Aggregation Filters

Overview

The FILTER clause is a nice addition to the SQL syntax in Oracle 26ai (23.26.1). It allows to restrict the rows considered by aggregate functions.

An aggregate query divides a set of records into groups, and returns a single result row for each group.
By default, the input for every aggregate function in the query includes all the records of the group.
However, sometimes we want the input to a specific aggregate function in the query to be only a subset of the group’s records. The FILTER clause provides an elegant solution for this requirement.

Example

As an example, I’ll use the EUROVISION_WINNERS table that I created for my Pattern Matching presentation.
This table contains a record for every year of the Eurovision Song Contest, with some details about the winner of that year.

SQL> select * from eurovision_winners order by year;

      YEAR WINNER          SONG                                PERFORMER                                  POINTS LANGUAGE      HOST
---------- --------------- ----------------------------------- -------------------------------------- ---------- ------------- --------------
      1956 Switzerland     Refrain                             Lys Assia                                         French        Switzerland
      1957 Netherlands     Net als toen                        Corry Brokken                                  31 Dutch         Germany
      1958 France          Dors, mon amour                     André Claveau                                  27 French        Netherlands
      1959 Netherlands     Een beetje                          Teddy Scholten                                 21 Dutch         France
      1960 France          Tom Pillibi                         Jacqueline Boyer                               32 French        United Kingdom
      1961 Luxembourg      Nous les amoureux                   Jean-Claude Pascal                             31 French        France
      1962 France          Un premier amour                    Isabelle Aubret                                26 French        Luxembourg
      1963 Denmark         Dansevise                           Grethe and Jørgen Ingmann                      42 Danish        United Kingdom
      1964 Italy           Non ho l'età                        Gigliola Cinquetti                             49 Italian       Denmark
      1965 Luxembourg      Poupée de cire, poupée de son       France Gall                                    32 French        Italy
      1966 Austria         Merci, Chérie                       Udo Jürgens                                    31 German        Luxembourg
      1967 United Kingdom  Puppet on a String                  Sandie Shaw                                    47 English       Austria
      1968 Spain           La, la, la                          Massiel                                        29 Spanish       United Kingdom
      1969 Netherlands     De troubadour                       Lenny Kuhr                                     18 Dutch         Spain
      1970 Ireland         All Kinds of Everything             Dana                                           32 English       Netherlands
      1971 Monaco          Un banc, un arbre, une rue          Séverine                                      128 French        Ireland
      1972 Luxembourg      Après toi                           Vicky Leandros                                128 French        United Kingdom
      1973 Luxembourg      Tu te reconnaîtras                  Anne-Marie David                              129 French        Luxembourg
      1974 Sweden          Waterloo                            ABBA                                           24 English       United Kingdom
      1975 Netherlands     Ding-a-dong                         Teach-In                                      152 English       Sweden
      1976 United Kingdom  Save Your Kisses for Me             Brotherhood of Man                            164 English       Netherlands
      1977 France          L'oiseau et l'enfant                Marie Myriam                                  136 French        United Kingdom
      1978 Israel          A-Ba-Ni-Bi                          Izhar Cohen and the Alphabeta                 157 Hebrew        France
      1979 Israel          Hallelujah                          Gali Atari and Milk and Honey                 125 Hebrew        Israel
      1980 Ireland         What's Another Year                 Johnny Logan                                  143 English       Netherlands
      1981 United Kingdom  Making Your Mind Up                 Bucks Fizz                                    136 English       Ireland
      1982 Germany         Ein bißchen Frieden                 Nicole                                        161 German        United Kingdom
      1983 Luxembourg      Si la vie est cadeau                Corinne Hermès                                142 French        Germany
      1984 Sweden          Diggi-Loo Diggi-Ley                 Herreys                                       145 Swedish       Luxembourg
      1985 Norway          La det swinge                       Bobbysocks!                                   123 Norwegian     Sweden
