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Shrink Space
Here’s a lovely effect looking at v$lock (on 11.2.0.4) select sid, type, id1, id2, lmode, request, ctime, block from V$lock where sid in ( select sid from V$session where username = 'TEST_USER' ) order...
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RLS bug
RLS – row level security, aka VPD (virtual private database) or FGAC (fine grained access control) has a critical bug in 11g. The bug is unpublished, but gets mentioned in various other documents, so...
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Easy – Oops.
A question came up on OTN today asking for suggestions on how to enforce uniqueness on a pair of columns only when the second column was not null. There’s an easy and obvious solution – but I decided...
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Cache anomaly
Just a quick heads-up for anyone who likes to play around with the Keep and Recycle caches. In 11g Oracle introduced the option for serial direct path reads for tablescans on tables that was...
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Coincindences
I had another of those odd timing events today that make me think that Larry Ellison has access to a time machine. I found (yet another example of a) bug that had been reported on MoS just a few days...
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Bigfiles
It’s always the combinations that catch you out. Bigfile tablespaces have their uses – especially in big systems Materialized views have their users – especially in big systems There’s absolutely no...
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Costing Bug
It’s amazing how you can find little bugs (or anomalies) as soon as you start to look closely at how things work in Oracle. I started to write an article for All Things Oracle last night about...
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Subquery with OR
Prompted by a pingback on this post, followed in very short order by a related question (with a most gratifying result) on Oracle-L, I decided to write up a note about another little optimizer...
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Ignoring Hints
Does Oracle ignore hints – not if you use them correctly, and sometimes it doesn’t ignore them even when you use them incorrectly! Here’s an example that I’ve run on 11.2.0.4 and 12.1.0.1 create table...
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Securefiles
A few weeks ago someone emailed me about a problem they had importing securefiles – it was very slow. Such things are never easy to address by email, of course, but there were three features to...
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Delete Costs
One of the quirky little anomalies of the optimizer is that it’s not allowed to select rows from a table after doing an index fast full scan (index_ffs) even if it is obviously the most efficient (or,...
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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'; select /*...
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Plan depth
A recent posting on OTN reminded me that I haven’t been poking Oracle 12c very hard to see which defects in reporting execution plans have been fixed. The last time I wrote something about the problem...
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first_rows(10)
No, not the 10th posting about first_rows() this week – whatever it may seem like – just an example that happens to use the “calculate costs for fetching the first 10 rows” optimizer strategy and does...
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12c Downgrade
No, not really – but sometimes the optimizer gets better and gives you worse performance as a side effect when you upgrade. Here’s an example where 11.2.0.4 recognised (with a few hints) the case for a...
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Invalidation
Someone who attended my sessions at the Bucharest Oracle Summit earlier on this year sent me an example of a quirky little bug, possibly related to the newer “fine-grained” invalidation mechanisms,...
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Subquery Factoring (10)
What prompted me to write my previous note about subquerying was an upgrade to 12c, and a check that a few critical queries would not do something nasty on the upgrade. As ever it’s always interesting...
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IN/EXISTS bugs
Here’s a simple data set – I’m only interested in three of the columns in the work that follows, but it’s a data set that I use for a number of different models: execute dbms_random.seed(0) create...
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Result Cache
Yesterday I thought I’d spend half an hour before breakfast creating a little demonstration of a feature; some time about midnight I felt it was time to stop because I’d spent enough time chasing...
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12c Downgrade
No, not really – but sometimes the optimizer gets better and gives you worse performance as a side effect when you upgrade. Here’s an example where 11.2.0.4 recognised (with a few hints) the case for a...
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