Why does my query have a MERGE JOIN CARTESIAN?

Yes, the mention of Cartesian Joins usually makes a DBA's heart skip a beat. The cartesian joins caused by missing join conditions are definitely a pain to deal with - these are the types of joins that can "blow up" temp space and cause all types of alarms to go off.

I didn't find anything in Oracle's official 11g documentation on this particular join method but I did find plenty of articles about issues with it in their support DB. I have chased down a few of these in the past couple of weeks and here is what I found.

SQL Tuning Advisor - what profile am I accepting?

When working on a SQL Tuning issue I often kick off a SQL Tuning Advisor job on the SQL while I gather info about statistics and other things I like to check when tuning. After I get an idea of where performance might be suffering I use the SQL Tuning Advisor results for a second opion. Unfortunately more often than not the SQL Tuning Advisor reports it can dramatically improve the performance of a SQL Statement by using a SQL Profile to force a different plan.

"There was a problem creating a SQL tuning task. ORA-13780: SQL statement does not exist. ORA-06512"

I recently got this error while trying to run a SQL Tuning Task against a SQL statement that was available in AWR but not currently in the library cache.

I got around it by locating the SQL in OEM and using SQL Worksheet to execute it:

Finding Query Block names for a SQL that you can't modify...

I recently had an issue where a query being submitted to the database had a large change in performance. I could attribute the change to a recent change in object statistics, but the query wasn't behaving because of a combination of a view, subquery, UNION, and LIKE operators. After spending too much time on this statement I decided to hint it to make it perform like it did before the most recent stats change.

Food for thought - Who Should Tune SQL - DBA or Developer?

Iggy Fernandez covers the topic of who is better suited to tune SQL - Developers or DBAs. Also listed are 5 Dangerous Beliefs which may influence the answer to that question.



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