      1986 Belgium         J'aime la vie                       Sandra Kim                                    176 French        Norway
      1987 Ireland         Hold Me Now                         Johnny Logan                                  172 English       Belgium
      1988 Switzerland     Ne partez pas sans moi              Céline Dion                                   137 French        Ireland
      1989 Yugoslavia      Rock Me                             Riva                                          137 Croatian      Switzerland
      1990 Italy           Insieme: 1992                       Toto Cutugno                                  149 Italian       Yugoslavia
      1991 Sweden          Fångad av en stormvind              Carola                                        146 Swedish       Italy
      1992 Ireland         Why Me?                             Linda Martin                                  155 English       Sweden
      1993 Ireland         In Your Eyes                        Niamh Kavanagh                                187 English       Ireland
      1994 Ireland         Rock 'n' Roll Kids                  Paul Harrington and Charlie McGettigan        226 English       Ireland
      1995 Norway          Nocturne                            Secret Garden                                 148 Norwegian     Ireland
      1996 Ireland         The Voice                           Eimear Quinn                                  162 English       Norway
      1997 United Kingdom  Love Shine a Light                  Katrina and the Waves                         227 English       Ireland
      1998 Israel          Diva                                Dana International                            172 Hebrew        United Kingdom
      1999 Sweden          Take Me to Your Heaven              Charlotte Nilsson                             163 English       Israel
      2000 Denmark         Fly on the Wings of Love            Olsen Brothers                                195 English       Sweden
      2001 Estonia         Everybody                           Tanel Padar, Dave Benton and 2XL              198 English       Denmark
      2002 Latvia          I Wanna                             Marie N                                       176 English       Estonia
      2003 Turkey          Everyway That I Can                 Sertab Erener                                 167 English       Latvia
      2004 Ukraine         Wild Dances                         Ruslana                                       280 Ukrainian     Turkey
      2005 Greece          My Number One                       Helena Paparizou                              230 English       Ukraine
      2006 Finland         Hard Rock Hallelujah                Lordi                                         292 English       Greece
      2007 Serbia          Molitva                             Marija Šerifovic                              268 Serbian       Finland
      2008 Russia          Believe                             Dima Bilan                                    272 English       Serbia
      2009 Norway          Fairytale                           Alexander Rybak                               387 English       Russia
      2010 Germany         Satellite                           Lena                                          246 English       Norway
      2011 Azerbaijan      Running Scared                      Ell & Nikki                                   221 English       Germany
      2012 Sweden          Euphoria                            Loreen                                        372 English       Azerbaijan
      2013 Denmark         Only Teardrops                      Emmelie de Forest                             281 English       Sweden
      2014 Austria         Rise Like a Phoenix                 Conchita Wurst                                290 English       Denmark
      2015 Sweden          Heroes                              Måns Zelmerlöw                                365 English       Austria
      2016 Ukraine         1944                                Jamala                                        534 Crimean Tatar Sweden
      2017 Portugal        Amar pelos dois                     Salvador Sobral                               758 Portuguese    Ukraine
      2018 Israel          Toy                                 Netta Barzilai                                529 English       Portugal
      2019 Netherlands     Arcade                              Duncan Laurence                               498 English       Israel
      2021 Italy           Zitti e buoni                       M†neskin                                      524 Italian       Netherlands
      2022 Ukraine         Stefania                            Kalush Orchestra                              631 Ukrainian     Italy
      2023 Sweden          Tattoo                              Loreen                                        583 English       United Kingdom
      2024 Switzerland     The Code                            Nemo                                          591 English       Sweden
      2025 Austria         Wasted Love                         JJ                                            436 English       Switzerland

