On Apr 29, 2011, at 10:25 AM, James Mansion <ja...@mansionfamily.plus.com> 
wrote:
> Robert Haas wrote:
>> The server can and does measure hit rates for the PG buffer pool, but to my 
>> knowledge there is no clear-cut way for PG to know whether read() is 
>> satisfied from the OS cache or a drive cache or the platter.
>> 
>>  
> Does the server know which IO it thinks is sequential, and which it thinks is 
> random? 

No. It models this in the optimizer, but the executor has no clue.  And 
sometimes we model I/O as partly random, partly sequential, as in the case of 
heap fetches on a clustered index.  So the answer isn't even a Boolean.

...Robert
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