Category: Oracle

Oracle DB’s Gradual Password Rollover Feature

It is good security practice to change passwords regularly. When changing a database password, however, the problem arises that applications that access this database have to be reconfigured if the password changes. If multiple applications or services use the same database user, then they all need to be reconfigured at once, typically during a scheduled downtime.

Oracle 21c introduced a new feature called Gradual Password Rollover that can help make such a password change less disruptive. The feature was also backported to Oracle 19c. If this feature is switched on for a user profile, a transition time is granted when the password is changed, during which both the old and the new password are valid. The applications can then change their configuration to the new password within this period according to their own schedule.

How to enable it

You must first be logged in as a privileged user who is allowed to manage users. The grace period for which both passwords should be valid after a password change is set via a user profile. A user profile is a set of limits on the database resources and the user password. The profile setting for this feature is called PASSWORD_ROLLOVER_TIME. You either create a new profile and specify this setting as a limit, or you adjust an existing profile. Here are both variants:

-- Create a new profile ...
CREATE PROFILE example_profile LIMIT PASSWORD_ROLLOVER_TIME 1;

-- ... or alter an existing profile
ALTER PROFILE example_profile LIMIT PASSWORD_ROLLOVER_TIME 1;

The unit of this setting is days. The minimum value is one hour (1/24) and the maximum value is 60 (days). You can assign this profile to a user with the following statement:

ALTER USER example_user PROFILE example_profile;

Now change the user’s password:

ALTER USER example_user IDENTIFIED BY thenewpassword;

Now you should be able to log in as this user with both the old and the new password. You can query the current status from the dba_users table:

SELECT username, account_status, profile
  FROM  dba_users
  WHERE username='example_user';

The value of the account_status column should have changed from OPEN to OPEN & IN ROLLOVER. This indicates that the user account is in the password rollover phase, and two passwords are active at the same time. You can end this period early with the following command:

ALTER USER example_user EXPIRE PASSWORD ROLLOVER PERIOD;

A final note: If you change the password again during the rollover period only the original password (the one before the rollover period was started) and the latest password are valid, which means a user account can’t have more than two valid passwords at the same time.

Materialized views in Oracle

Most relational database management systems (RDBMs) support not only views, but also materialized views. Materialized views and normal views are both database objects used to present data to users, but they work in different ways.

A database view is a virtual table or a named query that presents data from one or more tables in a specific way. They are not physical tables and do not store data directly, but instead retrieve data from the underlying tables based on a specified query.

Materialized views are similar to normal views, but they store pre-computed results. When a materialized view is created, the results of the underlying query are computed and stored in the database. One advantage of materialized views is faster query performance by avoiding the need to compute the same results repeatedly. This is especially useful for complex and time-consuming queries, as the results can be stored and accessed quickly.

The syntax for creating a normal view in an Oracle database is as follows:

CREATE VIEW view_name AS SELECT … FROM … WHERE …;

To create a materialized view instead of a normal view you add the MATERIALIZED keyword:

CREATE MATERIALIZED VIEW view_name AS SELECT … FROM … WHERE …;

When creating a materialized view you should think about and decide on three aspects of materialized views:

  1. the refresh method,
  2. the refresh interval,
  3. and the storage properties

The refresh method

The refresh method determines how the data in the materialized view is updated or refreshed to reflect changes in the base tables.

  • COMPLETE: This one completely rebuilds the materialized view from scratch. It drops the existing contents of the materialized view and then re-executes the query to populate it with fresh data. This method can be resource-intensive and slow, especially for large materialized views.
  • FAST: Updates only the rows in the materialized view that have changed since the last refresh. It uses the materialized view logs on the base tables to identify the changed rows and then applies the changes to the materialized view. It can be much faster than a complete refresh, especially if there are only a few changes to the data.
  • FORCE: Tries to perform a fast refresh if possible, but falls back to a complete refresh if necessary. This method is useful if you want to try to perform a fast refresh, but you’re not sure if it will be possible due to the nature of the data or the query.

You can specify the refresh method when creating the materialized view using the REFRESH keyword:

CREATE MATERIALIZED VIEW view_name
  REFRESH FAST
  AS SELECT ...;

If you do not specify a refresh mode FORCE is the default.

The refresh interval

The refresh interval controls how often the materialized view is automatically refreshed. It determines how frequently the materialized view is updated to reflect changes in the underlying data.

Some refresh interval options in Oracle are:

  • ON COMMIT: The materialized view is refreshed automatically every time a transaction that modifies the underlying data is committed. This interval is useful when you need to keep the materialized view up-to-date in near real-time.
  • ON DEMAND: The materialized view is refreshed only when you explicitly request a refresh using the DBMS_MVIEW.REFRESH.
  • START WITH … NEXT: With this interval, the materialized view is refreshed automatically at regular intervals. It is useful when you want to balance the need for up-to-date data with the resources required to refresh the materialized view.