69 rows selected.

I want to analyze the languages of the winning songs. For each language, I’d like to get: Continue reading “Aggregation Filters”

Mixed Feelings About MERGE

I have a love-hate relationship with the MERGE SQL statement.
Well, maybe it’s a bit harsh. Let’s make it a like-dislike relationship.

Why I Like It

Because it provides an elegant way to perform an “insert if not exists, update if exists” logic in a single statement. It has a rich syntax with various options for different use cases. One of the very first posts I wrote in this blog, in 2013, was a primer of the MERGE statement.

Why I Dislike It

I don’t like how it deals with concurrent transactions that try to handle the same key.
Let me show you what I mean exactly.

Demo Setup

I have a table t, with a primary key on the id column. Continue reading “Mixed Feelings About MERGE”

EBR – Part 13: The Trouble with Foreign Keys with ON DELETE Clause and Related Triggers

This is part 13 of a post series about Oracle Edition-Based Redefinition.

Visit the index page for all the parts of the series

Introduction

As I wrote in Part 7: Editioning Views:

Editioning views are the interface between the application and the tables

This is one of the most important rules when using EBR.
Code objects should never refer to tables.
Instead of referring to a table, code objects should refer to the corresponding editioning view – the editioning view that covers that table.

Code objects include packages, procedures, functions, views, regular (i.e., not cross-edition) triggers, etc. You can see the full list in Part 6: Editionable and Non-Editionable, Editioned and Non-Editioned.

Cross-edition triggers are excluded from this list, as they are not really part of our code base – they are temporary objects that their lifespan is limited to the upgrade timeframe (as soon as no client uses the pre-upgrade edition anymore, they are dropped).
And, by definition, they should refer to the base tables.

However, there is one case (that I can think of) of a code object that has to refer to the base table, as referring to the editioning view is not enough.

The ON DELETE Clause

Let’s say we have two tables with a foreign key constraint between them. The foreign key is defined with the ON DELETE clause (either CASCADE or SET NULL), which means that whenever a record is deleted from the parent table, the corresponding child records are automatically deleted (if the ON DELETE CASCADE option is used) or updated (if the ON DELETE SET NULL is used). Continue reading “EBR – Part 13: The Trouble with Foreign Keys with ON DELETE Clause and Related Triggers”

Happy New Square Year

At a family dinner a few days ago, we realized that the upcoming year – 2025 – is a square number 🤩
Promptly the kids checked and announced that it’s the only year in the 21st century that is a square number.
We wondered how many square years there are per century over time, and, as always, such questions are answered using a simple SQL query.

with
   n as (select 30 centuries from dual),
   c as (select rownum century from n connect by level <= n.centuries),
   y as (select rownum*rownum year from n connect by level * level <= n.centuries*100),
   yc as (select y.year, to_number(to_char(to_date(y.year,'yyyy'),'cc')) century from y)
select
  c.century,
  count(yc.year) number_of_sqaure_years,
  decode(count(yc.year), 1, any_value(yc.year)) the_only_sqaure_in_the_century
from c, yc
where yc.century(+) = c.century
group by c.century
order by c.century;

   CENTURY NUMBER_OF_SQAURE_YEARS THE_ONLY_SQAURE_IN_THE_CENTURY
---------- ---------------------- ------------------------------
         1                     10
         2                      4
         3                      3
         4                      3
         5                      2
         6                      2
         7                      2
         8                      2
         9                      2
        10                      1                            961
        11                      2
        12                      1                           1156
        13                      2
        14                      1                           1369
        15                      1                           1444
        16                      2
        17                      1                           1681
        18                      1                           1764
        19                      1                           1849
        20                      1                           1936
        21                      1                           2025 -- We Are Here!
        22                      1                           2116
        23                      1                           2209
        24                      1                           2304
        25                      2
        26                      0
        27                      1                           2601
        28                      1                           2704
        29                      1                           2809
        30                      1                           2916

30 rows selected.

Looking at the first 30 centuries, the 10th century was the first to have only one square year. Since the 17th century, each century has exactly one square year, except for the 25th that has two and the 26th that has none.

Happy new year!

EBR – Even If Downtime Is Acceptable

So, You Don’t Need Online Upgrades

EBR is a set of features that enables patching and upgrading live Oracle-based applications in an online fashion, with zero downtime.

If you are responsible for an Oracle-based application, and you have a maintenance window, during which users do not use the application, you don’t need EBR.
You can upgrade in an offline fashion, by taking the system down during the maintenance window.

But, although you don’t have to use EBR, using EBR may improve your quality of life and your users’ satisfaction.

Improve Your Users’ Satisfaction

The website of the bank that manages my account is “temporarily unavailable” every night between midnight and 4am. I guess this is their maintenance window, during which they do (among other tasks) offline application upgrades.

My user experience is usually ok.
Rarely do I want to check my balance or to do some transactions in the middle of the night, and even if I do want, it’s not a matter of life and death – I’ll wait for the morning.
But if the bank’s website was available during nights as well, then my experience as a user would be excellent, rather than just ok.

