A nice Descending Index Range Scan

I’ve been aware of some of the ways that Oracle database optimises index accesses for queries, but I’m also aware that you have to test each critical query to ensure that the expected optimisations are taking effect.

I had this simple query, the requirement of which is to get the “previous status” for a record from a journal table. Since the journal table records all inserts, updates and deletes, and this query is called immediately (more...)

ORA-00937: not a single-group group function Solution

Have you received an ORA-00937: not a single-group group function error? Learn what it is and how to fix it in this article. ORA-00937 Cause The cause of the ORA-00937 error is that a SELECT statement is trying to be executed, and the SELECT statement has an aggregate function (e.g. COUNT, MIN, MAX, SUM, or […]

60 SQL Interview Questions and Answers

Are you going for a job where you need to know SQL, such as a Database Developer or Database Administrator? Brush up on your interview questions with this extensive list of SQL interview questions. This collection of interview questions on SQL has been collated from my experience with SQL and from various websites. It contains […]

Bad, Bad data




One should feel really sorry about anyone who will rely on filtering and making a decision based on bad, bad data. It is going to be a bad decision.


This is serious stuff. I read the other day a recent study by IBM which shows that "Bad Data" costs US $3.1 trillion per year!


OK, let's say you don't mind the money and have money to burn, how about the implications of using the (more...)

ORA-00907: missing right parenthesis Solution

Did you get an ORA-00907: missing right parenthesis error? Learn what caused it and how to resolve it in this article. ORA-00907 Cause When working with Oracle SQL, all left parenthesis (the “(” character) must be paired with a right parenthesis character (the “)” character). If there are more left parentheses than right parentheses, then […]

Reset Oracle Password

This blog entry shows you how to reset the system password for an Oracle Database. It uses a Linux image running Oracle Database 11g Express Edition. It assumes the student user is the sudoer user.

After you sign on to the student user account, you open a Terminal session and you should see the following:

[student@localhost python]$ 

The oracle user account should be configured to prevent a login. So, you should use the su (more...)

web scale patterns in the bol.com back office – Mixed SQL – NoSQL

web scale patterns in the bol.com back office – Mixed SQL – NoSQL

In the previous weeks, we started a series of blog posts that show you how we use “web scale” patterns to achieve scalability and flexibility in our back office software. The previous patterns discussed were Event Sourcing and CQRS. This week we will dive into mixed SQL – NoSQL. Showing you how this doesn’t just solve a technical problem, they help (more...)

Truncating LISTAGG prior to 12.2

I've been following the LISTAGG problem for a while because I find the function useful.

The problem being this error:
ORA-01489: result of string concatenation is too long

It relates to when the result set is over 4000 characters, eg:
with data as (select rpad('x',400, 'x') str from dual connect by level <=10)
select listagg(str,', ') within group (order by null)
from data
Ten rows of 400 characters equals 4000, add the concatenation operator and you get (more...)

Formatting results from ORE script in a SELECT statement

This blog post looks at how to format the output or the returned returns from an Oracle R Enterprise (ORE), user defined R function, that is run using a SELECT statement in SQL.

Sometimes this can be a bit of a challenge to work out, but it can be relatively easy once you have figured out how to do it. The following examples works through some scenarios of different results sets from a user defined (more...)

Script: Inactive Parallel QC Holding Parallel Processes

Working on a data warehouse system can be quite challenging, as I mentioned in the post from yesterday. One of the things we need to take care of is the amount of parallel processes that are in used at all times. Yesterday I wrote about how to locate downgraded sessions. Today we will look at another aspect – who “steals” parallel processes and what can we do to solve it.

One of the biggest thieves (more...)

Script: Finding Session With Downgraded Parallel Execution

I was working with the data warehouse team at a customer site and at some point we realized that some parallel executions are not getting enough resources (downgraded).

