I have a table with a GUID as the primary key.

The default is newsequentialid() to keep the GUID's in order.

With only 65,000 rows, the primary key is 1.5 GB! There's a similar table which was 2GB with only 3,000 rows.

I've truncated the second table in a test environment and it has reduced the table size and DB size by that amount.

How do I troubleshoot this? What causes the indexes to be so large with so little data?

It's a SQL Server 2016 standard database on a Windows 2012 R2 server.

The data is from running the script below.

;with SpaceInfo(ObjectId, IndexId, TableName, IndexName
    ,Rows, TotalSpaceMB, UsedSpaceMB)
        t.object_id as [ObjectId]
        ,i.index_id as [IndexId]
        ,s.name + '.' + t.Name as [TableName]
        ,i.name as [Index Name]
        ,sum(p.[Rows]) as [Rows]
        ,sum(au.total_pages) * 8 / 1024 as [Total Space MB]
        ,sum(au.used_pages) * 8 / 1024 as [Used Space MB]
        sys.tables t with (nolock) join 
            sys.schemas s with (nolock) on 
                s.schema_id = t.schema_id
            join sys.indexes i with (nolock) on 
                t.object_id = i.object_id
            join sys.partitions p with (nolock) on 
                i.object_id = p.object_id and 
                i.index_id = p.index_id
            cross apply
                    sum(a.total_pages) as total_pages
                    ,sum(a.used_pages) as used_pages
                from sys.allocation_units a with (nolock)
                where p.partition_id = a.container_id 
            ) au
        i.object_id > 255
    group by
        t.object_id, i.index_id, s.name, t.name, i.name
    ObjectId, IndexId, TableName, IndexName
    ,Rows, TotalSpaceMB, UsedSpaceMB
    ,TotalSpaceMB - UsedSpaceMB as [ReservedSpaceMB]
order by
    TotalSpaceMB desc
option (recompile)
  • Is your primary key a clustered index? Jun 24, 2017 at 7:52
  • Yes, just setup as a default SQL Server PK
    – RemarkLima
    Jun 24, 2017 at 7:56
  • 1
    "How do I stop the PK including all the columns" - define it as non-clustered. You deployed the defaults, which makes it clustered, which creates a B+ Tree index for the PK, which includes all columns. Jun 24, 2017 at 9:50
  • 5
    Defining the PK as non-clustered would convert the table from clustered to a heap. You'll still need the same (or around) amount of disk space for the table to be stored. It's not that a clustered index duplicates the data. The clustered index is the table. Jun 24, 2017 at 10:28
  • 2
    What is your table definition?
    – Joe Obbish
    Jun 24, 2017 at 16:24

1 Answer 1


A clustered index in MSSQL causes sequential data rows to be stored together in the data file, including when data in a row is updated. You don't have to have one, but, unless you make it different, a primary key will be created with a clustered index. As we've already said, really you're changing your entire table into the clustered index. Basically a tree whose leaves are the data rows. You can also have a clustered index which isn't the primary key.

A nonclustered index is a separate copy of specified table column data, organised in a different key order. It includes the clustered index key, if there is a clustered index, to link back to the table row. This makes it economical to use a small size clustered index key. GUID isn't ideal, but there's worse. Data can be fetched from a nonclustered index without touching the stored table at all. Of course, when table data is updated, the server has the job of automatically updating the data in a nonclustered index as well.

  • With everyone's help and the comments that the cluster index IS the table allowed me to find the source - the message queue table was storing the entire MIME message with all attachments encoded in there... Each message generally having 10 - 50 MB of attachments. The vast growth makes sense in this context, and it wasn't an index, but the raw data.
    – RemarkLima
    Jul 10, 2017 at 9:35

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