Why my Azure SQL Database indexes are still fragmented?

Question

My company has committed the sin of using GUIDs as Primary Keys on our Azure SQL Database tables (it is actually worse than that: we used VARCHAR(36) instead of UNIQUEIDENTIFIER). As such, we end up with fragmented indexes. They looked like this:

CREATE TABLE OldTable (
  Id VARCHAR(36) PRIMARY KEY CLUSTERED NOT NULL DEFAULT NEWID(),
  CreateTime DATETIME2 NOT NULL,
  ...
)

I "fixed" the problem by creating new tables. This time, I used an immutable, ever-increasing DATETIME2 (e.g. CreateTime) column for CLUSTERED INDEX, and kept the VARCHAR(36) as PRIMARY KEY but this time, NONCLUSTERED. Like this:

CREATE TABLE NewTable (
  Id VARCHAR(36) PRIMARY KEY NONCLUSTERED NOT NULL DEFAULT NEWID(),
  CreateTime DATETIME2 NOT NULL INDEX IX_NewTable_CreateTime CLUSTERED,
)

Then I "copied" rows from old table to new table using INSERT INTO NewTable SELECT * FROM OldTable. Finally, I renamed tables and dropped the old one. Life seemed good.

For my surprise, couple of weeks later, I found out NewTable has many fragmented indexes, with avg fragmentation as high as 80%! Even the IX_NewTable_CreateTime reports fragmentation of 18%.

Did the INSERT INTO fragmented the index? Will REBUILD index solve the problem, for good?

Fragmentation is inevitable,over time indexes will be fragmeneted.You have to rebuild indexes,since SQL azure wont do it for you — TheGameiswar

Matias Quaranta Matias Quaranta · Accepted Answer · 2016-02-12T15:14:29

Fragmentation will depend on the insert/update frequency on the indexed fields and the size of the Index page.

For maintenance purposes, you can use Azure Automation and create a recurring script that checks for fragmented indexes and optimizes it.

There's a Runbook in the Gallery just for that:

The best thing about this is that Automation is free as long as you don't go over the 500 running minutes per month, time your executions well and you won't have to pay :)

I made a custom improvement to the gallery script, feel free to use it too:

<#
.SYNOPSIS 
    Indexes tables in a database if they have a high fragmentation

.DESCRIPTION
    This runbook indexes all of the tables in a given database if the fragmentation is
    above a certain percentage. 
    It highlights how to break up calls into smaller chunks, 
    in this case each table in a database, and use checkpoints. 
    This allows the runbook job to resume for the next chunk of work even if the 
    fairshare feature of Azure Automation puts the job back into the queue every 30 minutes

.PARAMETER SqlServer
    Name of the SqlServer

.PARAMETER Database
    Name of the database

.PARAMETER SQLCredentialName
    Name of the Automation PowerShell credential setting from the Automation asset store. 
    This setting stores the username and password for the SQL Azure server

.PARAMETER FragPercentage
    Optional parameter for specifying over what percentage fragmentation to index database
    Default is 20 percent

 .PARAMETER RebuildOffline
    Optional parameter to rebuild indexes offline if online fails 
    Default is false

 .PARAMETER Table
    Optional parameter for specifying a specific table to index
    Default is all tables

.PARAMETER SqlServerPort
    Optional parameter for specifying the SQL port 
    Default is 1433

.EXAMPLE
    Update-SQLIndexRunbook -SqlServer "server.database.windows.net" -Database "Finance" -SQLCredentialName "FinanceCredentials"

.EXAMPLE
    Update-SQLIndexRunbook -SqlServer "server.database.windows.net" -Database "Finance" -SQLCredentialName "FinanceCredentials" -FragPercentage 30

.EXAMPLE
    Update-SQLIndexRunbook -SqlServer "server.database.windows.net" -Database "Finance" -SQLCredentialName "FinanceCredentials" -Table "Customers" -RebuildOffline $True

.NOTES
    AUTHOR: Matias Quaranta
    LASTEDIT: Jan 10th, 2015 
#>
workflow MyRunBook
{
    param(
        [parameter(Mandatory=$True)]
        [string] $SqlServer,

        [parameter(Mandatory=$True)]
        [string] $Database,

        [parameter(Mandatory=$True)]
        [string] $SQLCredentialName,

        [parameter(Mandatory=$False)]
        [int] $FragPercentage = 20,

        [parameter(Mandatory=$False)]
        [int] $SqlServerPort = 1433,

        [parameter(Mandatory=$False)]
        [boolean] $RebuildOffline = $False,

        [parameter(Mandatory=$False)]
        [string] $Table

    )

    # Get the stored username and password from the Automation credential
    $SqlCredential = Get-AutomationPSCredential -Name $SQLCredentialName
    if ($SqlCredential -eq $null)
    {
        throw "Could not retrieve '$SQLCredentialName' credential asset. Check that you created this first in the Automation service."
    }

    $SqlUsername = $SqlCredential.UserName 
    $SqlPass = $SqlCredential.GetNetworkCredential().Password

    InlineScript{

        # Define the connection to the SQL Database
        $Conn = New-Object System.Data.SqlClient.SqlConnection("Server=tcp:$using:SqlServer,$using:SqlServerPort;Database=$using:Database;User ID=$using:SqlUsername;Password=$using:SqlPass;Trusted_Connection=False;Encrypt=True;Connection Timeout=30;")

        # Open the SQL connection
        $Conn.Open()

        # SQL command to find tables and their average fragmentation
        $SQLCommandString = @"
        SELECT a.object_id, b.name, (select name from sys.tables t where t.object_id = b.object_id) as tablename, avg_fragmentation_in_percent
        FROM sys.dm_db_index_physical_stats (
               DB_ID(N'$Database')
             , OBJECT_ID(0)
             , NULL
             , NULL
             , NULL) AS a
        JOIN sys.indexes AS b 
        ON a.object_id = b.object_id AND a.index_id = b.index_id;
"@
        # Return the tables with their corresponding average fragmentation
        $Cmd=new-object system.Data.SqlClient.SqlCommand($SQLCommandString, $Conn)
        $Cmd.CommandTimeout=120

        # Execute the SQL command
        $FragmentedTable=New-Object system.Data.DataSet
        $Da=New-Object system.Data.SqlClient.SqlDataAdapter($Cmd)
        [void]$Da.fill($FragmentedTable)

 # Return the table names that have high fragmentation
        ForEach ($FragTable in $FragmentedTable.Tables[0])
        {

            If ($FragTable.avg_fragmentation_in_percent -ge $Using:FragPercentage)
            {
                Write-Verbose ("Index found : " +  $FragTable.name + " on table:" + $FragTable.tablename)

                $SQLCommandString = "EXEC('ALTER INDEX "+$FragTable.name+" ON "+$FragTable.tablename+" REBUILD')"

                $Cmd2=new-object system.Data.SqlClient.SqlCommand($SQLCommandString, $Conn)
                # Set the Timeout to be less than 30 minutes since the job will get queued if > 30
                # Setting to 25 minutes to be safe.
                $Cmd2.CommandTimeout=1500

                Try 
                {
                    $Ds=New-Object system.Data.DataSet
                    $Da=New-Object system.Data.SqlClient.SqlDataAdapter($Cmd2)
                    [void]$Da.fill($Ds)
                }
                Catch
                {
                    Write-Verbose ($FragTable.name +" on table "+$FragTable.tablename+" could NOT be indexed.")
                }
            }
        }

        $Conn.Close()
    }
    Write-Verbose "Finished Indexing"
}

Why my Azure SQL Database indexes are still fragmented?

1 Answers