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rebuild index worsens query performance Expand / Collapse
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Posted Saturday, November 17, 2012 11:08 AM


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jgenovese (11/17/2012)
I believe it is multi-line

Note that the query did not change -- the emphasis is on what could have happened that weekend that caused the query to double in time


You are being tunnel-visioned. It may not be just what happened that weekend. Maybe you reached a tipping point with the data that caused the query to slow down, not just the index rebuild and stats updates. As Jeff indicated, you need to look at this as if the query did change and needs to be tuned.



Lynn Pettis

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Post #1385962
Posted Saturday, November 17, 2012 11:27 AM
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I have attached the results of sp_help's on the tables, and the sql statement

  Post Attachments 
sp_helps.txt (7 views, 268.19 KB)
trxnrefupdtest.txt (7 views, 5.57 KB)
Post #1385965
Posted Saturday, November 17, 2012 11:59 AM


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Just an FYI, your parse routing is slow. Take a look at this one, then read the article and its discussion that is referenced in the comments.

Be sure to tread the comments.


/****** Object: UserDefinedFunction [dbo].[DelimitedSplit8K] Script Date: 11/17/2012 11:57:10 ******/
IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[DelimitedSplit8K]') AND type in (N'FN', N'IF', N'TF', N'FS', N'FT'))
DROP FUNCTION [dbo].[DelimitedSplit8K]
GO

/****** Object: UserDefinedFunction [dbo].[DelimitedSplit8K] Script Date: 11/17/2012 11:57:10 ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE FUNCTION [dbo].[DelimitedSplit8K]
/**********************************************************************************************************************
Purpose:
Split a given string at a given delimiter and return a list of the split elements (items).

Notes:
1. Leading a trailing delimiters are treated as if an empty string element were present.
2. Consecutive delimiters are treated as if an empty string element were present between them.
3. Except when spaces are used as a delimiter, all spaces present in each element are preserved.

Returns:
iTVF containing the following:
ItemNumber = Element position of Item as a BIGINT (not converted to INT to eliminate a CAST)
Item = Element value as a VARCHAR(8000)

Statistics on this function may be found at the following URL:
http://www.sqlservercentral.com/Forums/Topic1101315-203-4.aspx

