Maintenance Tasks for SQL¶
Below are common SQL Server maintenance tasks that you should perform regularly to avoid performance degradation.
Defragment and rebuild index¶
Index fragmentation happens when logical order based on the key value doesn't match physical order within pages. Database Engine automatically modifies indexes when inserting, updating or deleting data. For example, the addition of rows in a table may cause existing pages in rowstore indexes to split to make room for the insertion of new key values. Modifications over time lead to scattered information and fragmented indexes in the database. Heavily fragmented indexes can degrade query performance due to additional I/O required to locate data to which the index points. More I/O slows down the application, particularly during scan operations.
Note
Rebuilding is recommended if avg_fragmentation_in_percent is greater than 30%.
To start index defragmentation and rebuilding:
- Run the query to find out the percentage of fragmentation:
SELECT DB_NAME() AS DBName ,OBJECT_NAME(ps.object_id) AS TableName ,i.name AS IndexName ,ips.index_type_desc ,ips.avg_fragmentation_in_percent FROM sys.dm_db_partition_stats ps INNER JOIN sys.indexes i ON ps.object_id = i.object_id AND ps.index_id = i.index_id CROSS APPLY sys.dm_db_index_physical_stats(DB_ID(), ps.object_id, ps.index_id, null, 'LIMITED') ips ORDER BY ips.avg_fragmentation_in_percent desc, ps.object_id, ps.index_id - Run the query to rebuilding indexes. Use the SQL maintenance script or the simplest one:
DECLARE @TableName varchar(255) DECLARE TableCursor CURSOR FOR ( SELECT '[' + IST.TABLE_SCHEMA + '].[' + IST.TABLE_NAME + ']' AS [TableName] FROM INFORMATION_SCHEMA.TABLES IST WHERE IST.TABLE_TYPE = 'BASE TABLE' ) OPEN TableCursor FETCH NEXT FROM TableCursor INTO @TableName WHILE @@FETCH_STATUS = 0 BEGIN PRINT('Rebuilding Indexes on ' + @TableName) Begin Try EXEC('ALTER INDEX ALL ON ' + @TableName + ' REBUILD with (ONLINE=ON)') End Try Begin Catch PRINT('Cannot do rebuild with Online=On option, taking table ' + @TableName+' down for douing rebuild') EXEC('ALTER INDEX ALL ON ' + @TableName + ' REBUILD') End Catch FETCH NEXT FROM TableCursor INTO @TableName END CLOSE TableCursor DEALLOCATE TableCursor
Find and create missing index¶
SQL Server keeps up with index statistics behind the scenes. When you use Entity Framework, very easy to skip the required index and decrease the performance of the solution.
The higher the improvement_measure is, the better is your performance. If you see a missing index with a high user_seeks value, which is the number of times the index could have been used, add it.
Note
Do not create more than 5-10 indexes per table.
To find missing indexes, run the query and copy SQL command to create them:
SELECT CONVERT (varchar, getdate(), 126) AS runtime,
mig.index_group_handle,
mid.index_handle,
CONVERT (decimal (28,1),
migs.avg_total_user_cost *
migs.avg_user_impact *
(migs.user_seeks + migs.user_scans))
AS improvement_measure,
'CREATE INDEX missing_index_' +
CONVERT (varchar, mig.index_group_handle) +
'_' +
CONVERT (varchar, mid.index_handle) +
' ON ' +
mid.statement +
' (' + ISNULL (mid.equality_columns,'') +
CASE WHEN mid.equality_columns IS NOT NULL
AND mid.inequality_columns IS NOT NULL
THEN ','
ELSE ''
END + ISNULL (mid.inequality_columns, '') +
')' +
ISNULL (' INCLUDE (' + mid.included_columns + ')',
'') AS create_index_statement,
migs.*,
mid.database_id, mid.[object_id]
FROM sys.dm_db_missing_index_groups mig
INNER JOIN sys.dm_db_missing_index_group_stats migs
ON migs.group_handle = mig.index_group_handle
INNER JOIN sys.dm_db_missing_index_details mid
ON mig.index_handle = mid.index_handle
WHERE CONVERT (decimal (28,1),
migs.avg_total_user_cost *
migs.avg_user_impact *
(migs.user_seeks + migs.user_scans)) > 10
ORDER BY migs.avg_total_user_cost *
migs.avg_user_impact *
(migs.user_seeks + migs.user_scans) DESC
Control and eliminate long-running queries¶
To find slow and long-running queries, run the script:
SELECT TOP 20 total_worker_time/execution_count AS [avg_cpu_time],
(SELECT TOP 1 SUBSTRING(s2.text,statement_start_offset / 2+1 ,
((CASE WHEN statement_end_offset = -1
THEN
(LEN(CONVERT(nvarchar(max),s2.text)) * 2)
ELSE
statement_end_offset
END)
- statement_start_offset) / 2+1)) AS sql_statement,
execution_count,
plan_generation_num,
last_execution_time,
total_worker_time,
last_worker_time,
min_worker_time,
max_worker_time,
total_physical_reads,
last_physical_reads,
min_physical_reads,
max_physical_reads,
total_logical_writes,
last_logical_writes,
min_logical_writes,
max_logical_writes,
s1.sql_handle
FROM sys.dm_exec_query_stats AS s1
CROSS APPLY sys.dm_exec_sql_text(sql_handle) AS s2
WHERE s2.objectid is null
ORDER BY (total_worker_time/execution_count) DESC
Prune PlatformOperationLog and NotificationMessage tables¶
The recommended retention period for PlatformOperationLog and NotificationMessage is 45 days. This is usually enough to find the reason for the issues. Create a task and run it either nightly or weekly.
