Oracle SQL Monitor not monitoring my SQL

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.sql_monitor

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.

save_sqlmon

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!

Which Oracle Release are you using?

Post Date: August 2018!

Recently an awesome Oracle Guru friend of mine questioned someone who was installing 11.2.0.4 with the word “seriously”, which is think shows that Oracle staff sometimes don’t live in the same technological world as the rest of business.

My response was:

11.2.0.4 is normal. In the real world:

– large corps mostly use old versions
– consultants look at current versions
– Oracle staff look at unreleased versions

I have known instances of Oracle staff blogging about how a feature works when, in the officially released versions, it didn’t. It only worked that way in a version which was released some months later. There was no reference to the release and the fact that there was a significant functional change between releases (but I suppose that’s a blog and not “official” documentation – the official documentation said nothing at all about how that particular feature worked. Nothing! So thank you mystery blogger.)

Anyway, the point of this post was I then did a small twitter poll to my most excellent and cosy band of followers to see what Oracle releases people were using. I asked 2 questions (because twitter is limited) and here’s the results:

oracle_version_highest

So more people have some form of 12 in the DB, but only 7% have 18 in Production. This at a time when most Oracle staff are thinking about Oracle 20 and 21, as Oracle 19 is done and just awaiting release. Think about that, Oracle… Whenever I am at a presentation by an Oracle PM, I think “wow – I might be able to use those new features in 2-5 years”.

oracle_version_lowest

So very few people have 12.x as their lowest version (which would include 18 as that’s really 12.2.0.2) and MORE have 9, 8 or 7 as their major headache! Yes – there are more on 9, 8 and 7 than are using 18 in Production. Lets say that once more. There are more on 9, 8 and 7 than are using 18 in Production

So why upgrade? Very few databases take advantage of all of the latest sexy features. I suspect that many of the applications still being produced could run on Oracle 7.3.4. – more so as the proliferation of ORM’s like Hibernate has left a generation of developers with little appreciation of the database and how to take advantage of it**. So why upgrade? These days? Security. Patches. Support. Without those 3 things, you are living on hope, hope that nothing goes wrong as you’ll struggle to find anyone to fix it – including Oracle. Hoping that nobody tries to hack your 8.1.7 database as it’s a Swiss Cheese of vulnerabilities, like all 7, 8, 8i, 9i, 10G DB’s. Not that we hear about systems being compromised every day on the news.

Anecdote #1: By coincidence I was talking to a client at about the same time and whilst they are a mostly 12.1 shop, they still had an old 8i database hanging around… as usual it was going to  be “retired soon” (which in my experience means sometime in the next 15-20 years) and wasn’t worth the time and effort to be upgraded or even do a business case to upgrade it!

**Anecdote #2: At a client a few years ago, an excellent Java Developer asked me to put an index on a flag column. I pointed out that with only 3 values that an index wouldn’t help, and as this was OLTP a bitmap index wasn’t appropriate due to concurrency issues. He said that with 3 values indexed, his query would be 3 times faster! We sat down and I explained some database fundamentals to him, at which point he said “don’t put an index on there – that would be a stupid idea”. A few weeks later he came back over and asked about SQL queries “I’m trying to aggregate this data – can the database help?”. I spent 30 minutes showing him in-line views and windowing analytic functions and we wrote the code he needed for his output. “Wow! You have just saved me 3 days of Java coding…” – he was going to pull everything into Java and process it there, so as well as 3 days of coding, we also saved the SAN, the network and a whole bunch of CPU by dealing with data at the database layer – which is always the most efficient place to deal with it!

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