CROSS APPLY Usage Examples and Tests:
--=====================================================================================================================
-- TEST 1:
-- This tests for various possible conditions in a string using a comma as the delimiter. The expected results are
-- laid out in the comments
--=====================================================================================================================
--===== Conditionally drop the test tables to make reruns easier for testing.
-- (this is NOT a part of the solution)
IF OBJECT_ID('tempdb..#JBMTest') IS NOT NULL DROP TABLE #JBMTest
;
--===== Create and populate a test table on the fly (this is NOT a part of the solution).
-- In the following comments, "b" is a blank and "E" is an element in the left to right order.
-- Double Quotes are used to encapsulate the output of "Item" so that you can see that all blanks
-- are preserved no matter where they may appear.
SELECT *
INTO #JBMTest
FROM ( --# & type of Return Row(s)
SELECT 0, NULL UNION ALL --1 NULL
SELECT 1, SPACE(0) UNION ALL --1 b (Empty String)
SELECT 2, SPACE(1) UNION ALL --1 b (1 space)
SELECT 3, SPACE(5) UNION ALL --1 b (5 spaces)
SELECT 4, ',' UNION ALL --2 b b (both are empty strings)
SELECT 5, '55555' UNION ALL --1 E
SELECT 6, ',55555' UNION ALL --2 b E
SELECT 7, ',55555,' UNION ALL --3 b E b
SELECT 8, '55555,' UNION ALL --2 b B
SELECT 9, '55555,1' UNION ALL --2 E E
SELECT 10, '1,55555' UNION ALL --2 E E
SELECT 11, '55555,4444,333,22,1' UNION ALL --5 E E E E E
SELECT 12, '55555,4444,,333,22,1' UNION ALL --6 E E b E E E
SELECT 13, ',55555,4444,,333,22,1,' UNION ALL --8 b E E b E E E b
SELECT 14, ',55555,4444,,,333,22,1,' UNION ALL --9 b E E b b E E E b
SELECT 15, ' 4444,55555 ' UNION ALL --2 E (w/Leading Space) E (w/Trailing Space)
SELECT 16, 'This,is,a,test.' --E E E E
) d (SomeID, SomeValue)
;
--===== Split the CSV column for the whole table using CROSS APPLY (this is the solution)
SELECT test.SomeID, test.SomeValue, split.ItemNumber, Item = QUOTENAME(split.Item,'"')
FROM #JBMTest test
CROSS APPLY dbo.DelimitedSplit8K(test.SomeValue,',') split
;
--=====================================================================================================================
-- TEST 2:
-- This tests for various "alpha" splits and COLLATION using all ASCII characters from 0 to 255 as a delimiter against
-- a given string. Note that not all of the delimiters will be visible and some will show up as tiny squares because
-- they are "control" characters. More specifically, this test will show you what happens to various non-accented
-- letters for your given collation depending on the delimiter you chose.
--=====================================================================================================================
WITH
cteBuildAllCharacters (String,Delimiter) AS
(
SELECT TOP 256
'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789',
CHAR(ROW_NUMBER() OVER (ORDER BY (SELECT NULL))-1)
FROM master.sys.all_columns
)
SELECT ASCII_Value = ASCII(c.Delimiter), c.Delimiter, split.ItemNumber, Item = QUOTENAME(split.Item,'"')
FROM cteBuildAllCharacters c
CROSS APPLY dbo.DelimitedSplit8K(c.String,c.Delimiter) split
ORDER BY ASCII_Value, split.ItemNumber
;
-----------------------------------------------------------------------------------------------------------------------
Other Notes:
1. Optimized for VARCHAR(8000) or less. No testing or error reporting for truncation at 8000 characters is done.
2. Optimized for single character delimiter. Multi-character delimiters should be resolvedexternally from this
function.
3. Optimized for use with CROSS APPLY.
4. Does not "trim" elements just in case leading or trailing blanks are intended.
5. If you don't know how a Tally table can be used to replace loops, please see the following...
http://www.sqlservercentral.com/articles/T-SQL/62867/
6. Changing this function to use NVARCHAR(MAX) will cause it to run twice as slow. It's just the nature of
VARCHAR(MAX) whether it fits in-row or not.
7. Multi-machine testing for the method of using UNPIVOT instead of 10 SELECT/UNION ALLs shows that the UNPIVOT method
is quite machine dependent and can slow things down quite a bit.
-----------------------------------------------------------------------------------------------------------------------
Credits:
This code is the product of many people's efforts including but not limited to the following:
cteTally concept originally by Iztek Ben Gan and "decimalized" by Lynn Pettis (and others) for a bit of extra speed
and finally redacted by Jeff Moden for a different slant on readability and compactness. Hat's off to Paul White for
his simple explanations of CROSS APPLY and for his detailed testing efforts. Last but not least, thanks to
Ron "BitBucket" McCullough and Wayne Sheffield for their extreme performance testing across multiple machines and
versions of SQL Server. The latest improvement brought an additional 15-20% improvement over Rev 05. Special thanks
to "Nadrek" and "peter-757102" (aka Peter de Heer) for bringing such improvements to light. Nadrek's original
improvement brought about a 10% performance gain and Peter followed that up with the content of Rev 07.

