You’ve probably heard about adaptive cursor sharing, and possibly you’ve wondered why you haven’t seen it happening very often on your production systems (assuming you’re running a modern version of Oracle). Here’s a little bug/fix that may explain the non-appearance.
MOS Doc ID 9532657.8 Adaptive cursor sharing ignores SELECTs which are not fully fetched.
This bug is confirmed in 126.96.36.199, and fixed in 188.8.131.52. The problem is that the ACS code doesn’t process the statistical information from the cursor unless the cursor reaches “end of fetch” – i.e. if you don’t select all the data in your query, Oracle doesn’t consider the statistics of that execution when deciding whether or not to re-optimise a statement.
Listening to a presentation by Paul Matuszyk on extended statistics yesterday, I learned something that I should have spotted ages ago. Here’s a little demo script to introduce the point:
create table t1 as with generator as ( select --+ materialize rownum id from dual connect by level <= 1000) select rownum id, mod(rownum,100) n1, mod(rownum,100) n2, mod(rownum,100) n3, lpad(rownum,10,'0') small_vc, rpad('x',100) padding from generator v1, generator v2 where rownum <= 1000000; create index t1_i1 on t1(n1, n2, n3); -- collect stats, no histograms.
When I wrote a note last week about the fixes to the subquery factoring optimizer code in 184.108.40.206, I finished with a comment about having more to say on the test case if I materialized the subquery. Today’s the day to talk about it. As a reminder, here’s the query, but with the /*+ materialize */ hint in place:
with subq as ( select /*+ materialize */ outer.* from emp outer where sal > 1000000 and outer.sal > ( select avg(inner.sal) from emp inner where inner.dept_no = outer.dept_no ) ) select * from subq ;
Here are the three plans – from 10.2.0.5, 220.127.116.11, and 18.104.22.168 respectively:
Time for another of those little surprises that catch you out after the upgrade.
Take a look at this “Top N” from a standard AWR report, from an instance running 22.214.171.124
Top 5 Timed Foreground Events ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Avg wait % DB Event Waits Time(s) (ms) time Wait Class ------------------------------ ------------ ----------- ------ ------ ---------- direct path read 3,464,056 6,593 2 33.5 User I/O DB CPU 3,503 17.8 db flash cache single block ph 2,293,604 3,008 1 15.3 User I/O db file sequential read 200,779 2,294 11 11.6 User I/O enq: TC - contention 82 1,571 19158 8.0 Other
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 126.96.36.199″ 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.)