RE: High mbrc in aux_stats$

  • From: "Christian Antognini" <Christian.Antognini@xxxxxxxxxxxx>
  • To: <jonathan@xxxxxxxxxxxxxxxxxx>
  • Date: Tue, 14 Mar 2006 18:31:24 +0100


>Working hypothesis: Oracle has finally separated the two functions
>of the db_file_Mulitblock_read_count: one as the value to be used
>by the optimizer (in the absence of an observed MBRC) for the arithmtic,
>and one to be attempted at run time when executing the query). The MBRC
>you finally gather will be probably be based on the actual run-time
>activity, and will then over-ride the _db_file_optimizer_read_count
>next time around the optimization cycle.

So far this matches my observations.

I'll just add that db_file_multiblock_read_count and 
_db_file_optimizer_read_count are "recalculated" when either the buffer cache 
size or the maximum number of sessions changes. This is because, according to 
my observations, the parameters are set to the value computed with a formula 
like the following one: db_cache_size / (sessions * db_block_size).

For example in my test database, where auto SGA management is on, after almost 
every bounce a new value is available.


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