Did someone already mention the optimizer_index_cost_adj setting? Sets the relative cost of index use versus table scans, Set it at something like 200 would probably force full table scans. I have never tried that though. On Tue, Sep 14, 2010 at 4:05 AM, Niall Litchfield < niall.litchfield@xxxxxxxxx> wrote: > Well, system stats generally boil down to what happens to sreadtim and > mreadtim and the ratio between them. I'm not clear what would happen on a > Database Machine, but I can imagine it *might* break the assumptions of the > system stat costing model, not least that Multi Block reads are slower than > single block reads. > > I personally on a non-exadata enabled machine would deal with the stability > issue that Greg mentions by grabbing system stats repeatedly over a period > (a week or a month most likely) to a user defined stattab and then sticking > them in a spreadsheet to determine what values are representative of the > system. You can then just set the stats once and for all. After all I/O and > CPU capability *rarely *changes in a normal server (VMs of course blow > this assumption out of the water). I may be overly optimistic but I > personally believe (and it is mostly a matter of logic/faith sadly I don't > have enough actual data to say one way or the other) that representative > system statistics are likely to be a sensible choice for *most* systems - > certainly ones where the OICA and OIC tricks are being used. > > On 13 Sep 2010 21:43, "Job Miller" <jobmiller@xxxxxxxxx> wrote: > > Niall, > > I don't know how system stats are calculated, but if they are measured from > the perspective of the database server, wouldn't they reflect the benefits > of exadata storage? > > > The server knows it needs to do a FTS, the high water mark says that > requires reading N blocks, exadata storage skips 50% of the blocks via the > storage indexes, and the multi-block read times from the servers perspective > look really fast because storage optimized away some of the i/o. > > If that's true, the system stats would measure the typical system benefit > from exadata storage experienced during the system stats workload capture. > > Does that make sense? What am I missing? > > > > How does one maintain how much of a table is on disk vs memory? A > > costing model that considers all those cases becomes increasingly more > > > difficult to program and manage. > The costing model doesn't really attempt to consider all that though does > it? I just thought that it measures the average system-wide performance > based on your workload during capture. Hopefully what happens during that > workload represents your cache ratios for data, your data placement ratios > and your true typical multi-block average read rates. > > That average system-wide multi-block read should lead to pretty good plans > for everything. instead of good only for a few special cases. > > --- On Mon, 9/13/10, Greg Rahn <greg@xxxxxxxxxxxxxxxxxx> wrote: > > > From: Greg Rahn <greg@xxxxxxxxxxxxxxxxxx> > > Subject: Re: Optimizer issue - cost of full table scans > > To: "Niall Litchfield" <niall.litchfield@xxxxxxxxx> > > Cc: "ORACLE-L" <oracle-l@xxxxxxxxxxxxx> > > Date: Monday, September 13, 2010, 2:16 PM > > > There are no special changes as of > > today. When I made the reference > > to representative stats, ... > > > -- > > //www.freelists.org/webpage/oracle-l > > > > > > > > > -- Andrew W. Kerber 'If at first you dont succeed, dont take up skydiving.'