Okay, it’s a little early in the day (for me at least) to say “night” – but here’s a fun little detail I picked up in Prague yesterday. What do you think will happen when you try to execute the following two queries:
select 0/0 from dual;
select count(*) from (select 0/0 from dual);
I’ve only tried it on 220.127.116.11 and 18.104.22.168 – I could imagine the results might be different if you’re still running 8i or 9i.
If those are too easy, you might want to think about an example that Julian Dontcheff produced at OpenWorld:
select power(0,0) from dual;
What SHOULD the answer be, and what do you think Oracle will supply ?
It probably won’t surprise many people to hear me say that the decode() function can be a bit of a nuisance; and I’ll bet that quite a lot of people have had trouble occasionally trying to get function-based indexes that use this function to behave properly. So (to put it all together and support the general directives that case is probably a better choice than decode() and that the cast() operator is an important thing to learn) here’s an example of how function-based indexes don’t always allow you to work around bad design/code. (Note: this is a model of a problem I picked up at a client site, stripped to a minimum – you have to pretend that I’m not allowed to fix the problem by changing code).
Sorted Hash Clusters have been around for several years, but I’ve not yet seen them being used, or even investigated in detail. This is a bit of a shame, really, because they seem to be engineered to address a couple of interesting performance patterns.
Here’s a funny little problem I came across some time ago when setting up some materialized views. I have two tables, orders and order_lines, and I’ve set up materialized view logs for them that allow a join materialized view (called orders_join) to be fast refreshable. Watch what happens if I refresh this view just before gathering stats on the order_lines table.
I’ve written a few notes about anomalies in subquery factoring (with subquery) in the past, principally making a fuss about the fact that moving an inline view into a “with subquery” can cause a plan to change even when the internal code moves the subquery back in line. With the arrival of 12c one of my first sets of tests was to rerun all the examples to see how many of them had been addressed. I hadn’t written about as many examples as I had thought, and some of them had been fixed before 12c, but here are few references to a couple of outstanding items that I thought worth a mention:
[Further reading on “subquery factoring”]
Now that 12c is out, here’s an idea that might save you some time even if you have no intention of migrating to, or even testing, the product for a couple of years. Download the “List of bugs fixed in 12c”: you may find that it’s the best starting point when you’re trying to solve a problem in your current version of Oracle.
A slightly more sophisticated version of the same thing – download and install the product, then take a dump of v$system_fix_control – that may also give you some insight into anomalies (that are not necessarily declared as bugs) in the way Oracle – and the optimizer in particular – behave. I updated the referenced note to add in a couple of figures for 12.1 – but one figure that’s not there is the number of database parameters: now at 368 in the v$ and 3,333 in the x$ (in my Beta 3 release).
I’ll probably have to file this one under “Optimizer ignoring hints” – except that it should also go under “bugs”, and that’s one of the get-out clauses I use in my “hints are not hints” argument.
Sometimes an invisible index isn’t completely invisible.
Actually it’s probably not the NOT IN that’s nasty, it’s the thing you get if you don’t use NOT IN that’s more likely to be nasty. Just another odd little quirk of the optimizer, which I’ll demonstrate with a simple example (running under 22.214.171.124 in this case):
Here’s a funny little bug – which probably won’t cause any damage – that may remind you that (most of) the people who work for Oracle are just ordinary people like you and me who make ordinary little mistakes in their programming. It’s a bug I discovered by accident because I just wanted to check something about how a particular undo tablespace had been defined, and I called dbms_metadata instead of querying dba_tablespaces. Here’s the cut-n-paste from an SQL*Plus session on 126.96.36.199:
Here’s a suggestion to help you avoid wasting time. If you ever include the rowid in a query – not that that should happen very commonly – make sure you give it an alias, especially if you’re using ANSI SQL. If you don’t, you may find yourself struggling to work out why you’re getting an irrational error message. Here’s an example that appeared recently on the OTN forum, with the output cut-n-pasted from a system running 188.8.131.52:
For your entertainment – there’s nothing up my sleeves, this was a simple cut-n-paste after real-time typing with no tricks:
20:39:51 SQL> create table t1 (t1 timestamp);
20:39:55 SQL> insert into t1 values(systimestamp);
1 row created.
20:39:59 SQL> select t1 - systimestamp from t1;
1 row selected.
From time to time I’ve looked at an AWR report and pointed out to the owner the difference in work load visible in the “SQL ordered by” sections of the report when they compare the summary figure with the sum of the work done by the individual statements. Often the summary will state that the captured SQL in the interval represents some percentage of the total workload in the high 80s to mid 90s – sometimes you might see a statement that the capture represents a really low percentage, perhaps in the 30s or 40s.
You have to be a little sensible about interpreting these figures, of course – at one extreme it’s easy to double-count the cost of SQL inside PL/SQL, at the other you may notice that every single statement reported does about the same amount of work so you can’t extrapolate from a pattern to decide how significant a low percentage might be. Nevertheless I have seen examples of AWR reports where I’ve felt justified in suggesting that at some point in the interval some SQL has appeared, worked very hard, and disappeared from the library cache before the AWR managed to capture it.
Now, from Nigel Noble, comes another explanation for why the AWR report might be hiding expensive SQL – a bug, which doesn’t get fixed until 12.2 (although there are backports in hand).
Here’s an oddity that I ran into a little while ago while trying to prepare a sample trace file showing a particular locking pattern; it was something that I’d done before, but trace files can change with different versions of Oracle so I decided to use a copy of 184.108.40.206 that happened to be handy at the time to check if anything had changed since the previous (11gR1) release. I never managed to finish the test; here are the steps I got through:
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:
Just a quick note to say that I found a blog over the weekend with a number of interesting posts, so I thought I’d pass it on: http://www.bobbydurrettdba.com/
There’s a really cute example (complete with test case) of an optimizer bug (possibly only in 11.1) in the December archive: http://www.bobbydurrettdba.com/2012/12/04/index-causes-poor-performance-in-query-that-doesnt-use-it/