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11.2 New views

It looks like Laimutis Nedzinskas has spent some time producing  a list of new views, and changes to existing views, for trouble-shooting specialists. The lists cover the dynamic performance views and the AWR history views, and the differences between 10.2 and 11.2. The lists include, where relevant, convenient hyperlinks to the view definitions in the online copy of  the 11g Server Reference manual.

I haven’t checked the lists in detail, but I can see myself  taking advantage of his efforts from time to time:

Nice Additions For Troubleshooting

This is just a short note that Oracle has added several nice details to and respectively that can be helpful for troubleshooting.

ASH, PGA Memory And TEMP Consumption

Since the V$ACTIVE_SESSION_HISTORY view (that requires Enterprise Edition plus Diagnostic License) contains the PGA_ALLOCATED and TEMP_SPACE_ALLOCATED columns.

In particular the latter closes an instrumentation gap that always bothered me in the past: So far it wasn't easy to answer the question which session used to allocate TEMP space in the past. Of course it is easy to answer while the TEMP allocation was still held by a session by looking at the corresponding V$ views like V$SORT_USAGE, but once the allocation was released answering questions like why was my TEMP space exhausted three hours ago was something that couldn't be told by looking at the information provided by Oracle.

Index Hash

You might think from the title that this little note is going to be about the index hash join – you would be half right, it’s also about how the optimizer seems to make a complete hash of dealing with index hash joins.

Let’s set up a simple data set and a couple of indexes so that we can take a closer look:


Here’s a set of Instance Activity stats I’ve never seen before, and I’d rather never see again. From an active standby running on AIX:

Easy and fast environment framework

Getting tired of typing and setting your ORACLE_HOME, ORACLE_SID and troubleshooting your . oraenv ?
going/connecting to different client sites with different platforms and server environments sometimes it is frustrating just how long it takes to get a proper SQL*Plus environment

So I always make use of this script

and for every server that I access I just have to do three bits of typing:

Visualizing Active Session History (ASH) Data With R

One of the easiest ways to understand something is to see a visualization. Looking at Active Session History (ASH) data is no exception and I’ll dive into how to do so with R and how I used R plots to visually present a problem and confirm a hypothesis. But first some background…


Frequently DBAs use the Automatic Workload Repository (AWR) as an entry point for troubleshooting performance problems and in this case the adventure started the same way. In the AWR report Top 5 Timed Foreground Events, the log file sync event was showing up as the #3 event. This needed deeper investigation as often times the cause for longer log file sync times is related to longer log file parallel write times.

Correlation oddity

This one’s so odd I nearly posted it as a “Quiz Night” – but decided that it would be friendlier simply to demonstrate it. Here’s a simple script to create a couple of identical tables. It’s using my standard environment but, apart from fiddling with optimizer settings, I doubt if there’s any reason why you need to worry too much about getting the environment exactly right.

How To Cancel A Query Running In Another Session

This is not really anything new - in fact Tanel Poder has already blogged about it a while ago. Tanel has specifically covered the handling of "urgent" TCP packets and how this could be used to signal a "cancel" to another process, however this only works on Unix environments and not with Windows SQL*Plus clients. In Tanel's article it is also mentioned that there is an officially documented way of doing this via the Resource Manager if you happen to have an Enterprise Edition license.

In my quick tests however the call to DBMS_RESOURCE_MANAGER.SWITCH_CONSUMER_GROUP_FOR_SESS using "CANCEL_SQL" as consumer group only errors out with ORA-29366 saying that the specified consumer group is invalid.

Star Transformation And Cardinality Estimates

If you want to make use of Oracle's cunning Star Transformation feature then you need to be aware of the fact that the star transformation logic - as the name implies - assumes that you are using a proper star schema.

Here is a nice example of what can happen if you attempt to use star transformation but your model obviously doesn't really correspond to what Oracle expects:

drop table d;

purge table d;

drop table t;

purge table t;

create table t
rownum as id
, mod(rownum, 100) + 1 as fk1
, 1000 + mod(rownum, 10) + 1 as fk2
, 2000 + mod(rownum, 100) + 1 as fk3
, rpad('x', 100) as filler
connect by
level <= 1000000

exec dbms_stats.gather_table_stats(null, 't')

create bitmap index t_fk1 on t (fk1);

What the heck is the SQL Execution ID – SQL_EXEC_ID?

Ok, I think it’s time to write another blog entry. I’ve been traveling and dealing with jetlag from 10-hour time difference, then traveling some more, spoken at conferences, drank beer, had fun, then traveled some more, trained customers, hacked some Exadatas and now I’m back home.

Anyway, do you know what is the SQL_EXEC_ID in V$SESSION and ASH views?

Oh yeah, it’s the “SQL Execution ID” just like the documentation says … all clear. Um … is it? I’d like to know more about it – what does it actually stand for?! Is it session level, instance level or a RAC-global counter? And why does it start from 16 million, not 1?