In part 1 of this mini-series we looked at the effects of costing a tablescan serially and then parallel when the maxthr and slavethr statistics had not been set.
In part 2 we looked at the effect of setting just the maxthr - and this can happen if you don’t happen to do any parallel execution while the stats collection is going on.
In part 3 we’re going to look at the two variations the optimizer displays when both statistics have been set. So here are the starting system stats:
Actually, there hasn’t been a “maxthr – 1″, I called the first part of this series“System Stats”. If you look back at it you’ll see that I set up some system statistics, excluding the maxthr and slavethr values, and described how the optimizer would calculate the cost of a serial tablescan, then I followed this up with a brief description of how the calculations changed if I hinted the optimizer into a parallel tablescan.
Several years ago I wrote the following in “Cost Based Oracle – Fundamentals” (p.47):
The maxthr and slavethr figures relate to throughput for parallel execution slaves. I believe that the figures somehow control the maximum degree of parallelism that any given query may operate at by recording the maximum rate at which slaves have historically been able to operate—but I have not been able to verify this.
Browsing the internet recently, I discovered that that no-one else seems to have published anything to very my comment, so I decided it was about time I did so myself. I’m going to work up to it in two blog notes , so if you do happen to know of any document that describes the impact of maxthr and slavethr on the optimizer’s costing algorithms please give me a reference in the comments – that way I might not have to write the second note.
A quick collation – and warning – for 11.2
Bottom line – be careful about what you do with system stats on 11.2
Footnote: the MOS link is a search string producing a list of references. I set it up like that because one of the articles referencing the bug is called “Things to consider before upgrade to 18.104.22.168″ and it’s worth reading.
Addendum: one of the people on the two-day course I’ve just run in Berlin sent me a link for a quick note on how to set your own values for the system stats if you hit this bug. It’s actually quite a reasonable thing to do whether or not you hit the bug given the way that gathering the stats can produce unsuitable figures anyway: setting system stats. (I’ve also added their company blog to the links on the right, they have a number interesting items and post fairly regularly.)
I wrote an article about system statistics / CPU Costing for Oracle magazine a few years ago – and last week I realised that I’ve never supplied a link to it in the notes and comments I’ve made about system statistics. So I’ve just run a search through the Oracle website trying to find it – and discovered that it’s no longer available. Apparently the editors have decided that any technical articles over a certain age should be withdrawn in case they are out of date and misleading. (Clearly they’ve read my blog on trust – I wish the people maintaining Metalink would do the same as the magazine editors – but they probably have a much larger volume to worry about).
If you want an original English version – dated April 2004, which is when I sent it in to Oracle Magazine, and before it underwent some editing – I’ve posted it as a pdf file.