Oracle SQL Monitor not monitoring my SQL
22/01/2019 Leave a comment
I needed to monitor a SQL statement in 11.2.0.3 (the limits mentioned below are the same in 12.1, 12.2, 18.4 and 19C) to determine what is was doing and why it was slow.
Usually I would use SQL Monitor [NOTE: You need to license the Oracle Tuning Pack to use SQL Monitor] for this but the SQL was not appearing in there, despite running for over 5 seconds, and being a parallel SQL (both of which qualify to be included in SQL Monitor). So I asked Twitter why, and thought I’d share the output here.
https://twitter.com/ChandlerDBA/status/1075692070952677376
It was nailed immediately by Jonathan Lewis, with added help from Ivica Arsov. (thank you!)
There is a hidden parameter “_sqlmon_max_planlines” which states that any SQL with a plan in excess of 300 lines should not be monitored (see below for SQLMon hidden parameters – and change them at your own risk, preferably with the backing of an SR from Oracle Support). This execution plan had well over 300 lines. The solution is to change either the session or the system to allow monitoring to happen when the plan is over 300 lines.
e.g. alter system set "_sqlmon_max_planlines"=500 scope=memory sid='*'; or alter session set "_sqlmon_max_planlines"=500;
The negative side effect it that the monitoring will use more resources (primarily memory and CPU), which is why there are default limits on this feature. You might want to change it back when you’re finished to conserve resources.
Note that if you change the system parameter whilst the SQL is running, it will start to monitor the SQL at that point, so you will only get a partial picture of what is taking place, which is less valuable.
select ksppinm, ksppstvl, ksppdesc
from sys.x$ksppi a, sys.x$ksppsv b
where a.indx=b.indx
and lower(ksppinm) like lower('%sqlmon%')
order by ksppinm;
KSPPINM KSPPSTVL KSPPDESC
------------------------- --------- --------------------------------------------------------------------------------
_sqlmon_binds_xml_format default format of column binds_xml in [G]V$SQL_MONITOR
_sqlmon_max_plan 640 Maximum number of plans entry that can be monitored. Defaults to 20 per CPU
_sqlmon_max_planlines 300 Number of plan lines beyond which a plan cannot be monitored
_sqlmon_recycle_time 60 Minimum time (in s) to wait before a plan entry can be recycled
_sqlmon_threshold 5 CPU/IO time threshold before a statement is monitored. 0 is disabled
You may also notice a few other parameters in there. The “_sqlmon_recycle_time” hows the amount of time that the SQLMon plan will be guaranteed to be retained. Any retention time after that will be a bonus and depend upon the amount of SQL needing to be monitored. I see monitoring plans disappearing after 2-3 minutes in some systems, so you need to be quick, and you should save the plans down to disk.
The mad thing is that I was aware of this restriction before I posted by request for help on Twitter but I’d completely forgotten about it. So here’s the blog post to help me remember!
Neil Chandler's DB Blog 