I also thank whoever wrote the first article I ever saw on "numbers tables" which is located at the following URL
and to Adam Machanic for leading me to it many years ago.
http://sqlserver2000.databases.aspfaq.com/why-should-i-consider-using-an-auxiliary-numbers-table.html
-----------------------------------------------------------------------------------------------------------------------
Revision History:
Rev 00 - 20 Jan 2010 - Concept for inline cteTally: Lynn Pettis and others.
Redaction/Implementation: Jeff Moden
- Base 10 redaction and reduction for CTE. (Total rewrite)

Rev 01 - 13 Mar 2010 - Jeff Moden
- Removed one additional concatenation and one subtraction from the SUBSTRING in the SELECT List for that tiny
bit of extra speed.

Rev 02 - 14 Apr 2010 - Jeff Moden
- No code changes. Added CROSS APPLY usage example to the header, some additional credits, and extra
documentation.

Rev 03 - 18 Apr 2010 - Jeff Moden
- No code changes. Added notes 7, 8, and 9 about certain "optimizations" that don't actually work for this
type of function.

Rev 04 - 29 Jun 2010 - Jeff Moden
- Added WITH SCHEMABINDING thanks to a note by Paul White. This prevents an unnecessary "Table Spool" when the
function is used in an UPDATE statement even though the function makes no external references.

Rev 05 - 02 Apr 2011 - Jeff Moden
- Rewritten for extreme performance improvement especially for larger strings approaching the 8K boundary and
for strings that have wider elements. The redaction of this code involved removing ALL concatenation of
delimiters, optimization of the maximum "N" value by using TOP instead of including it in the WHERE clause,
and the reduction of all previous calculations (thanks to the switch to a "zero based" cteTally) to just one
instance of one add and one instance of a subtract. The length calculation for the final element (not
followed by a delimiter) in the string to be split has been greatly simplified by using the ISNULL/NULLIF
combination to determine when the CHARINDEX returned a 0 which indicates there are no more delimiters to be
had or to start with. Depending on the width of the elements, this code is between 4 and 8 times faster on a
single CPU box than the original code especially near the 8K boundary.
- Modified comments to include more sanity checks on the usage example, etc.
- Removed "other" notes 8 and 9 as they were no longer applicable.

Rev 06 - 12 Apr 2011 - Jeff Moden
- Based on a suggestion by Ron "Bitbucket" McCullough, additional test rows were added to the sample code and
the code was changed to encapsulate the output in pipes so that spaces and empty strings could be perceived
in the output. The first "Notes" section was added. Finally, an extra test was added to the comments above.

Rev 07 - 06 May 2011 - Peter de Heer, a further 15-20% performance enhancement has been discovered and incorporated
into this code which also eliminated the need for a "zero" position in the cteTally table.
**********************************************************************************************************************/
--===== Define I/O parameters
(@pString VARCHAR(8000), @pDelimiter CHAR(1))
RETURNS TABLE WITH SCHEMABINDING AS
RETURN
--===== "Inline" CTE Driven "Tally Table" produces values from 0 up to 10,000...
-- enough to cover NVARCHAR(4000)
WITH E1(N) AS (
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
), --10E+1 or 10 rows
E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front
-- for both a performance gain and prevention of accidental "overruns"
SELECT TOP (ISNULL(DATALENGTH(@pString),0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
),
cteStart(N1) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter)
SELECT 1 UNION ALL
SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(@pString,t.N,1) = @pDelimiter
),
cteLen(N1,L1) AS(--==== Return start and length (for use in substring)
SELECT s.N1,
ISNULL(NULLIF(CHARINDEX(@pDelimiter,@pString,s.N1),0)-s.N1,8000)
FROM cteStart s
)
--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.
SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
Item = SUBSTRING(@pString, l.N1, l.L1)
FROM cteLen l
;

GO





Lynn Pettis

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Post #1385973
Posted Saturday, November 17, 2012 12:08 PM


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Also, no one is really going to dive into what you recently posted in an effort to help you figure out what be going on. We really need the information posted as DDL scripts, plus you should put together some sample data (i.e. NOT REAL data) for the tables that is representative of the problem. One other thing would really help, the actual execution plan.


Also, do you really drop the temporary table as soon as it is populated as it appears in the code you posted?



