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Message Posted: Tue, 27 Jul 2004 @ 22:47:28 GMT

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Subj:   Re: AMP Worker Tasks and Skewed Queries
From:   Christie, Jon

The amp worker tasks responds to messages sent from the dispatcher. The dispatcher coordinates execution of the plan (which you can see if you get an explain). Each step that you see in the explain corresponds to a message sent from the dispatcher to one or more amps. An amp worker task receives the message, does whatever is appropriate for that message, then signals it's done. Sometimes that will be done by sending a message back to the dispatcher, but more often there's some inter-amp, end-of-step communication which, I'm afraid, is beyond my ken. (I'm an optimizer guy, and this amp magic is something I accept on faith.)

But the important thing in relation to your question is that the scope of the worker task is no greater than a single step.

The story on "Skewed Queries" is that when a few amps get most of the data (because of uneven distribution) you will have a few amps working real hard while most of the amps a loafing, and you're only a fast as your slowest amp.

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