A reader of this blog, VijayS, asked me to share the script I use to demo ITL deadlocks that I mentioned in this comment. Since other readers might be interested, here is the script. SET TERMOUT ON FEEDBACK ON VERIFY OFF SCAN ON ECHO ON @connect SELECT * FROM v$version WHERE rownum = 1; REM [...]

Extended SQL trace (a.k.a. debugging event 10046 at a level higher than 1) is one of the key features provided by Oracle to troubleshoot applications using Oracle Database. For many years the available levels were always the same (4, 8 and 12). In fact, since I wrote my first paper about it in May 2000...

I regularly use the system-level activity chart available in Enterprise Manager. In my opinion it is a simple and effective way to know how much a specific database is loaded at a specific time. This is for example an interesting way for observing how a specific load is processed (see this post for an...

Recently I used the COMMIT_WAIT and COMMIT_LOGGING parameters for solving (or, better, working around) a problem I faced while optimizing a specific task for one of my customers. Since it was the first time I used them in a production system, I thought to write this post not only to shortly explain the...

Recently I had to analyse a row lock contention problem that can be illustrated by the following test case: A session (let’s call it #1) creates a table and inserts a row into it (note that “n” is the primary key of the table): SQL> CREATE TABLE t (n NUMBER PRIMARY KEY); SQL> VARIABLE n NUMBER...

Most execution plans can be interpreted by following few basic rules (in TOP, Chapter 6, I provide such a list of rules). Nevertheless, there are some special cases. One of them is when an index scan, in addition to the access predicate, has a filter predicate applying a subquery. The following execution...

Challenges and Chances of the 11g Query Optimizer is the name of a presentation I gave at several events (e.g. Trivadis Performance Days, Oracle OpenWorld, DOAG Konferenz, UKOUG Conference) throughout 2011. Its abstract is the following: With every new release, the query optimizer is enhanced. Oracle...

Recently I was involved in a project where I had to trace the database calls of an application based on Oracle Portal 10.1.4. The basic requirements were the following: Tracing takes place in the production environment Tracing has to be enable for a single user only Instrumentation code cannot be added...

In 2003 I published a paper entitled Debugging PL/SQL and Java Stored Procedures with JPDA. Its aim was to describe how to debug PL/SQL and Java code deployed into the database with JDeveloper 9i. Two weeks ago a reader of my blog, Pradip Kumar Pathy, contacted me because he tried, without success, to...

The book Der Oracle DBA (Hanser, 2011), which was written in German, is at last available! I say “at last” because the authors worked on this project for not less than two years. Who are the authors? Several colleagues of mine at Trivadis (Mirko Hotzy, Konrad Häfeli, Daniel Steiger, Sven Vetter, Peter...

The STATISTICS_LEVEL parameter controls a bunch of features. In addition to the documentation, also the V$STATISTICS_LEVEL view provides a list of the ones it controls. SQL> SELECT statistics_name, description, activation_level 2 FROM v$statistics_level 3 ORDER BY 3 DESC, 1; STATISTICS_NAME [...

At page 189 of TOP I wrote the following piece of text: In summary, with the initialization parameter optimizer_secure_view_merging set to TRUE, the query optimizer checks whether view merging could lead to security issues. If this is the case, no view merging will be performed, and performance could...

A reader of this blog, Paresh, asked me how I was able to find out the logic behind ITL waits without having access to Oracle code. My reply was: I wrote a test case that reproduce ITL waits and a piece of code that monitors them. Since other readers might be interested, here is the [...]

In recent releases Oracle has silently changed the behavior of ITL waits. The aim of this post it to describe what has changed and why. But, first of all, let’s review some essential concepts about ITLs and ITL waits. Interested Transaction List The Oracle database engine locks the data modified by...

In this post I pointed out that I like to have a copy of the documentation in PDF format on my notebook. In the same post, and its comments, I also described how I generate the scripts I use to download the files. Recently I updated the scripts and, as a result, I thought to [...]

At page 383 of my book I wrote the following sentence (BTW, the same information is also provided by Table 9-3 at page 381): With B-tree indexes, IS NULL conditions can be applied only through composite B-tree indexes when several SQL conditions are applied and at least one of them is not based on IS...

While writing a post about the TKPROF new features in 11.2.0.2 I noticed that I didn’t write one about an important change introduced in 11.2.0.1. So, before finishing the other one, let’s have a look to what changed in 11.2.0.1. One problem with TKPROF up to 11gR1 is that when the AGGREGATE parameter...

Even though in general parallel full table scans performs direct reads, some exceptions exist. The aim of this post is to show such an exception. For test purposes I build in my own schema a copy of the SH.SALES table (the one distributed by Oracle with the demo schemas…). On that table I build an ...

When two tables are equi-partitioned on their join keys, the query optimizer is able to take advantage of partition-wise joins. To make sure that the tables are equi-partitioned, as of Oracle Database 11g reference partitioning can be used. In fact, per definition, with reference partitioning all “related...

During an evolution the database engine compares the performance of two execution plans. The aim is to find out which one provides the better performance. For that purpose it has to run the SQL statement on which the SQL plan baseline is based and compare some execution statistics. The following output...

The aim of this post is to describe a strange (buggy) situation that I observed recently. But before doing that, I shortly summarize what a parent cursor and a child cursor are as well as when they can be shared. By the way, I borrowed this description from the pages 20/21 of my book. Hence, [...]

In the first and second post of this series I shared with you some basics about smart scan and gave some details about projection and restriction. The aim of this post is to cover the third basic technique: join filtering. Join filtering is not something specific to the Exadata Storage Server. In fact...

In the first post of this series I shared with you some basics about smart scan and gave some details about projection. The aim of this post is to cover the second basic technique: restriction. Simply put, the aim of this technique is to offload to a cell the processing of predicates found in the [....

Almost one year ago Iggy Fernandez asked me to write a short text for the Ask the Oracles column of the NoCOUG Journal. The topic was “Bind Variable Peeking: Bane or Boon?”. My text along with the ones of Wolfgang Breitling, Dan Tow and Jonathan Lewis were published in the August issue. For some (unknown...

Even though the utilization of an Exadata Storage Server should be transparent for the query optimizer, when you look at execution plans generated in an environment using it you might notice slight differences. The purpose of this series of post is to summarize the differences I was able to observe....