SQL

So, what do you think is the most fundamental difference between NESTED LOOPS and HASH JOINS?

This is not a trick question. You’re welcome to write your opinion in the comments section – and I’ll follow up with an article about it (my opinion) later today…

Update: The answer article is here:

http://blog.tanelpoder.com/2010/10/06/a-the-most-fundamental-difference-between-hash-and-nested-loop-joins/

Here’s a little trick question. Check out the execution plan below.

What the hell, shouldn’t the INDEX/TABLE access be the other way around?!

Also, how come it’s TABLE ACCESS FULL (and not by INDEX ROWID) in there?

This question is with a little gotcha, but can you come up with a query which produced such plan? ;-)

----------------------------------------------
| Id  | Operation          | Name   | E-Rows |
----------------------------------------------
|   0 | SELECT STATEMENT   |        |        |
|*  1 |  INDEX RANGE SCAN  | PK_EMP |      1 |
|*  2 |   TABLE ACCESS FULL| EMP    |      1 |
----------------------------------------------

You most likely have seen this error before: ORA-01719: outer join operator (+) not allowed in operand of OR or IN Cause: An outer join appears in an or clause. Action: If A and B are predicates, to get the effect of (A(+) or B), try (select where (A(+) and not B)) union all (select [...]

This week I’ve seen an issue with a CONNECT BY query: for some reason Oracle 10.2.0.4 decided to build a weird plan (the query is weird too, but that’s not my point here ). An explanation of why that happened looks interesting, so here it is. Set up: drop table t2 cascade constraints purge; drop [...]

Gwen Shapira has written an article about a good example of a non-trivial performance problem.

I’m not talking about anything advanced here (such as bugs or problems arising at OS/Oracle touchpoint) but that sometimes the root cause of a problem (or at least the reason why you notice this problem now) is not something deeply technical or related to some specific SQL optimizer feature or a configuration issue. Instead of focusing on the first symptom you see immediately, it pays off to take a step back and see how the problem task/application/SQL is actually used by the users or client applications.

In other words, talk to the users, ask how exactly they experience the problem and then drill down from there.

In my Beyond Oracle Wait interface article I troubleshooted a test case where an execution plan somehow went “crazy” and started burning CPU, lots of logical IOs and the query never completed.

I have uploaded the test case I used to my new website, to a section where I will upload some of my demo scripts which I show at my seminars (and people can download & test these themselves too):

http://tech.e2sn.com/oracle-seminar-demo-scripts

Basically what I do is this:

1. I run the query with bind variable values where only a handful of rows match the filter condition. Thus Oracle picks nested loop join (and indexed access path)
2. Then I run the same query with different bind values, where a lot of rows match the filter condition. Oracle reuses existing execution plan (with nested loops!!!). Oracle ends up looping through a lot of blocks again and again (because nested loop visits the “right” side of the join once for every row coming from the “left” side of the join).

Using nested loops over lots of rows is a sure way to kill your performance.

And an interesting thing with my script is that the problem still happens in Oracle 11.1 and 11.2 too!

Oracle 11g has Adaptive Cursor Sharing, right? This should take care of such a problem, right? Well no, adaptive bind variable peeking is a reactive technique – it only kicks in after the problem has happened!

Many times it occurs that an inappropriate execution plan is used which was produced by using the current values of bind variables provided at the time of the hard parse. But later on the variables change so much that another execution plan would be required. Unfortunately there is no automatism in 9i and 10g that would spot this fact. Oracle finally resolved this problem in 11g.

The trick is to virtually set the statistics for the object which is involved in the query. What I mean by virtually is that I read the current statistics and store the same statistics back what makes no harm but the side effect is that the cursor is invalidated and hence it will be re-parsed and hopefully this time optimized for the right values of bind variables.

Here is the code:

Enrollment is open for the course taught by Christopher J. Date that we'll host 26–28 January in the Dallas/Fort Worth area. For many of us, this will be a once-in-a-lifetime opportunity to sit in a classroom for three days with one of the pioneers who created the field we live in each day.

I'm looking forward to this course myself. It is so easy to use Oracle in non-relational ways. But not understanding how to use SQL relationally leads to countless troubles and unnecessary complexities. Chris's focus in this course will be the discipline to use Oracle in a truly relational way, the mastery of which will make your applications faster, easier to prove, and more fun to write, maintain, and ehnance.

Note added 3 April 2009: When I wrote this post, we were counting down toward 2 April as the date for our preliminary go/no-go decision. That date is now behind us, and we have made the preliminary decision to Go. We are accepting further enrollments. —Cary Millsap

Thursday 2 April 2009 is our last call for enrollment in C. J. Date's course, "How to write correct SQL, and know it: a relational approach to SQL." I'm looking forward to this course more eagerly than anything I've attended in the past ten years, ...maybe twenty.

SQL and I never really got along too well. When I first joined Oracle Corporation in 1989, I was new to relational databases. I had done one hierarchical database project in college. I enjoyed the project ok, but it wasn't something I ever wanted to do again. When I joined Oracle, I didn't know much about relational technology or SQL. In my formative first couple of years at Oracle, though, I just never learned to like the SQL language. Prior to my Oracle career, I designed languages and wrote compilers for a living. From a language design standpoint, it just seemed that SQL (at least "Oracle SQL") could have become something really cool, but it didn't. For Oracle to treat an empty string as NULL, for example, is a decision which I still can't believe made it into the light of day...

I had a lot of respect over the years for the people I met who knew how to make SQL do what they wanted it to do. Dominic Delmolino was one of the first people I ever met who could make SQL do things I had no idea it could do. I'm still amazed when I see the things that Tom Kyte can do with SQL. I was never one of the SQL people.