SELECT TOP $processRowsCount Id FROM [dbo].[$tableName] WHERE CreatedDate < '$thresholdDate' ORDER BY CreatedDate
"@
Write-Output "Parameters set to remove $processRowsCount rows from $tableName table in $Database database with created date < $thresholdDate"
$records = @()
try {
$records = Invoke-Sqlcmd -ServerInstance $SqlServer -Database $Database -Username $SqlUsername -Password $SqlPass -Query $sqlQuery -ConnectionTimeout $ConnectionTimeout
} catch {
Write-Error "$error"
throw "$error"
}
if ($records.count -eq 0) { Write-Output "No records found fitting the parameters"}
$c = 0
foreach ($entry in $records) {
if ($VerboseOutput -eq 1){Write-Output "Removing record with Id $($records[$c].Id)"}
$sqlQuery2 = @"
DELETE FROM [dbo].[$tableName] WHERE Id = '$($records[$c].Id)'
"@
try {
Invoke-Sqlcmd -ServerInstance $SqlServer -Database $Database -Username $SqlUsername -Password $SqlPass -Query $sqlQuery2 -ConnectionTimeout $ConnectionTimeout
} catch {
Write-Error "$error"
throw "$error"
}
$c = $c + 1
}
Write-Output "Removed $c rows"
}
Control table size¶
It is very important to control the size of tables. It takes more time to apply any operation to a larger table. If you have temporary or log tables, don't forget to clean up the expired data.
Run the script to return the size of the table in Mbytes:
select
sys.objects.name, sum(reserved_page_count) * 8.0 / 1024
from
sys.dm_db_partition_stats, sys.objects
where
sys.dm_db_partition_stats.object_id = sys.objects.object_id
group by sys.objects.name
ORDER BY 2 DESC
Use Azure SQL Elastic Pool and multiple databases instead of single one¶
Azure SQL pools are well suited for Virto Commerce solutions. Virto Commerce allows you to create a unique connection string for each module. The more databases you can add to a Pool, the more you're going to save. Depending on your application usage patterns, it's possible to see savings with as few as two S3 databases.
To set own connection string for every module, use Module Id as a name of a connection string. You can find Module Id in the module.manifest file. For example, Module Id for the Order module is as follows:
<?xml version="1.0" encoding="utf-8" ?>
<module>
<id>VirtoCommerce.Orders</id>
<version>2.17.30</version>
<platformVersion>2.13.42</platformVersion>
<dependencies>
<dependency id="VirtoCommerce.Core" version="2.25.24" />
<dependency id="VirtoCommerce.Catalog" version="2.12.0" />
<dependency id="VirtoCommerce.Pricing" version="2.11.0" />
<dependency id="VirtoCommerce.Customer" version="2.11.0" />
<dependency id="VirtoCommerce.Store" version="2.11.0" />
</dependencies>
Set your own connection strings for:
- VirtoCommerce.Cart.
- VirtoCommerce.Catalog.
- VirtoCommerce.Customer.
- VirtoCommerce.Orders.
- VirtoCommerce.Pricing.
- VirtoCommerce - Default database for other modules.
It decreases database size and cost, increases performance and simplifies maintenance tasks.
Use Azure tools¶
Azure supports a big list of the service to improve Maintenance Tasks for SQL Server.
Here is the automatic tuning list:
- Automated performance tuning of Azure SQL databases.
- Automated verification of performance gains.
- Automated rollback and self-correction.
- Tuning history.
- Tuning action T-SQL scripts for manual deployments.
- Proactive workload performance monitoring.
- Scale-out capability on hundreds of thousands of databases.
- Positive impact to DevOps resources and the total cost of ownership.
For more information, read Automatic tuning in Azure SQL Database and Azure SQL Managed Instance.
Find blocking queries in SQL Azure¶
To finding blocking queries in SQL Azure, run:
SELECT TOP 10 r.session_id,
r.plan_handle,
r.sql_handle,
r.request_id,
r.start_time,
r.status,
r.command,
r.database_id,
r.user_id,
r.wait_type,
r.wait_time,
r.last_wait_type,
r.wait_resource,
r.total_elapsed_time,
r.cpu_time,
r.transaction_isolation_level,
r.row_count,
st.text
FROM sys.dm_exec_requests r
CROSS APPLY sys.dm_exec_sql_text(r.sql_handle) as st
WHERE r.blocking_session_id = 0
and r.session_id in
(SELECT distinct(blocking_session_id) FROM sys.dm_exec_requests)
GROUP BY r.session_id, r.plan_handle, r.sql_handle,
r.request_id, r.start_time, r.status,
r.command, r.database_id, r.user_id, r.wait_type,
r.wait_time, r.last_wait_type, r.wait_resource,
r.total_elapsed_time, r.cpu_time,
r.transaction_isolation_level, r.row_count, st.text
ORDER BY r.total_elapsed_time desc
Set maximum number of concurrent requests and sessions¶
To set max concurrent request and sessions, run: