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My favourite format options for dbms_xplan.display_cursor().

This is another of those posts where I tell you about something that I’ve frequently mentioned but never documented explicitly as a good (or, at least, convenient) idea. It also another example of how easy it is to tell half the story most of the time when someone asks a “simple” question.

Tuning Database XQuery Statements (2)

So we concluded the post “Tuning Database XQuery Statements (1)” with the following SQL statement…

Tuning Database XQuery Statements (1)

I had a question of a colleague to have a look at a statement that…

Lunchtime quiz

There was a question on OTN a few days ago asking the following question:

Here’s a query that ran okay on 11g, but crashed with Oracle error “ORA-01843: not a valid month” after upgrade to 12c; why ?

The generically correct answer, of course, is that the OP had been lucky (or unlucky, depending on your point of view) on 11g – and I’ll explain that answer in another blog posting.

That isn’t the point of this posting, though. This posting is a test of observation and deduction. One of the respondants in the thread had conveniently supplied a little bit of SQL that I copied and fiddled about with to demonstrate a point regarding CPU costing, but as I did so I thought I’d show you the following and ask a simple question.’

Group By Bug

This just in from OTN Database Forum – a surprising little bug with “group by elimination” exclusive to 12c.

alter session set nls_date_format='dd-Mon-yyyy hh24:mi:ss';

       /* optimizer_features_enable('')*/
       trunc (ts,'DD') ts1, sum(fieldb) fieldb
from (
        ts, max(fieldb) fieldb
  from (
  select trunc(sysdate) - 1/24 ts, 1 fieldb from dual
  union all
  select trunc(sysdate) - 2/24 ts, 2 fieldb from dual
  union all
  select trunc(sysdate) - 3/24 ts, 3 fieldb from dual
  union all
  select trunc(sysdate) - 4/24 ts, 4 fieldb from dual
  union all
  select trunc(sysdate) - 5/24 ts, 5 fieldb from dual
  group by ts
group by trunc (ts,'DD')

You might expect to get one row as the answer – but this is the result I got, with the execution plan pulled from memory:

Hash Clusters – 3

This note is a quick summary of an oddity that came to light after a twitter conversation with Christian Antognini yesterday. First a little test script to get things going:


That’s SQL Tuning Sets, optimizer_features_enable, and SQL Plan Management.

There’s a recent post on OTN describing an issue when using SQL Tuning Sets to enforce plan stability when upgrading from to – it doesn’t always work. Here’s a very simple model to demonstrate the type of thing that can happen (the tables are cloned from a completely different demo, so don’t ask why I picked the data they hold):

dbms_xplan bug

Here’s a very long post (which is mainly an example) demonstrating a little bug in the “explain plan” functionality. It’s a variation of a bug which I thought had been fixed in 11g, but it still appears in some cases. Take a look at this execution plan, which comes from explaining “select * from dba_tab_cols” – the bit I want to emphasise is in lines 1 to 10:



DBMS_XPLAN.DISPLAY_CURSOR can be used to get more insights into the actual resource consumption on execution plan operation level when using the GATHER_PLAN_STATISTICS hint (from 10g on), or increasing the STATISTICS_LEVEL parameter to ALL (on session level, on system level the overhead is probably prohibitive).As soon as a SQL execution is done (either successfully, cancelled or with error) the corresponding extended data in the child cursor gets populated/updated and the additional information about the actual runtime profile can be accessed via V$SQL_PLAN_STATISTICS resp.

DOUG presentation on dbms_xplan

Please join us at the DOUG (DALLAS ORACLE USERS GROUP) Oracle Database Forum meeting on Thursday, October 25, 2012 from 5 pm – 7 pm.
Presented by Riyaj Shamsudeen, OraInternals, & Sahil Thapar:

“Out with the old way, Enter dbms_xplan: A Swiss army knife for performance engineers”

Rough outline:
(i) Ability to query access path from memory, AWR repository
(ii) Ability to use cardinality feedback method to understand access plan issues. Few tips from a real world experience will be provided too.
(iii) Ability to understand issues with database links etc.
(iv) Options such as ADVANCED, ALLSTATS etc
(v) Why should you choose dbmx_xplan over tkprof+sql_trace combination?
(vi) Disadvantages of dbms_xplan and a quick introduction to dbms_monitor.

Refreshments sponsored by me :)