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Wednesday, October 27, 2010

Adaptive Direct Path Reads

This is one of my favorite topics, as it keeps coming up.

Well as you read the title, you are probably going huh ?? Until you upgrade to 11g, you probably just think direct reads are for reading temp only.. Well it all changes with 11g. Full table scans of large tables turn from "db scattered read" to "direct path read"..


What does this mean ?? well the good news is it runs about 4 times faster than the old fasion reads.. How does it do this ?? It bypasses the SGA.. Is this good ? probably.. especially for full table scans.. it doesn't force anything out of your buffer cache, and you get faster reads. The only concern I would have is if you wanted to read a lot of data into memory, I don't know how to burn things into cache. I tried to disable it to do timings, but no luck. Even with the optimizer set back to 10g it still does direct path reads.

Here is some great information on it.

http://afatkulin.blogspot.com/2009/01/11g-adaptive-direct-path-reads-what-is.html

http://shallahamer-orapub.blogspot.com/2010/01/mystery-surrounding-11g-and-direct.html

I've had some bad experiences with Direct path reads, and concurrency. If you think about lots and lots concurrent sessions, doing direct path reads you start to imagine how this can reek havoc. None of these sessions share the results of the read, and they all independently read disk blocks.. Eventually you run out of runway for I/O.

Happy tuning.

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