This is the second part of a series of blogpost on Oracle database PGA usage. See the first part here. The first part described SGA and PGA usage, their distinction (SGA being static, PGA being variable), the problem (no limitation for PGA allocations outside of sort, hash and bitmap memory), a resolution for Oracle 12 (PGA_AGGREGATE_LIMIT), and some specifics about that (it doesn’t look like a very hard limit).
But this leaves out Oracle version (more...)
First things first: Thou shalt not explicitly set AQ_TM_PROCESSES=0 ! Unless, of course, you want to disable the Queue Manager Process (QMNC in Oracle 11.x). Which you may want to do during database upgrades to prevent Streams or Advanced Queueing from interfering with the upgrade process. However, if you don’t reset this parameter afterwards, you might run […]
If you're using dynamic SQL to source your menus and you're utilising the apex_application_pages dictionary view to build you list of menu options - I suggest you consider using Materialized Views
We have a reporting application that does just this; and the query also links to an authorisation table that decides what types of users see which reports.
You can create a snapshot (materialized view) using something as simple as
create materialized view apx_application_pages
APPEND hint in oracle is very useful, there are many benefits
Fragmented data loading – data will clustered above the HWM so that similar data will be together. Otherwise Oracle will place the records in blocks using PCT_FREE rule
- No undo will be generated – in large direct path insert will free up large amount of data blocks
- Less CBC latches (because of no UNDO blocks), less CPU usage
- Readers need not undo the records, (more...)
This post is about tracing logical and physical reads in Oracle using systemtap. You will find a few examples illustrating the main mechanisms used by Oracle to do physical and logical I/O and you will learn how to build systemtap scripts to further explore and troubleshoot Oracle I/O.Introduction:
Performing input/output to and from the storage media, i.e. physical I/O, is one of the critical tasks underlying all database engines. When accessing data (more...)
There have been rumblings from the HPC community indicating a general suspicion of and disdain for Big Data technology which would lead one to believe that whatever Google, Facebook and Twitter do with their supercomputers is not important enough to warrant seriousness—that social supercomputing is simply not worthy. A little of this emotion seems to […]
Yes, direct path reads again :) No worries I’m already a bit bored from digging in this algorithm, so I think this is going to be my last post about it. Till they change it again of course. And yes, in 184.108.40.206 they did. For introduction to DPR, you can check my previous post, because […]
This post is about using SystemTap
for investigating and troubleshooting Oracle RDBMS. In particular you will learn how to probe Oracle
processes and their userspace
functions. These techniques aim to be useful
as well as fun
to learn for those keen into peeking under the hood
of the technology and improve
their effectiveness in troubleshooting and performance
is a very powerful technique that can be used to complement the available (more...)
In Oracle Database 12c we can find many new and shiny things… So many that we can miss the little good things really easy. I think that this one, is one of them. Previously I made a post “All About HCC“, describing how HCC is working and some of the issues that we can hit […]
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 220.127.116.11 – 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 18.104.22.168 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...)
Today while I was scrolling my “WordPress reader” page I saw a post which reminds me to something I had fun before /and find it useful in many situations/: So if we have an index based on null column followed by not null column we can use it to filter the rows with null value. […]
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...)