Lynn Pettis

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Post #1385977
Posted Saturday, November 17, 2012 12:16 PM
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"One other thing would really help, the actual execution plan."

This IS the ACTUAL execution plan

"Also, do you really drop the temporary table as soon as it is populated as it appears in the code you posted?"

NO -- I extracted the query from the stored proc in which it resides
Post #1385982
Posted Saturday, November 17, 2012 12:28 PM
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A new development:
As previously stated, the "trans" and "asset" tables are subscribed tables
I re-initialized the subscription this morning
Now the query appears to be back to normal run time
(no changes in table definition [indexes, etc.] compared to before the purge/etc)
My understanding is that a re-initialization does the following:
- Drops and re-creates the table on the subscriber side
- Bulk inserts into the subscribed table
- Re-creates indexes, constraints, etc.
So what is the difference between doing all of the above, versus just running a maintenance plan to rebuild all the table's indexes?
I am suspending the weekly index rebuild/update stats for now -- I don’t want the table to get fragmented again, but I also don’t want this performance degradation to resurface. Also, a re-initialization of this subscription would be too time-consuming and disruptive to do on a regular basis.
Thoughts?
Post #1385988
Posted Saturday, November 17, 2012 12:55 PM


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jgenovese (11/17/2012)
"One other thing would really help, the actual execution plan."

This IS the ACTUAL execution plan

"Also, do you really drop the temporary table as soon as it is populated as it appears in the code you posted?"

NO -- I extracted the query from the stored proc in which it resides



As for this being an actual execution plan, no it isn't. All that is shown in the plan are estimates. If this were the actual execution plan there would be actual counts shown in addition to the the estimates.



Lynn Pettis

For better assistance in answering your questions, click here
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For more about Tally Tables, click here
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Post #1385993
Posted Saturday, November 17, 2012 1:02 PM


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jgenovese (11/17/2012)
A new development:
As previously stated, the "trans" and "asset" tables are subscribed tables
I re-initialized the subscription this morning
Now the query appears to be back to normal run time
(no changes in table definition [indexes, etc.] compared to before the purge/etc)
My understanding is that a re-initialization does the following:
- Drops and re-creates the table on the subscriber side
- Bulk inserts into the subscribed table
- Re-creates indexes, constraints, etc.
So what is the difference between doing all of the above, versus just running a maintenance plan to rebuild all the table's indexes?
I am suspending the weekly index rebuild/update stats for now -- I don’t want the table to get fragmented again, but I also don’t want this performance degradation to resurface. Also, a re-initialization of this subscription would be too time-consuming and disruptive to do on a regular basis.
Thoughts?



Takes me back to what I originally stated. After rebuilding the indexes you rebuilt the the statistics on those indexes using a sample size intead of a full scan. This means that by rebuilding the statistics on those indexes, you made them less accurate than there were immediately following the index rebuild, which essentially rebuilds the statistics for the index using a full scan.




Lynn Pettis

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Post #1385994
Posted Saturday, November 17, 2012 1:16 PM
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the stats which were rebuilt were COLUMN statistics

I thought table index rebuild only updated TABLE statistics (fullscan)

Are you trying to tell me that this is not the case, i.e. table index rebuild updates both table AND column stats at 100%?
Post #1385997
Posted Saturday, November 17, 2012 1:19 PM


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jgenovese (11/17/2012)
the stats which were rebuilt were COLUMN statistics

I thought table index rebuild only updated TABLE statistics (fullscan)

Are you trying to tell me that this is not the case, i.e. table index rebuild updates both table AND column stats at 100%?


An index rebuild rebuilds the statistics on the columns for the index just as a statistics rebuild with a full scan.



Lynn Pettis

For better assistance in answering your questions, click here
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For more about Cross Tabs and Pivots, click here and here
Managing Transaction Logs

SQL Musings from the Desert Fountain Valley SQL (My Mirror Blog)
Post #1385998
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