Online upgrades mean less downtime, and less downtime means better user experience.
Even if downtime is acceptable, users are happier with no (or less) downtime.

Improve Your Quality Of Life

Upgrades can be done at any time

With EBR you are not limited to do upgrades only during the “maintenance window”.

Upgrades can be done at any time. We don’t need to look for off-peak hours, that are usually at nights and weekends – when we are tired and frustrated. We can do it when it’s most convenient to us – during work hours.

But it’s not just about our convenience.
If something goes wrong with an upgrade that is executed during work hours, rather than at nights/weekends, then solving the problem is much easier and usually faster. Our colleagues that can help with the problems are there. We don’t need to wake someone in the middle of the night, or to try tracking someone on a weekend family trip.

And most importantly, in my opinion, is that we can deploy features as soon as they are ready.
With offline upgrades, waiting for the maintenance window – which may be once a week, once a month, once a quarter, or any other frequency – means that your users do not get the best service.
EBR allows for agile development – you deploy features (and bug fixes) as soon as they are ready.

Upgrades can take as long as needed

The maintenance window is usually limited to several hours. What if the upgrade takes more than that? With EBR, the upgrade takes as long as needed. It is not limited to some arbitrary period of time.

When the upgrade is limited in time, we tend to be stressed, because we’re in a race against the clock. This is obviously not a good thing. And this stress may cause us to make some errors, which means the upgrade will take even longer, with a higher risk of not finishing it on time, which makes the whole situation even more stressful.

With EBR the upgrades can be done properly, with no pressure.

We can start an upgrade with a new edition in the morning, and then something more urgent comes up and we handle it, and get back to the upgrade later on – maybe even in the evening – and complete it.

Or, if we start an upgrade before lunch time, there is no problem having lunch before finishing the upgrade.

If during the upgrade there are some “half-baked” objects, or even invalid objects, in the production schema, it doesn’t matter.
Not even if it’s true for many hours.
As long as these objects are in the new (unexposed) edition, nobody knows it, and the end users (that are exposed only to the pre-upgrade edition) keep using the application uninterruptedly.

Database-side upgrades can be done independently of server-side upgrade readiness

With EBR, we can prepare a new edition, expose it to a new service, and the app server upgrade can be done at any time afterwards.
If I’m brave enough, I can even go on vacation once I expose the new edition, and the server may start using it even days later 🙂

Flexible exposure of new versions

With EBR, we don’t have to expose the new edition to all the servers at once. We can take advantage of that, and first use it only by a dedicated testing server. All the “real” users continue using the old edition during this time, without being affected.

Another flexibility we gain with EBR is that different types of app servers may use different versions (i.e., editions). Sometimes a database change affects several types of app servers, and only some of them are ready to upgrade. No problem – they can continue using an older edition, while the app servers that are ready can upgrade and start using the new edition.

Summary

EBR is not just about reducing downtime.
It gives several benefits that can improve the quality of life for both the developers and the end users.
And, at least based on my experience, the additional efforts that are required to develop and deploy with EBR are not high.

My First Decade in the International Oracle Community

Exactly ten years ago, in November 2014, I gave my first presentation at a conference outside my country and not in my native language.

At that point, I already had over twenty years of career experience in software development, focusing on databases – my long-lasting passion. I’ve always believed in knowledge sharing, and during those years, I enjoyed giving occasional presentations – sharing from what I learned and from the experience I gained – but I always presented in Hebrew and always in Israel: in companies I worked for, in the historical DBA Forum of ilOUG (Israel Oracle User Group) and at some local conferences.

Sharing and Learning

I discovered that when you build a presentation and prepare to deliver it, you learn a tremendous amount yourself. For example, in 2013, I took it upon myself to deliver a full-day seminar on the new features for developers in Oracle 12c. Since this version had just been released at the time, no one here had the experience and knowledge – and consequently, the desire – to build and deliver such a seminar in Israel. I volunteered to do it and spent many days preparing the seminar, during which I learned a lot. It’s likely that without the goal of preparing and delivering the seminar, I wouldn’t have learned so much new (and useful) content in a relatively short time period and at such a deep level.
To learn, I also used materials published by members of the international Oracle community – both Oracle users and Oracle employees.
Here’s a post I published then, containing the list of topics I included in that seminar.

