Bxdty

On a server with 32GB we are running SQL Server 2014 SP2 with a max memory of 25GB we have two tables, here you find a simplified structure of both tables:

CREATE TABLE [dbo].[Settings](

    [id] [int] IDENTITY(1,1) NOT NULL,

    [resourceId] [int] NULL,

    [typeID] [int] NULL,

    [remark] [varchar](max) NULL,

    CONSTRAINT [PK_Settings] PRIMARY KEY CLUSTERED ([id] ASC)

) ON [PRIMARY]

GO



CREATE TABLE [dbo].[Resources](

    [id] [int] IDENTITY(1,1) NOT NULL,

    [resourceUID] [int] NULL,

 CONSTRAINT [PK_Resources] PRIMARY KEY CLUSTERED ([id] ASC)

) ON [PRIMARY]

GO

with following non-clustered indexes:

CREATE NONCLUSTERED INDEX [IX_UID] ON [dbo].[Resources]

(

    [resourceUID] ASC

)



CREATE NONCLUSTERED INDEX [IX_Test] ON [dbo].[Settings]

(

    [resourceId] ASC,

    [typeID] ASC

)

The database is configured with compatibility level 120.

When I run this query there are spills to tempdb.
This is how I execute the query:

exec sp_executesql N'

select r.id,remark

FROM Resources r

inner join Settings on resourceid=r.id

where resourceUID=@UID

ORDER BY typeID',

N'@UID int',

@UID=38

If don't select the [remark] field no spills occurs. My first reaction was that the spills occurred due to the low number of estimated rows on the nested-loop operator.

So I add 5 datetime and 5 integer columns to the settings table and add them to my select statement. When I execute the query no spills are happening.

Why are the spills only happening when [remark] is selected? It has probably something to do with the fact that this is a varchar(max).
What can I do to avoid spilling to tempdb?

There are going to be several possible workarounds here.

You can manually adjust the memory grant, though I probably wouldn't go that route.

You can also use a CTE and TOP to push the sort lower, before grabbing the max length column. It will look something like below.

WITH CTE AS (

SELECT TOP 1000000000 r.ID, s.ID AS ID2, s.typeID

FROM Resources r

inner join Settings s on resourceid=r.id

where resourceUID=@UID

ORDER BY s.typeID

)

SELECT c.ID, ca.remark

FROM CTE c

CROSS APPLY (SELECT remark FROM dbo.Settings s WHERE s.id = c.ID2) ca(remark)

ORDER BY c.typeID

Proof-of-concept dbfiddle here
Sample data would still be appreciated!

Why are the spills only happening when [remark] is selected?

The spill is happening when you include that column because you don't get a big enough memory grant for the large string data being sorted.

You don't get a big enough memory grant because the actual number of rows is 10x more than the estimated number of rows (1,302 actual vs 126 estimated).

Why is the estimate off? Why does SQL Server think there's only one row in dbo.Settings with a resourceid of 38?

It could be a statistics issue, which you can check by running DBCC SHOW_STATISTICS('dbo.Settings', 'IX_Test') and see the counts for that histogram step. But the execution plan seems to indicate that the stats are as complete and up-to-date as they could be.

Since stats aren't helping, your best bet is probably a query rewrite - which Forrest has covered in his answer.