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Parallelism in queries [message #527838]

Thu, 20 October 2011 06:51

manojbe09
Messages: 16
Registered: October 2011
Location: MUMBAI

Junior Member

does parallel hint in query can create bottleneck/Slowdown/crash
database..??

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Re: Parallelism in queries [message #527842 is a reply to message #527838]

Thu, 20 October 2011 06:53

Michel Cadot
Messages: 68716
Registered: March 2007
Location: Saint-Maur, France, https...

Senior Member
Account Moderator

Crash no (but due to a bug), bottleneck and slowdown, yes, as always when someone asks for more resources the other ones have less.

Regards
Michel

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Re: Parallelism in queries [message #527848 is a reply to message #527842]

Thu, 20 October 2011 06:58

manojbe09
Messages: 16
Registered: October 2011
Location: MUMBAI

Junior Member

Thanks a ton..

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Re: Parallelism in queries [message #528651 is a reply to message #527848]

Wed, 26 October 2011 05:09

Kevin Meade
Messages: 2103
Registered: December 1999
Location: Connecticut USA

Senior Member

sounds like someone was looking for a reason to tell people not use parallel query because it is a bad thing. Lets ask some questions of this poster, it will be fun:

manojbe09

1) do you understand what parallel query is and how it works? If so then you should understand that if you have for example two processes scanning one table at the same time because you are doing a parallel query with degree of 2 to scan the table, you will be using twice the I/O and twice the CPU and twice the memory (more or less) to scan the table in the hopes that you will scan it in half the time (more or less). Imagine what would happen if you turned every query on your box on for PX (parallel execution). You would basically be asking for twice the resources to get consumed in the hopes that everything would get done in half the time. IS THIS GOOD OR BAD?

2) well that depends... is your box under-utilized? Do you have twice the resources available on the box to use? Is your CPU at say 40% utilization or less? Is your memory at 40% utilization or less? Is your I/O system at 40% utilizatio or less? If so then maybe it is good and everything will go twice as fast. If not then it is bad for somewhere you will hit a bottleneck and everything will slow down instead of speading up.

Get the point? Kevin

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Re: Parallelism in queries [message #529300 is a reply to message #528651]

Sun, 30 October 2011 23:55

manojbe09
Messages: 16
Registered: October 2011
Location: MUMBAI

Junior Member

Thanks kevin,
Got your point.. Smile

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