You can specify the refresh interval when creating the materialized view by adding it to the REFRESH clause when creating the view:

CREATE MATERIALIZED VIEW view_name
  REFRESH FAST ON COMMIT
  AS SELECT ...;

The following materialized view gets refreshed every hour:

CREATE MATERIALIZED VIEW view_name
  REFRESH FAST START WITH SYSDATE NEXT SYSDATE + 1/24
  AS SELECT ...;

Storage properties

Storage properties affect how the data in the materialized view is stored and accessed. In Oracle, some of these are:

  • CACHE: The data is stored in the database buffer cache, which is a portion of memory used to cache frequently accessed data. It improves query performance by reducing disk I/O, but it can consume a significant amount of memory.
  • LOGGING: Changes to the materialized view data are logged in the database redo logs. This property ensures that changes to the materialized view can be recovered in case of a system failure but can result in additional overhead.
  • TABLESPACE: Allows you to specify the tablespace where the materialized view data is stored.

Again, you can specify these properties when creating the materialized view:

CREATE MATERIALIZED VIEW view_name
  CACHE
  LOGGING
  TABLESPACE tablespace_name
AS SELECT ... FROM ... WHERE ...;

Now you know the basics for creating materialized views in an Oracle database when needed. There is still more to learn about them. You can find the full reference here.

Partitioning in Oracle Database: Because Who Wants to Search an Endless Table?

As data volumes continue to grow, managing large database tables and indexes can become a challenge. This is where partitioning comes in. Partitioning is a feature of database systems that allows you to divide large tables and indexes into smaller, more manageable parts, known as partitions. This can improve the performance and manageability of your database. Aside from performance considerations, maintenance operations, such as backups and index rebuilds, can become easier by allowing them to be performed on smaller subsets of data.

This is achieved by reducing the amount of data that needs to be scanned during query execution. When a query is executed, the database can use the partitioning information to skip over partitions that do not contain the relevant data, instead of having to scan the entire table. This reduces the amount of I/O required to execute the query, which can result in significant performance gains, especially for large tables.

There are several types of partitioning available in Oracle Database, including range partitioning, hash partitioning, list partitioning, and composite partitioning. Each type of partitioning is suited to different use cases and can be used to optimize the performance of your database in different ways. In this blog post we will look range partitioning.

Range partitioning

Here is an example of range-based partitioning in Oracle:

CREATE TABLE books (
  id NUMBER,
  title VARCHAR2(200),
  publication_year NUMBER
)

PARTITION BY RANGE (publication_year) (
  PARTITION p_before_2000 VALUES LESS THAN (2000),
  PARTITION p_2000s VALUES LESS THAN (2010),
  PARTITION p_2010s VALUES LESS THAN (2020),
  PARTITION p_after_2020 VALUES LESS THAN (MAXVALUE)
);

In this example, we have created a table called books that stores book titles, partitioned by the year of publication. We have defined four partitions, p_before_2000, p_2000s, p_2010s, and p_after_2020.

Now, when we insert data into the books table, it will automatically be placed in the appropriate partition based on the year of publication:

INSERT INTO books (id, title, publication_year)
  VALUES (1, 'Nineteen Eighty-Four', 1949);

This book will be inserted into partition p_before_2000, as the year of publication is before 2000. The following book will be placed into partition p_2000s:

INSERT INTO books (id, title, publication_year)
  VALUES (2, 'The Hunger Games', 2008);

When we query the books table, the database will only access the partitions that contain the data we need. For example, if we want to retrieve data for books published in 2015 and 2016, the database will only access partition p_2010s.

SELECT * FROM books WHERE publication_year>=2015 AND publication_year<=2016:

However, you should be aware that while partitioning can improve query performance for some types of queries, it can also negatively impact query performance for others, especially if the partitioning scheme does not align well with the query patterns. Therefore, you should tailor the partitioning to your needs and check if it brings the desired effect.

Re-ordering table columns in an Oracle database

In an Oracle database, once a table is created, there is no obvious way to change the order of its columns. Sometimes you add a new column to an existing table and want it to be displayed in a different position by default for query results via SELECT *. Imagine you add an ID column to a table after the fact. Wouldn’t it be nice if this appeared in the first position?

Of course you can drop the whole table and create it again with the new column order. But this is cumbersome and potentially dangerous if the table is already filled with data. However, there is a trick that allows you to rearrange the columns without having to recreate the table.

The key to this is an Oracle feature that allows invisible columns. The feature itself is interesting in its own right, but it has a useful side effect that we’ll exploit. The documentation says:

When you make an invisible column visible, the column is included in the table’s column order as the last column. When you make a visible column invisible, the invisible column is not included in the column order, and the order of the visible columns in the table might be re-arranged.

So the plan is to make the appropriate columns invisible first by clever choice, and then to make them visible again in the desired order. This is how it works:

First we have a table with the following columns.