Not getting enough parallel processes in such a complex environment is really bad. That means that since everybody is hogging the CPU, some sessions will not be able to complete inside the night ETL time frame. If that happens – some ETLs will go on into the (more...)

Explicit Semantic Analysis setup using SQL and PL/SQL

In my previous blog post I introduced the new Explicit Semantic Analysis (ESA) algorithm and gave an example of how you can build an ESA model and use it. Check out this link for that blog post.

In this blog post I will show you how you can manually create an ESA model. The reason that I'm showing you this way is that the workflow (in ODMr and it's scheduler) may not be for everyone. (more...)

Oracle Diagnostic Queries

It’s always a challenge when you want to build your own Oracle SQL Tools. I was asked how you could synchronize multiple cursors into a single source. The answer is quite simple, you write an Oracle object type to represent a record structure, an Oracle list of the record structure, and a stored function to return the list of the record structure.

For this example, you create the following table_struct object type and a table_list (more...)

Explicit Semantic Analysis in Oracle 12.2c Database

A new Oracle Data Mining algorithm in the Oracle 12.2c Database is called Explicit Semantic Analysis.

[The following examples are built using Oracle Data Miner 4.2 (SQL Developer 4.2) and the Oracle 12.2 Database cloud service (extreme edition) ]

The Explicit Semantic Analysis algorithm is an unsupervised algorithm used for feature extraction. ESA does not discover latent features but instead uses explicit features based on an existing knowledge base. There (more...)

A Recipe for Summoning the RBO Monster (even in Oracle 12c): On Delete Cascade, Function-Based Index and Missing Table Statistics

The last version of Oracle in which CHOOSE was officially supported as an OPTIMIZER_MODE parameter value was 9.2.
This is what the documentation of Oracle 9.2 says about it:

choose
The optimizer chooses between a cost-based approach and a rule-based approach based on whether statistics are available.
If the data dictionary contains statistics for at least one of the accessed tables, then the optimizer uses a cost-based approach and optimizes with a goal (more...)

Inserting data in SQL*Plus correctly

When inserting data into the database, it is occasionally forgotten (especially by English-speakers) that we need to take steps to ensure we are inserting data correctly and without unexpected character translation.

For example, in SQL*Plus we need to ensure we set the NLS_LANG environment variable to the correct setting for our database before we initiate SQL*Plus.

Here’s a quick example showing what can go wrong:

[oracle@ORA122 ~]$ echo $NLS_LANG

[oracle@ORA122 ~]$ sqlplus neil/neil
SQL*Plus: Release  (more...)

Auditing Oracle Data Mining model usage

In a previous blog post I talked about how you can rename and comment your Oracle Data Mining models. This is to allow you to easily to see and understand the intended use of the data mining model.

Another feature available to you is to audit the usage of the the data mining models. As your data mining environment grows to many 10s or more typically 100s of models, you will need to have some (more...)

The “Control Freak Trigger” Design Pattern

Suppose that every time we add records into the T1 table we have to do some additional stuff.
One option to implement this is by using an AFTER INSERT trigger that will perform this additional stuff, but I really dislike this option (mainly because the code becomes hidden in a way, and there may be a negative impact on performance).
I prefer writing a procedure that inserts the records into T1 and performs this additional (more...)

Renaming & Commenting Oracle Data Mining Models

As your company evolves with their data mining projects, the number of models produced and in use in production will increase dramatically.

Care needs to be taken when it comes to managing these. This includes using meaningful names, adding descriptions of what the model is about or for, and being able to track their usage, etc.

I will look at tracking the usage of the models in another blog post, but the following gives examples (more...)

#UKOUG_Tech16: Ranges, Ranges Everywhere

I presented this topic on at Tech 16 on Monday afternoon. The latest version is on the Tech 16 website and on http://www.slideshare.net/StewAshton1/ranges-ranges-everywhere-oracle-sql Thanks to the organisers, staff, presenters and delegates who made this a great conference and a wonderful experience!