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sql-server performance execution-plan azure-sql-database cpu
Updated Fri, 29 Jul 2022 05:44:26 GMT

What is causing high CPU usage from this query/execution plan?

I have an Azure SQL Database that powers a .NET Core API app. Browsing the performance overview reports in the Azure Portal suggests that the majority of the load (DTU usage) on my database server is coming from CPU, and one query specifically:

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As we can see, query 3780 is responsible for nearly all of the CPU usage on the server.

This somewhat makes sense, since query 3780 (see below) is basically the entire crux of the application and is called by users quite often. It is also a rather complex query with many joins necessary to get the proper dataset needed. The query comes from a sproc that ends up looking like this:

            @UserId IS NULL
            OR C.[Favorites] IS NULL
            OR JSON_VALUE(C.[Favorites], CONCAT('$."', @UserId, '"')) IS NULL
        THEN 0
        ELSE 1
    END [Favorite],
            @UserId IS NULL
            OR C.[Folders] IS NULL
        THEN NULL
    END [FolderId],
        WHEN C.[UserId] IS NOT NULL OR OU.[AccessAll] = 1 OR CU.[ReadOnly] = 0 OR G.[AccessAll] = 1 OR CG.[ReadOnly] = 0 THEN 1
        ELSE 0
    END [Edit],
        WHEN C.[UserId] IS NULL AND O.[UseTotp] = 1 THEN 1
        ELSE 0
    END [OrganizationUseTotp]
    [dbo].[Cipher] C
    [dbo].[Organization] O ON C.[UserId] IS NULL AND O.[Id] = C.[OrganizationId]
    [dbo].[OrganizationUser] OU ON OU.[OrganizationId] = O.[Id] AND OU.[UserId] = @UserId
    [dbo].[CollectionCipher] CC ON C.[UserId] IS NULL AND OU.[AccessAll] = 0 AND CC.[CipherId] = C.[Id]
    [dbo].[CollectionUser] CU ON CU.[CollectionId] = CC.[CollectionId] AND CU.[OrganizationUserId] = OU.[Id]
    [dbo].[GroupUser] GU ON C.[UserId] IS NULL AND CU.[CollectionId] IS NULL AND OU.[AccessAll] = 0 AND GU.[OrganizationUserId] = OU.[Id]
    [dbo].[Group] G ON G.[Id] = GU.[GroupId]
    [dbo].[CollectionGroup] CG ON G.[AccessAll] = 0 AND CG.[CollectionId] = CC.[CollectionId] AND CG.[GroupId] = GU.[GroupId]
    C.[UserId] = @UserId
    OR (
        C.[UserId] IS NULL
        AND OU.[Status] = 2
        AND O.[Enabled] = 1
        AND (
            OU.[AccessAll] = 1
            OR CU.[CollectionId] IS NOT NULL
            OR G.[AccessAll] = 1
            OR CG.[CollectionId] IS NOT NULL

If you care, full source for this database can be found on GitHub here. Sources from the query above:

I've spent some time on this query over the months tuning the execution plan as best I know how, ending up with it's current state. Queries with this execution plan are fast across millions of rows (< 1 sec), but as noted above, are eating up server CPU more and more as the application grows in size.

I have attached the actual query plan below (not sure of any other way to share that here on stack exchange), which shows an execution of the sproc in production against a returned dataset of ~400 results.

Some points I am looking for clarification on:

  • Index Seek on [IX_Cipher_UserId_Type_IncludeAll] takes 57% of total cost of the plan. My understanding of the plan is that this cost is related to IO, which makes since being that the Cipher table contains millions of records. However, Azure SQL performance reports are showing me that my problems stem from CPU on this query, not IO, so I am unsure if this is actually a problem or not. Plus it is already doing an index seek here, so I am not really sure there is any room for improvement.

  • The Hash Match operations from all the joins seem to be what is showing significant CPU usage in the plan (I think?), but I am not really sure how this could be made better. The complex nature of how I need to get the data necessitates lots of joins across several tables. I already short-circuit many of these joins if possible (based on results from a previous join) in their ON clauses.

Download full execution plan here:

I feel like I can get better CPU performance out of this query, but I am at a stage where I am not sure how to proceed on tuning the execution plan any further. What other optimizations could be had to decrease CPU load? What operations in the execution plan are the worst offenders of CPU usage?


You can view operator level CPU and elapsed time metrics in SQL Server Management Studio, although I can't say how reliable they are for queries that finish as quickly as yours. Your plan only has row mode operators so the time metrics apply to that operator as well as the operators in the subtree below it. Using the nested loop join as an example, SQL Server is telling you that entire subtree took 60 ms of CPU time and 80 ms of elapsed time:

subtree costs

Most of that subtree time is spent on the index seek. Index seeks take CPU too. It looks like your index has exactly the columns needed so it isn't clear how you could reduce the CPU costs of that operator. Other than the seeks most of the CPU time in the plan is spent on the hash matches which implement your joins.

This is a huge oversimplification, but the CPU taken by those hash joins is going to depend on the size of the input for the hash table and the number of rows processed on the probe side. Observing a few things about this query plan:

  • At most 461 returned rows have C.[UserId] = @UserId. These rows don't care about the joins at all.
  • For the rows that do need the joins, SQL Server isn't able to apply any of the filtering early (except for OU.[UserId] = @UserId).
  • Nearly all of the processed rows are eliminated near the end of the query plan (reading from right to left) by the filter: [vault].[dbo].[Cipher].[UserId] as [C].[UserId]=[@UserId] OR ([vault].[dbo].[OrganizationUser].[AccessAll] as [OU].[AccessAll]=(1) OR [vault].[dbo].[CollectionUser].[CollectionId] as [CU].[CollectionId] IS NOT NULL OR [vault].[dbo].[Group].[AccessAll] as [G].[AccessAll]=(1) OR [vault].[dbo].[CollectionGroup].[CollectionId] as [CG].[CollectionId] IS NOT NULL) AND [vault].[dbo].[Cipher].[UserId] as [C].[UserId] IS NULL AND [vault].[dbo].[OrganizationUser].[Status] as [OU].[Status]=(2) AND [vault].[dbo].[Organization].[Enabled] as [O].[Enabled]=(1)

It would be more natural to write your query as a UNION ALL. The first half of the UNION ALL can include rows where C.[UserId] = @UserId and the second half can include rows where C.[UserId] IS NULL. You're already doing two index seeks on [dbo].[Cipher] (one for @UserId and one for NULL) so it seems unlikely that the UNION ALL version would be slower. Writing out the queries separately will allow you to do some of the filtering early, both on the build and the probe sides. Queries can be faster if they need to process less intermediate data.

I don't know if your version of SQL Server supports this, but if that doesn't help you could try adding a columnstore index to your query to make your hash joins eligible for batch mode. My preferred way is to create an empty table with a CCI on it and to left join to that table. Hash joins can be much more efficient when they run in batch mode compared to row mode.

Comments (1)

  • +0 – As suggested, I was able to re-write the sproc with 2 queries that UNION ALL (one for C.[UserId] = @UserId and one for C.[UserId] IS NULL AND ...). This reduced the join result sets and removed the need for hash matches altogether (now doing nested loops on small joins sets). The query is now much better on CPU. Thank you! — Apr 24, 2018 at 04:33