Plan Instability

  • From: "Kenneth Naim" <kennethnaim@xxxxxxxxx>
  • To: <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 23 Mar 2010 02:41:58 -0400

I have been called in to help on a instance on AIX with cursor
sharing set to force due to rare use of bind variables. I have one query
which goes from running in 5 seconds or less to taking over an hour and
erroring when it runs out of 15gb of temp space. The issue is caused by one
step in the plan where a hash join is replaced with a nested loop. I need to
stabilize this plan but cannot use a stored outline because the query has
both literal variables and bind variables so the literals come through even
with cursor sharing set to force. I cannot change the code as it is complied
in a delivered binary (am working with the vendor but I need a solution
asap). I have stopped gathering stats (all involved tables/indexes have
accurate stats), removed histograms, set dynamic sampling back to the
default level of 2 so neither plan is affected by it. I wanted to "fix" the
stats that cause the plan to change but after running a 10053 trace on a 9
table join query I couldn't figure out which stat was the culprit. The only
other item I can think of that would cause the plan to change is bind
variable peeking, so my question is what else can cause a plan to change?




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