbid
WHERE sd.dbid > 4
AND sd.name NOT IN (
'distribution'
,'LogShipping'
)
AND (
s.unique_compiles > 70000
OR s.user_seeks > 70000
OR s.user_scans > 70000
)
AND s.avg_user_impact > 80
ORDER BY sd.name
,s.user_seeks DESC
,s.user_scans DESCFill factor option
When creating (or rebuilding) an index, you can set the fill factor.
The fill factor specifies the percentage of each 8K data page to be filled at the ‘leaf’ level of the index.
Lowering this setting may improve performance by reducing page splits, but do not set the system-wide value for it, probably that will hurt more than help.
It would be best if you had a proper maintenance plan and only changed the fill factor for heavily fragmented indexes.
If needed, start reducing the fill factor gradually, going to 95%, and rebuilding the indexes in a planned change.
Unused indexes
Sometimes an index is never used to improve performance.
Regardless, SQL Server must maintain it during every INSERT, UPDATE, and DELETE operation.
So, why keep this index?
You can use the script below to find indexes that were updated more than 5000 times, never used, or with many updates and few lookups ( < 10 ):
- Save the ‘create_cmd’ output for the re-creation of the index script in case a rollback is needed.
- Execute the output “drop_cmd” to implement the change.
- Make sure there is not any query or store procedure forcing hint to the indexes that will be dropped.
SET NOCOUNT ON
DECLARE @DBName NVARCHAR(128)
,@cmd NVARCHAR(max) = N''
,@collation VARCHAR(100)
SELECT @collation = convert(VARCHAR, serverproperty('collation'))
IF object_id('tempdb..#indexes') IS NOT NULL
DROP TABLE #indexes
CREATE TABLE #indexes (
databasename SYSNAME NOT NULL
,tablename VARCHAR(200) NOT NULL
,indexname VARCHAR(200) NOT NULL
,drop_cmd VARCHAR(max) NOT NULL
,create_cmd VARCHAR(max) NOT NULL
);
DECLARE DBList CURSOR LOCAL FAST_FORWARD FOR
SELECT cast(name AS NVARCHAR(128))
FROM master.dbo.sysdatabases
WHERE dbid > 4
AND name NOT IN ('distribution', 'LogShipping')
AND databasepropertyex(name, 'Status') = 'ONLINE'
AND databasepropertyex(name, 'Updateability') = 'READ_WRITE'
OPEN DBList
FETCH NEXT FROM DBList INTO @dbname
WHILE @@fetch_status <> - 1
BEGIN
SET @cmd = N'
;WITH notusedindexes
AS (
SELECT db_name(db_id()) AS DBName
,ss.name + ''.'' + o.name AS [table_name]
,i.name AS [index_name]
,i.type_desc
,'' DROP INDEX '' + i.name + '' ON '' + ss.name + ''.'' + o.name AS drop_cmd
FROM [' + @DBName + '].sys.dm_db_index_usage_stats iu
INNER JOIN [' + @DBName + '].sys.indexes i ON iu.index_id = i.index_id
AND iu.object_id = i.object_id
INNER JOIN [' + @DBName + '].sys.objects o ON i.object_id = o.object_id
LEFT JOIN [' + @DBName + '].sys.objects n ON n.name = i.name
INNER JOIN [' + @DBName + '].sys.schemas ss ON o.schema_id = ss.schema_id
WHERE iu.database_id = db_id()
AND i.type <> 0
AND i.type_desc = '' NONCLUSTERED ''
AND (
n.TYPE IS NULL
OR n.type NOT IN ('' PK '', '' UQ '')
)
AND (
( iu.user_seeks = 0
AND iu.user_scans = 0
AND iu.user_lookups = 0
AND iu.user_updates <> 0
) -- only ever updated
OR ( iu.user_updates > 100000
AND iu.user_seeks < 10
AND iu.user_scans < 10
AND iu.user_lookups < 10
) -- loads of updates, few lookups
OR ( iu.user_seeks < 10
AND iu.user_scans < 10
AND iu.user_lookups < 10
AND iu.user_updates < 10
)
)
)
,-- hardly used at all and iu.user_updates > 5000), -- updated more than 5000 times
indexlist
AS (
SELECT DISTINCT n.table_name tablename
,n.index_name indexname
,n.drop_cmd
,i.is_unique
,(
SELECT DISTINCT stuff((
SELECT ''
,'' + c.name
FROM sys.index_columns ic1
INNER JOIN sys.columns c ON ic1.object_id = c.object_id
AND ic1.column_id = c.column_id
WHERE ic1.index_id = ic.index_id
AND ic1.object_id = i.object_id
AND ic1.index_id = i.index_id
AND ic1.is_included_column = 0
ORDER BY ic1.key_ordinal
FOR XML PATH('''')
), 1, 2, '''')
FROM sys.index_columns ic
WHERE object_id = i.object_id
AND index_id = i.index_id
) AS keycolumnlist
,(
SELECT DISTINCT stuff((
SELECT ''
,'' + c.name
FROM sys.index_columns ic1
INNER JOIN sys.columns c ON ic1.object_id = c.object_id
AND ic1.column_id = c.column_id
WHERE ic1.index_id = ic.index_id
AND ic1.object_id = i.object_id
AND ic1.index_id = i.index_id
AND ic1.is_included_column = 1
ORDER BY index_column_id
FOR XML PATH('''')
), 1, 2, '''')
FROM sys.index_columns ic
WHERE object_id = i.object_id
AND index_id = i.index_id
) AS includedcolumnlist
FROM sys.indexes i
INNER JOIN sys.index_columns ic ON i.object_id = ic.object_id
AND i.index_id = ic.index_id
INNER JOIN sys.objects o ON i.object_id = o.object_id
INNER JOIN notusedindexes n ON i.object_id = object_id(n.table_name)
AND i.name collate '
+ @collation + ' = n.index_name collate ' + @collation + '
WHERE o.is_ms_shipped = 0
AND i.is_primary_key = 0
AND i.is_disabled = 0
AND i.is_unique_constraint = 0
AND i.has_filter = 0
)
INSERT INTO #indexes
SELECT db_name() AS databasename
,tablename
,indexname
,drop_cmd
,''
CREATE '' + CASE
WHEN is_unique = 1
THEN '' UNIQUE ''
ELSE ''''
END + '' INDEX '' + indexname + '' ON '' + tablename + '' ('' + keycolumnlist + '') '' + CASE
WHEN includedcolumnlist IS NOT NULL
THEN + '' include('' + includedcolumnlist + '') ''
ELSE ''''
END + ''
WITH (data_compression = page) '' create_cmd
FROM indexlist'
PRINT @cmd
--exec sys.sp_executesql @cmd
FETCH NEXT
FROM DBList
INTO @DBName
END
CLOSE DBList
DEALLOCATE DBList
SELECT * FROM #indexes
DROP TABLE #indexes
