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 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...

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...

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...

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...

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...

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, [...]

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 a number of articles and blog posts have already been written on this topic (e.g. on Pete Finnigan’s site I found references dating back from 2003), from time to time I’m still asked “How to trace predicates generated by VPD?”. Hence, here’s yet another blog post about this topic… Let’s setup...

The short answer to this question is “yes”, it does. Unfortunately, the distribution costs are not externalized through the execution plans and, as a result, this limitation (yes, it is really a limitation in the current implementation, not a bug) confuses everyone that carefully look at...

You can add the COMPUTE STATISTICS clause to the CREATE INDEX statement. It instructs the SQL statement to gather and store index statistics in the data dictionary, while creating the index. This is useful because the overhead associated with the gathering of statistics while executing this SQL statement...

Up to Oracle Database 10g Release 2, direct-path inserts are supported only by INSERT INTO … SELECT … statements (including multitable inserts), MERGE statements (for the part inserting data), and applications using the OCI direct-path interface (for example, the SQL*Loader utility). At the statement...

In the first post about this topic I wrote: “What I don’t like about the column “A-Rows” (or the underlying columns LAST_OUTPUT_ROWS in the V$ views), is that for the operations modifying a table 0 is shown. By the way, according to the documentation it is not a bug.” What I forgot to mention is [....

Today’s post is dedicated to the Metalink SR identified by the number 6468252.994. I know, this number says noting to you. For me, however, it’s a very well known number. The reason is quite simple… Even if I open this SR almost two years ago (to be precise, September 5, 2007), it was closed few...

As you can read in the documentation, the columns MAX_TEMPSEG_SIZE and LAST_TEMPSEG_SIZE in the dynamic performance view V$SQL_WORKAREA provide information about the size of the temporary segment used for a specific workarea. The values are given in bytes. Let’s perform a test to check this information…...

Yesterday a colleague of mine asked the following question: “Is there a way to get information about free CPU resources within sqlplus?” When I read it, I immediately thought to V$OSSTAT. In fact, as of 10g that dynamic performance view provides system utilization statistics from the operating...

In the paper SQL Profiles (page 22) I described the data dictionary tables where the hints belonging to SQL profiles are stored. For example, with the following query it is possible to display the hints associate to the SQL profile named opt_estimate. SQL> SELECT attr_val 2 FROM sys.sqlprof$ p,...

It happened to me several times to being asked about the mysterious full table scan in CONNECT BY operations. In this post I would like to share with you some of the information I wrote about it in my book (pages 233 to 236) . The operation CONNECT BY WITH FILTERING is used to process hierarchical [...