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Werid Behaviour of TSQL Expand / Collapse
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Posted Tuesday, May 6, 2014 12:31 AM


SSChasing Mays

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I have complex view and try to fetch data from it.
In following code query 2 takes 1 sec to execute and query 1 keeps on running for hours.
I tried to reduce the time of query 1 to same as query 2 but no luck.I tried force order,hash join,creating clustered on temp table,type conversion etc.

This is one of the most complicated behavior I have seen in SQL server.any idea why it is happening so

SET statistics io ON
SET TRANSACTION isolation level READ uncommitted

DROP TABLE tempdb..#ps_temp

CREATE TABLE #ps_temp
(
a numeric(13,0),
b numeric(3,0)
)

DECLARE @a numeric(13,0),
@b numeric(3,0)

---fetches only 1 record
INSERT INTO #ps_temp
(a,
b)
SELECT a,
b
FROM tbl_x
WHERE d = 'L140502021514'


SELECT @a = a,
@b = b
FROM #ps_temp


-----------------query 1---------------------------
SELECT t.a,
t.b
FROM #ps_temp d
inner JOIN dbo.vw_complex AS t -- (hash)
ON t.a = d.a
AND t.b = d.b

-- option (force order)
-----------------query 2---------------------------
SELECT t.a,
t.b
FROM #ps_temp d
inner JOIN dbo.vw_complex AS t -- (hash)
ON t.a = @a
AND t.b = d.b


Pramod
SQL Server DBA | MCSA SQL Server 2012

in.linkedin.com/in/pramodsingla/
http://pramodsingla.wordpress.com/
Post #1567793
Posted Tuesday, May 6, 2014 1:50 AM
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First a quick question, why the dirty read (READ UNCOMMITTED)?

Without more information such as execution plans, view code, schema structure etc., it is hard to tell what the problem is.
Post #1567817
Posted Tuesday, May 6, 2014 2:31 AM


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psingla (5/6/2014)
I have complex view and try to fetch data from it.
In following code query 2 takes 1 sec to execute and query 1 keeps on running for hours.
I tried to reduce the time of query 1 to same as query 2 but no luck.I tried force order,hash join,creating clustered on temp table,type conversion etc....


Check the execution plan of query1 for timeout. You could post the actual execution plans for both queries if you are interested in more than guesses.


“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden
Exploring Recursive CTEs by Example Dwain Camps
Post #1567832
Posted Tuesday, May 6, 2014 5:41 PM


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psingla (5/6/2014)
I have complex view and try to fetch data from it.
In following code query 2 takes 1 sec to execute and query 1 keeps on running for hours.
I tried to reduce the time of query 1 to same as query 2 but no luck.I tried force order,hash join,creating clustered on temp table,type conversion etc.

This is one of the most complicated behavior I have seen in SQL server.any idea why it is happening so

SET statistics io ON
SET TRANSACTION isolation level READ uncommitted

DROP TABLE tempdb..#ps_temp

CREATE TABLE #ps_temp
(
a numeric(13,0),
b numeric(3,0)
)

DECLARE @a numeric(13,0),
@b numeric(3,0)

---fetches only 1 record
INSERT INTO #ps_temp
(a,
b)
SELECT a,
b
FROM tbl_x
WHERE d = 'L140502021514'


SELECT @a = a,
@b = b
FROM #ps_temp


-----------------query 1---------------------------
SELECT t.a,
t.b
FROM #ps_temp d
inner JOIN dbo.vw_complex AS t -- (hash)
ON t.a = d.a
AND t.b = d.b

-- option (force order)
-----------------query 2---------------------------
SELECT t.a,
t.b
FROM #ps_temp d
inner JOIN dbo.vw_complex AS t -- (hash)
ON t.a = @a
AND t.b = d.b


Without seeing the tables or the code for the view and the fact that the setup code refers to a "tbl_x" table that doesn't seem to be available, I'd have to say that the relationship of t.a = d.a produces a many-to-many join rivaliing a Cartesian Product. Look at your execution plan for counts on arrows that are much bigger than the number of rows expected from each table.


--Jeff Moden
"RBAR is pronounced "ree-bar" and is a "Modenism" for "Row-By-Agonizing-Row".

First step towards the paradigm shift of writing Set Based code:
Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column."

(play on words) "Just because you CAN do something in T-SQL, doesn't mean you SHOULDN'T." --22 Aug 2013

Helpful Links:
How to post code problems
How to post performance problems
Post #1568243
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