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_very_large_object_threshold tips

Oracle Database Tips by Donald BurlesonJune 7, 2015

 

Question:  I have noted the hidden parameter _very_large_object_threshold, and I was hoping that you could tell us how _very_large_object_threshold works in a production database.

 

Answer:  According to various sources in the web, _very_large_object_threshold is expressed in megabytes and defines the upper level thresholds for using direct path reads.

As we know, a direct path read will bypass the data buffer and Saibabu Devabhaktuni has these notes on the effects of _very_large_object_threshold:

"Event 10949 set to any level will disable direct path reads for serial scans only if the size of an object is less then 5 times the size of buffer cache.

The combination of setting "event 10949" and "_very_large_object_threshold" parameter will disable direct path reads for serial scans if the size of an object is less than 5 times the size of buffer cache or the value of "_very_large_object_threshold" is less than about 0.8 times the size of an object.

So, if you want to disable direct path reads for serial scans for any object, then set event 10949 at any level and set "_very_large_object_threshold" to greater than the size of largest object in MB."

 It is important to remember not to to change any hidden parameters unless directed by Oracle technical support.

   
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