I regularly read materials shared by community members (and learned a lot from them), and I heard about user group conferences around the world.
I realized I could develop more professionally, learn more, and share my knowledge with more people if I stepped out of my comfort zone, traveled to conferences worldwide, and presented in English – a language I naturally find more challenging to express myself in.
It wasn’t easy, it was even scary, but I took heart and decided to try. I submitted abstracts to several conferences, and finally two of them got accepted.

My Trial by Fire

And so, in November 2014, I arrived in Nuremberg for the DOAG conference, one of the largest Oracle user group conferences in the world.
It was a fascinating experience, somewhat frightening, and very enlightening.

I understood that this step meant much more than I had thought.

For instance, the opportunity to meet product managers face-to-face – the people directly responsible for the software and features I use. To talk with them, consult with them, complain to them. Even to have lunch with them, like what happened with Bryn Llewellyn, who was then the product manager for PL/SQL and EBR.

The first presentation I gave at that conference (and as mentioned, my first-ever presentation abroad, in English, and in front of a large audience) was about EBR.

As I was preparing on stage, just before starting the presentation, nervous and excited, someone approached me and said “Excuse me, I’d like to introduce myself: I’m Bryn, the product manager for the topic you’re about to present on.”
Of course, that wasn’t the best antidote for my nerves…

I replied that I knew very well who he was (I had read his articles, seen his videos), and that it was a great honor for me, as well as terrifying. He was incredibly nice and said “I just wanted to say I promise not to interfere, and if you want my help, you’re welcome to ask.”


The presentation went quite well, and afterward, I had lunch with Bryn, where he already solved one problem I had with EBR (how to run a job in a specific edition – you associate the job with a class, the class with a service, and the service with an edition).

Bryn helped me quite a lot afterward, we continued to meet at conferences and exchanged opinions. When he came to Israel he visited my workplace, and he encouraged me to attend Oracle’s big conference in San Francisco – Oracle OpenWorld – and we even gave a joint presentation there, and he was a staunch supporter.

None of this would have happened if I had stayed at home and only learn from what he and others published.

Access to Oracle product managers not only helps solve problems (like the advice I received at that lunch ten years ago) but also allows influencing the product’s future. There’s a lot of listening to users that translates into actions.
An example from my experience is the USER_TRIGGERS_AE dictionary view that was added in version 19c.
Another example is the support of both pre-update and post-update values in the DML RETURNING clause that was added in version 23ai.

The Community

Since DOAG 2014, I’ve participated in dozens of conferences in about 20 countries. I became part of a community of hundreds of people from around the world, with shared interests, united most of all by their desire to contribute to others, share their knowledge and experience, and collaborate with each other.

The Oracle ACE program has a big and important role in building and maintaining this community, and many of us are members of the program. The program recognizes, rewards, and promotes technical experts and community leaders for their contributions to the Oracle community.

Another group I’m a member of is Symposium 42, which was created by the Oracle database community to support the Oracle database community. All members are experts in creating & sharing technical content or organizing community events and activities.

I have many interactions with my community peers on professional topics.
There’s always someone to consult with and get guidance from.
Getting honest feedback on your post or presentation from people whose opinions you greatly value is priceless.
The opportunity to do a joint presentation with someone like Connor is something I wouldn’t have dared to dream about ten years ago.

The Friendship

But this community is much more than just a group of professionals with shared interests. Real friendships form, people support each other, and of course, enjoy drinking and eating together.

In times when there are many troubles and conflicts in the world, it’s heartwarming, and even hopeful, to see people from different countries and different cultures simply sit, drink, chat, and enjoy together.
Here for example – 7 people from 7 different countries:


During this journey that I started ten years ago, I gained a new circle of friends from many countries.
People I used to admire from afar for their knowledge and the way they share it in books, blogs, or videos – I’m proud today to call them friends (and still admire some of them 🙂).

I remember one evening many years ago at OOW in San Francisco, when I ran into Tim on the street, and he noticed I was a bit lost and down, and immediately took me to get something to eat. It was the most natural thing for him, and for me it emphasized how much this community is far beyond just sharing knowledge about features.

