Nigel Noble's Oracle Blog

28/03/2013

11.2.0.3 v$sqlstats.last_active_time stops changing and breaks AWR

Filed under: 11gR2, Monitoring — Nigel Noble @ 3:18 pm

My site uses a 3rd party SQL monitoring tool which collects data based on the Oracle view v$sqlstats.  The tool collects data for all sql statements which have been executed since the previous collection using the last_active_time column. A few months ago we noticed (after an upgrade to 11g) we would occasionally be missing some sql statements from the monitor graphs. We investigated the problem, re-produced a test case, raised a bug with Oracle and Support has just released an 11.2 patch.

Before I explain the issue and demonstrate the issue, I will explain what prompted me to post this blog item. Until now I always thought this just effected v$sqlstats (and my 3rd party monitoring tool) and not AWR. If you ever trace (10046) an AWR collection you will not find any references to v$sqlstats. If the data is missing from the 3rd Party tools “I can always rely on AWR?”…. I thought!

Well, I recently discovered that any time a sql statement drops off the 3rd party monitoring tool…… IT STOPS BEING MONITORED IN AWR’s DBA_HIST_SQLSTAT table as well. We have our AWR collection threshold set to collect as many sql statements as possible. This problem has nothing to do with the cost of the SQL statements and you could well find your most expensive sql statement just disappear from AWR for a period of time.

Let’s start at the very beginning….

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