Table partitioning and MAX queries performance problem

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Table partitioning and MAX queries performance problem

peterjonk

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March 26, 2010 at 7:56 am

#220949

Hi All,
Just implemented partitioning in SQL Server 2005. Which should be a great tool for better flexibility and performance. However I'm pretty disappointed running into performance problems while running simple queries like these:
select max(id) from tbl_test
If the table contains 50 miljons of rows. An index scan of 50 miljon rows occurs despite the existence of the index. When using an unpartitioned table an index seek is used as one would expect.
Then I found out this bug is also confirmed by Microsoft:
http://social.msdn.microsoft.com/forums/en-US/sqldatabaseengine/thread/ac2f13d7-875a-4f0c-b119-c654e4990ea7/
I'm pretty disappointed in Microsoft especially the fact they are not able to solve this problem.
I was wondering if some of you also ran into this problem and what workarounds you have used.
Best Regards, Peter (SQL Server DBA)
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November 29, 2016 at 4:26 am

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Bhuvnesh SSC Guru Points: 59351 More actions · Answer 1

peterjonk (3/26/2010)
select max(id) from tbl_test

first of all this query never go for index seek as it doesnt contain any where clause

and second, if this table has only one column (id) then above query should go for index scan

and if it has more then one column it will go for table scan for these cases i am assuming that we have non clustered index on id column

-------Bhuvnesh----------
I work only to learn Sql Server...though my company pays me for getting their stuff done;-)

peterjonk SSCommitted Points: 1655 More actions · Answer 2

Please read the link I posted. A max query does in fact use an index seek if the column is a primary key or indexed.

The query I provided was just to use a sample. In the real world the table has a clustered partitioned index. There is huge difference between the execution plans of a partitioned table (Index scan) and an unpartitioned table (Index seek).

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RBarryYoung SSC Guru Points: 143329 More actions · Answer 3

Have you tried a query like this?:

SELECT MAX(CAST(LTRIM(CAST(subID AS VARCHAR(MAX))) AS bigint)) AS id

FROM

(

SELECT

$PARTITION.YourPatitionFunction(YourPartitionColumn) AS [partitionNum]

, MAX(id) AS [subID]

FROM tbl_test

GROUP BY $PARTITION.YourPatitionFunction(YourPartitionColumn)

) AS a

I don't have partitioned tables to test this against, so it may have more opaqueness in the outer MAX than it needs.

[font="Times New Roman"]-- RBarryYoung[/font], [font="Times New Roman"] (302)375-0451[/font] blog: MovingSQL.com, Twitter: @RBarryYoung [font="Arial Black"]
Proactive Performance Solutions, Inc. [/font][font="Verdana"] "Performance is our middle name."[/font]

peterjonk SSCommitted Points: 1655 More actions · Answer 4

Thanks for you reply and suggestion. It seems that rewriting the queries is the only way to work around this problem.

Some of our .NET applications using ad-hoc queries (via OR mappers) or SP's to execute such queries receive time-outs. So we decided to rollback partitioning because we first need to investigate the impact of this problem. That's very time consuming if you have dozens of applications and stored procedures which need to be reviewed for query rewriting.

The problem is confirmed by Microsoft and still exists in SQL 2008

You've provided a very nice analysis that has shown several limitations in the optimizer related to queries on partitioned tables that use MIN and TOP. We're not in a position to fix these issues as a bug fix for SQL Server 2008 because they require some significant effort.

A very disappointing answer from Microsoft in my opinion.

This is the workaround I used which includes an extra where clause to limit the number of rows which must be scanned:

declare @numrows int

declare @return_value int

-- Get number of rows in table

SELECT @numrows=sum(p.rows) FROM sys.partitions p

inner join sys.indexes i on p.object_id=i.object_id and p.index_id=i.index_id

WHERE p.object_id=OBJECT_ID('TBL_TEST')

and name='PK_TBL_TEST_IDENTITY_ID'

-- Get max value of identity column

select max(IDENTITY_ID) from TBL_TEST

where IDENTITY_ID >= @numrows

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RBarryYoung SSC Guru Points: 143329 More actions · Answer 5

Ah, nice idea. I like that one.

[font="Times New Roman"]-- RBarryYoung[/font], [font="Times New Roman"] (302)375-0451[/font] blog: MovingSQL.com, Twitter: @RBarryYoung [font="Arial Black"]
Proactive Performance Solutions, Inc. [/font][font="Verdana"] "Performance is our middle name."[/font]

vidya_pande SSCertifiable Points: 5275 More actions · Answer 6

this has been improved in 2008. Seek happens at partition level and not at total table level.[Partition-Aware Seek Operation]

Paul White SSC Guru Points: 150468 More actions · Answer 7

vidya_pande (3/27/2010)
this has been improved in 2008. Seek happens at partition level and not at total table level.[Partition-Aware Seek Operation]

Doesn't help at all. Read the Connect item linked to previously in this thread.

Paul White
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@SQL_Kiwi

Paul White SSC Guru Points: 150468 More actions · Answer 8

This is an annoying issue. I used to work around this by creating an extra non-partitioned index on the partitioned table.

The extra index had to be dropped before, and re-created after, partition SWITCHing, of course - but partitions typically aren't switched around very frequently, and usually during a maintenance window.

Slightly inconvenient, but worth it for the other benefits of partitioning.

Paul White
All articles available on SQL.kiwi
@SQL_Kiwi