RE: Flushing Bad Plan - No Longer in Shared Pool

Thanks for the quick response.

Our goal was to avoid baselines as a permanent fix, since we could theoretically be creating these all the time. Our ideal solution would be fixing the root cause, so baselines as a work-around while we investigate, but in the issue we were facing today, we could see that it still would not use the baseline, even though it was fixed.

We actually have since found that the plan we were trying to use in the baseline is not reproducible, so we need to go back to app team to see if they’ve made indexing changes that would be preventing this plan from even being considered.

Thanks,
Scott

From: Svetoslav Gyurov [mailto:softice_at_gmail.com] Sent: Wednesday, November 18, 2015 3:20 PM To: Deas, Scott
Cc: oracle-l_at_freelists.org
Subject: Re: Flushing Bad Plan - No Longer in Shared Pool

Hi Scott,
Which version of the database you are running ? Why stay away from the baselines where this is the best solution to make sure a particular SQL will run with a particular plan, this will make sure it won't piuck up the bad plan anymore. You can also create a baseline with the "good" from you AWR repository. Indeed you can trace later the CBO to see why it's always picking up the bad plan. Regards,
Sve

On Wed, Nov 18, 2015 at 7:58 PM, Deas, Scott <Scott.Deas_at_lfg.com<mailto:Scott.Deas_at_lfg.com>> wrote: All,

We have an environment that has been experiencing some wandering plans. While we’d like to look into the details to see why the optimizer is choosing bad plans, we have immediate needs to get statements running with good plans that have been used in the past. When we’re contacted in time, we have been stopping the query, flushing the plan from the shared pool (using DBMS_SHARED_POOL.PURGE), gathering stats (where applicable) and re-running. The problem is that sometimes we’re not contacted until the statement has been running for so long that it’s now showing up in dba_hist_sql_plan, meaning the DBMS_SHARED_POOL.PURGE procedure will no longer flush it as an available plan for the optimizer.

We have tried setting baselines for these statements (although we’d prefer not to use them long term), but the optimizer continues to see these bad plans as cheaper options that would be a better choice.

Is there a way to completely eliminate or invalidate a past plan from being considered by the optimizer again?

Thanks,
Scott

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