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New Defaults, Old Side Effects

When 11.2 came out I posted about deferred segment creation at http://orawin.info/blog/2010/04/25/new-features-new-defaults-new-side-effects/ and a couple of odd side effects. Oracle published a Note  1050193.1 that makes the quite extraordinary claim that Sequences are not guaranteed to generate all consecutive values starting with the ‘START WITH’ value. It’s absolutely true that sequences don’t guarantee no gaps – but [...]

System Stats

A quick collation – and warning – for 11.2

Bottom line – be careful about what you do with system stats on 11.2

Footnote: the MOS link is a search string  producing a list of references. I set it up like that because one of the articles referencing the bug is called “Things to consider before upgrade to 11.2.0.2″ and it’s worth reading.

Addendum: one of the people on the two-day course I’ve just run in Berlin sent me a link for a quick note on how to set your own values for the system stats if you hit this bug. It’s actually quite a reasonable thing to do whether or not you hit the bug given the way that gathering the stats can produce unsuitable figures anyway:  setting system stats. (I’ve also added their company blog to the links on the right, they have a number interesting items and post fairly regularly.)

Trouble-shooting

How do you trouble-shoot a problem ? It’s not an easy question to answer when posed in this generic fashion; but perhaps it’s possible to help people trouble-shoot by doing some examples in front of them. (This is why I’ve got so many statspack/AWR examples – just reading a collection of different problems helps you to get into the right mental habit.)

So here’s a problem someone sent me yesterday. Since it only took a few seconds to read, and included a complete build for a test case, with results, and since it clearly displayed an Oracle bug, I took a look at it. (I’ve trimmed the test a little bit, there were a few more queries leading up to the error):


create table person (id number(2), name varchar2(10)) ;

insert into person values (1, 'Alpha') ;
insert into person values (2, 'Bravo') ;
insert into person values (3, 'Charlie') ;
insert into person values (4, 'Charles') ;
insert into person values (5, 'Delta') ;

create or replace view vtest as
select id, 'C' as letter from person where name like 'C%' ;

select p.id, p.name, v.id, v.letter
from person p
left join vtest v on v.id = p.id
order by p.id ;

The problem was that 10.2.0.4 and 11.2.0.2 gave different results – and the 11.2.0.2 result was clearly wrong. So the question was: “is there something broken with outer joins on views, or possibly ANSI outer joins?” (The ansswer to the last question is always “probably” as far as I’m concerned, but I wouldn’t turn that into a “yes” without checking first.) Here are the two results:

10.2.0.4:
========
        ID NAME               ID L
---------- ---------- ---------- -
         1 Alpha
         2 Bravo
         3 Charlie             3 C
         4 Charles             4 C
         5 Delta

11.2.0.2
========
        ID NAME               ID L
---------- ---------- ---------- -
         1 Alpha                 C
         2 Bravo                 C
         3 Charlie             3 C
         4 Charles             4 C
         5 Delta                 C

Clearly the extra ‘C’s in the letter column are wrong.

So what to do next ? Knowing that Oracle transforms ANSI SQL before evaluating an execution plan I decided to run the 10053 trace. Sometimes you get lucky and see the “unparsed SQL” in this trace file, a representation (though not necessarily 100% exact) image of the statement for which Oracle will generate a plan. I was lucky, this was the unparsed SQL (cosmetically enhanced):


SELECT
	P.ID ID,
	P.NAME NAME,
	PERSON.ID ID,
	CASE  WHEN PERSON.ROWID IS NOT NULL THEN 'C' ELSE NULL END  LETTER
FROM
	TEST_USER.PERSON P,
	TEST_USER.PERSON PERSON
WHERE
	PERSON.ID  (+) = P.ID
AND	PERSON.NAME(+) LIKE 'C%'
ORDER BY
	P.ID
;

So I ran this query, and found that the same error appeared – so it wasn’t about ANSI or views. So possibly it’s something about the CASE statement and/or the ROWID in the CASE statement, which I tested by adding three extra columns to the query:

        person.name,
        person.rowid,
        CASE  WHEN PERSON.name IS NOT NULL THEN 'C' ELSE NULL END  LETTER

With these extra columns I got the following results from the query:

        ID NAME               ID NAME       ROWID              L L
---------- ---------- ---------- ---------- ------------------ - -
         1 Alpha                                               C
         2 Bravo                                               C
         3 Charlie             3 Charlie    AAAT7gAAEAAAAIjAAC C C
         4 Charles             4 Charles    AAAT7gAAEAAAAIjAAD C C
         5 Delta                                               C

So the CASE did the right thing with the person.name column, but the wrong thing with the person.rowid column.
Time to get onto MOS (Metalink).

I searched the bug database with the key words: case rowid null
This gave me 2,887 hits, so I added the expression (with the double quotes in place) “outer join”
This gave me 110 hits, so from the “product category” I pick “Oracle Database Products”
This gave me 80 hits, and the first one on the list was:

Bug 10269193: WRONG RESULTS WITH OUTER JOIN AND CASE EXPRESSION OPTIMIZATION CONTAINING ROWID

The text matched my problem, so job done – except it’s reported as not fixed until 12.1

This isn’t a nice bug, of course, because the particular problem can be generated automatically in the transformation of ANSI outer joins to Oracle outer joins, so you can’t just change the code.

In passing, it’s taken me 31 minutes to write this note – that’s 10 minutes longer than it took to pin down the bug, but I have to say I got lucky on two counts: first, that the “unparsed SQL” was available, second that my choice of key words for MOS got me to the bug so quickly (which is where I usually find I waste most time).

Virtual bug

I’ve said in the past that one of the best new features, in my view, in 11g was the appearance of proper virtual columns; and I’ve also been very keen on the new “approximate NDV” that makes it viable to collect stats with the “auto_sample_size”.

Who’d have guessed that if you put them both together, then ran a parallel stats collection it would break :(

The bug number Karen quotes (10013177.8) doesn’t (appear to) mention extended stats – but since virtual columns, function-based indexes, and extended stats share a number of implementation details I’d guess that they might be affected as well.

php long numbers

I got a mail from long time Oracle guy Joel Garry regarding the twitter widget in the right hand column.

On orawin.info, you have a twitter feed, where each entry has a date/time link on it.  But those links look like http://twitter.com/nlitchfield/statuses/8.0773403426E+16 which look to me like something is translating a big number to scientific notation…?

Well Joel was right. Something was translating a large number to scientific notation. That something turns out to be php itself. Twitter status updates are (apparently) a simple ever increasing integer. There are now a lot of twitter status updates. As this page shows recent versions of php will automatically display that in scientific notation unless explicitly told otherwise. Now I personally think that this is an odd thing for php to choose to do, but us database folks surely recognize the folly of the plugin programmers relying on default formats. Anyway my version of the plugin is now fixed – based on this forum post - and thanks to Joel for the heads up. Anyone reading this who programs in php against databases that might return large numbers might wish to reveiw their web pages for appropriate results.

Oracle.com passwords

Just a quick note for anyone who has missed the Oracle Security alerts email. If you downloaded either Enterprise Manager 11g or Oracle Database 11.2.0.2 before November 17th last year then you downloaded a version of OUI that sent unencrypted passwords for your oracle.com SSO account to Oracle over the intertubes. Not Good. Oracle have [...]