When trying to understand why the optimiser might choose not to use an index to read rows from a table, I find the following diagram helpful.
The axis along the bottom represents the selectivity of the query (0% being no rows selected, 100% being all rows selected), and the axis up the side represents the cost of the chosen operation (more costly = more work to do).
When reading from a table without using an (more...)
Here is a step-by-step analysis of the deadlock which occurs due to unindexed foreign keys. This analysis was performed on Oracle XE 188.8.131.52 – results may vary on other versions.
A typical deadlock graph arising from an unindexed foreign key is as follows.
Resource Name process session holds waits process session holds waits
TM-0000508a-00000000 27 101 SX 28 12 SSX
TX-00090013-0000019b 28 12 X 27 101 S
This type of (more...)
I have been investigating “enq: TM – contention” wait events, which are almost certainly due to un-indexed foreign keys. One such extreme example which showed up in a Statspack report is as follows :
Counting the number of distinct values (NDV
) for a table column has important
applications in the database domain, ranging from query optimization to optimizing reports for large data warehouses. However the legacy
SQL method of using SELECT COUNT (DISTINCT <COL>) can be very slow
. This is a well known problem and Oracle 184.108.40.206 provides a new function APPROX_COUNT_DISTINCT implemented with a new-generation algorithm to address this issue by providing (more...)
One of the great things about working in IT is that you can often win an argument simply by being right. Not because of who you are or because you are more eloquent than others, but because the facts support your position. Almost every IT person I have ever met respects facts.
In order to win arguments this way, you of course need some facts to work with. And that’s where too many people fail. (more...)
June 8, 2014 It has been nearly two years since I last wrote a review of an Oracle Database related book, although I have recently written reviews of two Microsoft Exchange Server 2013 books and a handful of security cameras in the last two years. My copy of the second edition of the “Troubleshooting Oracle […]
It's been a while since I provided any public updates regarding Simora, our Oracle workload simulation product. It's finally time to unveil the status of Simora and our steps moving forwards. We have been working extensively on the Simora engine and infrastructure over the last several months, with a view to transforming it into a […]
This post is a hands-on introduction to using on-CPU Flame Graphs for investigating Oracle workloads. This technique is about collecting and analyzing sampled stack trace data to analyze and troubleshoot Oracle processes at the OS level (in particular applied to Linux).Motivations:
The techniques and tools described here can be used for performance investigations
to complement wait-event based information gathered from the Oracle engine, such as information available with ASH and sql monitoring. They (more...)
This blog entry is about investigating Oracle's DBMS_RESOURCE_MANAGER.CALIBRATE_IOSpoiler:
If you have reached this article in search of a tool for quantitative analysis of storage performance and in particular for measuring random read I/O in Oracle, I'd rather advise you to use tools that allow generating test workloads in a controlled manner, in a way that can be understood and measured and in particular with latency details together with IOPS measurements. For example (more...)
This post introduces the latest changes to OraLatencyMap and PyLatencyMap, two custom tools for collecting and displaying Oracle wait event latency details using heatmaps.OraLatencyMap
is a SQL*Plus tool, with a core written in PL/SQL, aimed at studying Oracle random I/O by displaying the latency drill-down of the wait event 'db file sequential read' using heatmaps. The tool can also be used to collect and display event latency histograms for any other Oracle wait (more...)
For anyone running Windows 2008 (or above), you can simply add the “Command Line” column to the Task Manager view. From there, the instance name will follow the “-s” startup option, for example: C:\…\Binn\sqlservr.exe” –sPREPROD If you’re on Windows 2000/2003 then it’s not quite as straight forward. You can either get the Process ID from
Random thoughts on a Friday afternoon…
We’ve all got problems. More to the point, every IT department or team has problems of some kind. It’s why we hire consultants, buy products, start long and arduous journeys into the great unknown depths of root cause analysis, and so on.
What fascinates me is the level at which we come to identify with our problems. When I’ve gone into an environment to deliver recommendations, the conversation usually (more...)
This post is about a performance analysis technique based on high frequency sampling of wait event history data in Oracle. Two scripts are provided for performing this type of analysis and two example cases are discussed applied to the study random read latency and read workload characterization.
In the context of performance analysis, for example for a study of I/O response time, I want to analyze the flow of all wait events
Recently while observing AWR reports, I’ve seen a very good example of how average value hides important pattern.
Here is a Workload Comparison section from an AWR diff report (generated with $ORACLE_HOME/rdbms/admin/awrddrpt.sql):
~~~~~~~~~~~~~~~~~~~ 1st Per Sec 2nd Per Sec %Diff 1st Per Txn 2nd Per Txn %Diff
--------------- --------------- ------ --------------- --------------- ------
DB time: 0.6 0.6 1.6 0.0 0.0 -50.0
CPU time: 0.3 0. (more...)
I have learned something new today and this blog post will be (primary) reminder to myself. I am sure that most Oracle DBA’s or Developers are familiar with this trick - so please skip this post if you are one of them :)
My test table will have 1000000 rows with 1000 NULL values for “OBJECT_NAME” column.
Tests are performed on 11gR1 version.
select c.table_name, c.column_name, c.data_type, c.num_nulls,
In the Oracle technical universe, it seems that the end of the calendar year is always eventful. First there’s OpenWorld: obviously significant for official announcements and insight into Oracle’s strategy. It’s also the week when many top engineers around the world meet up in San Francisco to catch up over (more...)
Recently I received an email with an simple content:
"It" is slow again :-)
The email explained a package to be slow.
Ok, I started with my "not enough information" reply and an email conversation began.
We are still not ready to identify the problem, but as there is no (more...)
Here are two interesting enhancements in Oracle database 12c PL/SQL. PL/SQL subprogram defined using WITH clause of a subquery – Oracle database 12c allows PL/SQL declaration section in the WITH clause. One can define PL/SQL function or procedure into a WITH clause. Functions declared in the PL/SQL declaration section can (more...)
Today was speaking at HighLoad++ 2013 Moscow. I had two presentation covering deep internals of JVM. One about JIT compilation and other concerning pauseless garbage collection algorithms.
Slide decks are below (in Russian)
Today I was speaking at CEE SECR 2013 at Moscow.
Below is a slide deck from presentation (in Russian)