DMF reading

vancas@dmt.com (Nick Vancas) said: 
 
>The first thing to try is to increase dmf.size_readahead to 16 and then measure
>again.  The result should be a reduction in the GREAD rate. 
> 
 
Exactly so, as this will also get it to read in bigger chunks at once. 
 
There is one more condition where the DMF will revert to group reads: when
sufficient single row probes have been on pages in sequence. So, for
example, reading every third row will eventually go through the group
buffers. 
 
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