Putting multiple sessions and optimizer choices aside I would like to know exactly why (assume 128 is max) 128 faster than 1 but not faster than value < 128. Perhaps understanding the reasons for this are not exactly clear with the complexities of the IO subsystem. I agree the best method is likely to run some tests to see what is the faster method to access a row given a full scan then balance that with the # of users and influence things will have on the optimizer. I just don't want to be one of these people who think the sky is falling every time there is a full scan. I can't tell you how many people come to me demanding I somehow magically remove all full scans from some sort of query plan because "all full scans are bad". -----Original Message----- From: Christian Antognini [mailto:Christian.Antognini@xxxxxxxxxxxx]=20 Sent: Wednesday, December 08, 2004 1:35 AM To: Post, Ethan Cc: Oracle-L@xxxxxxxxxxxxx; ryan_gaffuri@xxxxxxxxxxx Subject: RE: db_file_multiblock_read_count and performance - Bayesian Filter detected spam Hi=20 (Sorry for the delay... but I'm offline by a customer...) >This kind of brings up an interesting thought. There is the script on >Ixora to test the largest MBR size and then you are suppose to set the >value to that, Setting it to the largest value is not good on all I/O sub-systems and sometimes very poor for the optimizer. Usually a value that gives good performance (let's say 90-95% of the maximum) lead to much better execution plans. Notice that if system statistics are used, then they automatically "compensate" large values with much better one (e.g. on a real system it happens not very often that you can read 50-60 contiguous blocks with a FTS...). >maybe it would be a better practice to generate a huge >table, run tests at different sizes then set. For my tests I took a table of at least 1GB (on small systems) or 10GB (on big systems), i.e. I just reused a table with real data. >In theory the largest >size possible would be fastest but it would be interesting to find out >if this always was true. This is not true! You should really do some tests!!!! Chris -- //www.freelists.org/webpage/oracle-l