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using multiple UNION in query Expand / Collapse
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Posted Tuesday, May 14, 2013 6:14 AM
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Hello!
I have one stored proc which returns data by querying multiple tables using union clause without any search criteria.

select xyz as Col1 from table1 join table2 ...(total 8 joins)
union
select abc as Col1 from table1 join table3...(total 8 joins)
union
select pqras Col1 from table1 join table4...(total 8 joins)
union
select pqras Col1 from table1 join table5...(total 8 joins)

As per database design Table1 represent supertype and Table3,4,5 represent sub types. Table1 has 1146090 rows. Not sure what to do with this. It showed up as leading in average reads at 14 million per execution and runs for 35 seconds. It is going to get a lot worse looking at the tables it's selecting from. It is basically doing a select across lots of tables, some large tables, without a select criteria. It's doing a bunch of in clustered index scans... appreciate your help here.


Abhijit - http://abhijitmore.wordpress.com
Post #1452529
Posted Tuesday, May 14, 2013 7:11 AM
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Abhijit More (5/14/2013)
Hello!
I have one stored proc which returns data by querying multiple tables using union clause without any search criteria.

select xyz as Col1 from table1 join table2 ...(total 8 joins)
union
select abc as Col1 from table1 join table3...(total 8 joins)
union
select pqras Col1 from table1 join table4...(total 8 joins)
union
select pqras Col1 from table1 join table5...(total 8 joins)

As per database design Table1 represent supertype and Table3,4,5 represent sub types. Table1 has 1146090 rows. Not sure what to do with this. It showed up as leading in average reads at 14 million per execution and runs for 35 seconds. It is going to get a lot worse looking at the tables it's selecting from. It is basically doing a select across lots of tables, some large tables, without a select criteria. It's doing a bunch of in clustered index scans... appreciate your help here.


How do you want us to help?
Can you drop this proc and never use?
If not, what are the real requirements?
It's impossible to advise on something like that - too little information.



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Post #1452554
Posted Tuesday, May 14, 2013 7:58 AM
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How do you want us to help? - need help to optimize the query.
Can you drop this proc and never use? - not possible
If not, what are the real requirements? - requirement is to get all record that satisfy the join criteria
It's impossible to advise on something like that - too little information. - can u tell me what information do you need


Abhijit - http://abhijitmore.wordpress.com
Post #1452596
Posted Tuesday, May 14, 2013 8:16 AM


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Abhijit More (5/14/2013)
How do you want us to help? - need help to optimize the query.
Can you drop this proc and never use? - not possible
If not, what are the real requirements? - requirement is to get all record that satisfy the join criteria
It's impossible to advise on something like that - too little information. - can u tell me what information do you need


Seriously with 900 points you have surely at least one or two forum posts. We can't see your screen, we can't read your mind, we don't know your data structures, we don't know the business rules...in short we don't know anything about your system or your issue.

http://weblogs.sqlteam.com/jeffs/archive/2008/05/13/question-needed-not-answer.aspx


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Post #1452607
Posted Tuesday, May 14, 2013 8:18 AM
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Abhijit More (5/14/2013)
How do you want us to help? - need help to optimize the query.
Can you drop this proc and never use? - not possible
If not, what are the real requirements? - requirement is to get all record that satisfy the join criteria
It's impossible to advise on something like that - too little information. - can u tell me what information do you need


Details!

I guess you should start from here: http://www.sqlservercentral.com/articles/Best+Practices/61537/



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"The only true wisdom is in knowing you know nothing"
"O skol'ko nam otkrytiy chudnyh prevnosit microsofta duh!"
(So many miracle inventions provided by MS to us...)

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Post #1452610
Posted Thursday, May 16, 2013 3:56 AM
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sorry friends it my bad....i should have frame the question correctly...but for now I think i found the solution for my problem...sorry for the inconvenience

Abhijit - http://abhijitmore.wordpress.com
Post #1453395
Posted Thursday, May 16, 2013 7:09 AM
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Just for completeness sake, you never use more than one UNION, which is the last one. All the rest should be UNION ALL.
Post #1453503
Posted Thursday, May 16, 2013 7:41 AM


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lnardozi 61862 (5/16/2013)
Just for completeness sake, you never use more than one UNION, which is the last one. All the rest should be UNION ALL.


Why not?



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Post #1453537
Posted Thursday, May 16, 2013 7:58 AM


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Phil Parkin (5/16/2013)
lnardozi 61862 (5/16/2013)
Just for completeness sake, you never use more than one UNION, which is the last one. All the rest should be UNION ALL.


Why not?

I suppose because UNION eliminates duplicates you only need to do it once - as long as you make sure that UNION is the last one evaluated. I don't know whether the query optimizer is smart enough to do that implicitly if you use more than one UNION - so it may or may not make a difference from a performance point of view.

John
Post #1453555
Posted Thursday, May 16, 2013 8:14 AM


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John Mitchell-245523 (5/16/2013)
Phil Parkin (5/16/2013)
lnardozi 61862 (5/16/2013)
Just for completeness sake, you never use more than one UNION, which is the last one. All the rest should be UNION ALL.


Why not?

I suppose because UNION eliminates duplicates you only need to do it once - as long as you make sure that UNION is the last one evaluated. I don't know whether the query optimizer is smart enough to do that implicitly if you use more than one UNION - so it may or may not make a difference from a performance point of view.

John


But that is only good if you want to eliminate ALL duplicates. It is very possible that you have 3 tables and in the first 2 tables you want duplicates but if the same value shows up in the third table you want only distinct values. Other times maybe you want to include all duplicates.

The blanket statement that "you never use more than one UNION, which is the last one. All the rest should be UNION ALL." is just completely not true. There are time when that is correct but not always.


_______________________________________________________________

Need help? Help us help you.

Read the article at http://www.sqlservercentral.com/articles/Best+Practices/61537/ for best practices on asking questions.

Need to split a string? Try Jeff Moden's splitter.

Cross Tabs and Pivots, Part 1 – Converting Rows to Columns
Cross Tabs and Pivots, Part 2 - Dynamic Cross Tabs
Understanding and Using APPLY (Part 1)
Understanding and Using APPLY (Part 2)
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