And there are many more examples. Here are just a few of them:

The warm hospitality of Alex and his family when I came with my wife and children for a vacation in their country.

The first time I met Erik, when I came to a conference in the Netherlands, and he was so welcoming and warm that in an instant I felt comfortable in the company of people I hadn’t known before.

When Jože heard at a conference in Ireland that I also bake bread, and gave me half of the sourdough starter he had brought for Martin.
Actually, sourdough bread is a serious matter of deep discussions with Martin, Jonathan, Gerald, and others. Which also reminds me of the discussions about pizza ovens with Maria.

Driving a very small car in Slovenia with Heli, Javed and Kamran (once again, each one of us from a different country).

A very unique experience was traveling around India together with my son Nimrod at OCYatra, along with Sai, Mike, Connor, Sandesh, Dimitri, Pedro, and others.

In the last year – a challenging period in my country during which I haven’t traveled – many friends moved me by showing interest and concern for my family’s well-being.

Local User Groups

Many members of the international community are also active in their local user groups, which benefits the various organizations and allows even smaller organizations to bring international-level content to their members.

I, for example, as an active member in the Israeli organization, helped organize conferences and meetups in Israel where speakers from around the world participated.

The crown jewel for me was in 2020, the last conference we held in Israel before COVID, during which I had the honor of hosting my fellow speakers at my home.


I hope to continue being active and contributing more to the community. I’ll update again in 2034 🙂

Optimization of NOT NULL Constraint Creation

Several years ago I wrote a series of 8 posts about constraint creation optimization. I think it’s time to add some more posts to the series.
I showed there, among other things, that Oracle does a nice optimization – that may save plenty of time – when we add in a single ALTER TABLE statement a new (nullable with no default value) column and a corresponding:
– inline (a.k.a. “column-level”) check constraint
or
– foreign key constraint (either inline or out-of-line)

What about NOT NULL constraints?

Oracle has a special optimization for the case of adding a column with a NOT NULL constraint (and no default value).
It checks if the table contains at least one record. If it does, then we cannot add the new column, becuase the NOT NULL constraint will be violated for all the existing rows. If the table doesn’t contain any records, the new column can be added.

Let’s start with an empty table: Continue reading “Optimization of NOT NULL Constraint Creation”

DECODE With Ranges

Tim Hall wrote this post yesterday (as part of Joel Kallman Day 2022) with some examples for “the old way” vs. “the new way” to do some stuff.
One of these examples is DECODE and CASE Expressions. As Tim wrote:

CASE expressions were first released in Oracle 8i […] Prior to that if we wanted something similar to a CASE expression in our SQL, we had to use the DECODE function.

This reminded me of a trick I came up with many years ago, probably before 8i 🙂

A table T has a column X, and we need a query that one of the items it returns is a rank – either “Low”, “Medium” or “High”. This rank is determined by the value of X and two thresholds – LOW_TH and HIGH_TH.

With a (Searched) CASE Expression it’s as simple as this: Continue reading “DECODE With Ranges”

Eurovision Analysis #JoelKallmanDay

Here’s my contribution to Joel Kallman Day 2022.

The picture is from the last time I met Joel. It was taken at ilOUG Tech Days 2020, just before an invigorating keynote by Joel.

Four years ago, as part of ODC Appreciation Day 2018 (as the annual Oracle community appreciation day was called that year), I wrote about Pattern Matching in SQL. That post was not a technical one.

This time I decided to return to the same topic, with a technical post.
I recorded a video that explains Pattern Matching by analyzing the winners of the Eurovision Song Contest over the years.

Pre-19c Crossedition Triggers

As shown in the previous post, finding all the crossedition triggers in the database is easy using DBA_TRIGGERS_AE, but this can be done only as of Oracle 19c – the release in which this data dictionary view was added.

Here are two options that can be used before 19c.

A Proper Way

We can use DBA_OBJECTS_AE to find all the triggers across all editions, and then for each one to “jump” to its edition using dbms_sql and check whether it is a crosseedition trigger or not using DBA_TRIGGERS. Continue reading “Pre-19c Crossedition Triggers”