Operator used tempdb to spill data during execution with spill level 2

I am struggling to minimise the cost of sort operation on a query plan with the warning Operator usedtempdbto spill data during execution with spill level 2

I have found several posts related to spill data during execution with spill level 1, but not level 2. Level 1 seems to be caused bu outdated statistics, what about level 2? I could not find anything related to level 2.

I found this article very interesting related to Sort warnings:

Never Ignore a Sort Warning in SQL Server

My Sql Server?


Microsoft SQL Server 2014 (SP2) (KB3171021) - 12.0.5000.0 (X64) Jun
17 2016 19:14:09 Copyright (c) Microsoft Corporation Enterprise
Edition (64-bit) on Windows NT 6.3 (Build 9600: ) (Hypervisor)

My Hardware?

running the query below for find the harware:

-- Hardware information from SQL Server 2012

SELECT cpu_count AS [Logical CPU Count], hyperthread_ratio AS [Hyperthread Ratio],
cpu_count/hyperthread_ratio AS [Physical CPU Count], 
physical_memory_kb/1024 AS [Physical Memory (MB)], affinity_type_desc, 
virtual_machine_type_desc, sqlserver_start_time
FROM sys.dm_os_sys_info WITH (NOLOCK) OPTION (RECOMPILE);

currently allocated memory

SELECT
(physical_memory_in_use_kb/1024) AS Memory_usedby_Sqlserver_MB,
(locked_page_allocations_kb/1024) AS Locked_pages_used_Sqlserver_MB,
(total_virtual_address_space_kb/1024) AS Total_VAS_in_MB,
process_physical_memory_low,
process_virtual_memory_low
FROM sys.dm_os_process_memory;

when I run my query with one year scope I don't get any warning whatsoever, as per the picture below:

But when I run it only for 1 day scope I get this warning on the sort operator:

this is the query:

```
DECLARE @FromDate SMALLDATETIME = '19-OCT-2016 11:00'
DECLARE @ToDate SMALLDATETIME = '20-OCT-2016 12:00'

SELECT DISTINCT
a.strAccountCode ,
a.strAddressLine6 ,
a.strPostalCode ,
CASE WHEN a.strCountryCode IN ('91','92') THEN 'GB-

what about level 2? I could not find anything related to level 2.

As per this Old MS Doc the number in Tempdb spill signifies how many passes is required over data to sort the data. So Spill 1 means it has to pass 1 time to sort the data and 2 means it has to pass 2 times.

Quoting from the blog:


If a query involving a sort operation generates a Sort Warnings event class with a spill level value of 2, the performance of the query can be affected because multiple passes over the data are required to sort the data. In the below example we see a spill level value of 1, meaning that one pass over the data was enough to complete the sort.


why 70? I am using sql server 2014

This is because the compatibility level of database in picture is NOT 120 (which signifies compatibility level of 2014 database) since it is not 120 query will be processed using old cardinality estimation (CE) model which is referred to as CardinalityEstimationModelVersion="70". I am sure you are aware that from SQL Server 2014 we have new CE.


how do I get rid of that sort operator (if at all possible)?

The distinct command you are using is causing the sort operation. The data which is being sorted does not fit into memory so it is spilled to tempdb and when this happens a sort warning with yellow exclamation mark is given in execution plan. Sort warnings are not always a problem.

You can see in the execution plan that the estimated number of rows to be sorted is 1, but 16,353 are encountered at run time. The amount of memory reserved for the sort is based on the expected (estimated) size of the input, and cannot grow during execution (in this case).

The small memory grant for the query (1632KB) is also shared out among concurrently-executing memory-consuming operators (sort and 'optimized' loop joins). In your plan, that means 33.33% (544KB) is available to the sort while reading rows (input memory fraction). This is not enough memory to sort the 16,353 rows, so it spills to tempdb. A single level spill is not enough to complete the sort, so a second level of spilling is needed (see the reference at the end for more details on spill levels).

Sort properties as viewed in SQL Sentry Plan Explorer

Updating statistics will likely help with the cardinality estimation issue. You may be experiencing the ascending key problem, especially on table tblBOrder. A simple select from that table with the literal dates from your question will likely estimate one row right now.


I have seen page life expectation pretty low, apart adding more memory to this server, is there any other thing I can have a look at to see if I can prevent this warning?

PLE is indication of amount of I/O activity, has it increased ?. So does this happens often or only when you run certain query or has this occurred just today. Avoid knee jerk reaction, first we need to make sure that you really are facing memory pressure or some rogue query which is generating too much I/O is causing this. Anyways you already have 97 G memory assigned to SQL Server.

For more information on spill levels and the ascending key problem, see:

• The Sort that Spills to Level 15,000 by Paul White

• Statistics on Ascending Columns by Fabiano Amorim

StackExchange Database Administrators Q#152901, answer score: 21