CREATE TABLE t (a NUMBER, b NUMBER, c NUMBER, e NUMBER, f NUMBER);

Later we realize that we need a column d that should be between c and f. So we add it to the table:

ALTER TABLE t ADD (d NUMBER);

This is of course added at the end:

DESC t;
Name Null? Type   
---- ----- ------ 
A          NUMBER 
B          NUMBER 
C          NUMBER 
E          NUMBER 
F          NUMBER 
D          NUMBER

To get it in the right position, we first hide the columns e and f, and then make them visible again.

ALTER TABLE t MODIFY (e INVISIBLE, f INVISIBLE);
ALTER TABLE t MODIFY (e VISIBLE, f VISIBLE);

And voilà, we have our desired order:

DESC t;
Name Null? Type   
---- ----- ------ 
A          NUMBER 
B          NUMBER 
C          NUMBER 
D          NUMBER 
E          NUMBER 
F          NUMBER

Note that this doesn’t change the internal, physical layout of the table on the disk. It’s just a cosmetic change.

Commenting SQL database objects

Did you know that you can annotate database object like tables, views and columns with comments in many SQL database systems? By that I don’t mean comments in SQL scripts, indicated by double dashes (--), but comments attached to the objects themselves, stored in the database. These may be helpful to the database admin by providing context via a description text on what is stored in these objects.

For PostgreSQL and Oracle databases the syntax is as follows:

COMMENT ON TABLE [schema_name.]table_name IS '...';
COMMENT ON COLUMN [schema_name.]table_name.column_name IS '...';

For example:

COMMENT ON COLUMN books.author IS 'The main author''s last name';
COMMENT ON TABLE books IS 'Contains only the best books';

These comments can be viewed in database tools like SQL Developer:

Comments on columns
Comments on tables

You can also view the comments in psql:

db=# \d+ books
 Column |  Type   |          Description
--------+---------+------------------------------
id      | integer |
author  | text    | The main author''s last name
title   | text    |

And for a table:

db=# \dt+ books
                    List of relations
 Schema | Name  | Type  |     |        Description
--------+-------+-------+ ... +------------------------------
public  | books | table |     | Contains only the best books

In Oracle you can query the comments from the data dictionary views ALL_TAB_COMMENTS and ALL_COL_COMMENTS:

> SELECT * FROM all_col_comments WHERE table_name='BOOKS';
OWNER    TABLE_NAME  COLUMN_NAME  COMMENTS
--------------------------------------------------------------
LIBRARY	 BOOKS	     ID           (null)
LIBRARY	 BOOKS	     AUTHOR       The main author's last name
LIBRARY	 BOOKS	     TITLE        (null)

> SELECT * FROM all_tab_comments WHERE table_name='BOOKS';
OWNER    TABLE_NAME  TABLE_TYPE  COMMENTS
--------------------------------------------------------------
LIBRARY	 BOOKS	     TABLE       Contains only the best books

In Oracle comments are limited to tables, views, materialized views, columns, operators and indextypes, but in PostgreSQL you can attach comments to nearly everything. Another good use case for this are documentation comments on database functions:

COMMENT ON FUNCTION my_function IS $$
This function does something important.

Parameters:
...
Example usage:
...
$$;

Note: the $$ delimits multi-line strings (called dollar quoted string constants).

Converting character sets in an Oracle database

I recently had to correct text data stored in an Oracle database with a wrong character set. A bug in one of our applications caused text data to be stored in the ISO 8859-1 West European character encoding. The database, however, is configured to work with UTF-8 encoded strings. The bug was fixed, but the existing data had to be corrected afterwards. If you ever encounter the same problem you can use Oracle’s CONVERT() function to fix the data. The syntax is as follows:

CONVERT(text, to_charset, from_charset)

First you have to find out Oracle’s names of the desired source and target character sets. You can look them up in this table. In my case the the character set names are 'UTF8' and 'WE8ISO8859P1' (for Western European 8-bit ISO-8859-1). You can check the function by selecting from DUAL:

SELECT CONVERT('ä ö ü Ä Ö Ü ß', 'UTF8', 'WE8ISO8859P1') FROM DUAL;

If the result shows the same scrambled characters you see in the data you know that you have chosen the correct character set names:

The other direction should unscramble the characters again:

Finally, you can convert your data:

UPDATE example_table
  SET text_column=CONVERT(text_column, 'WE8ISO8859P1', 'UTF8');

Migrating a Grails application from Oracle to PostgreSQL

In my previous post I explained how to migrate an Oracle schema with data to a PostgreSQL database management system (DBMS). Besides the general tasks and issues there are additional topics to migrate a complete application using the database to the other DBMS.

In our specific case we have a grails application which we maintain since Grails 1.0 times for more than 12 years. During that time we did a ton of feature development with lots of refactoring and many database migrations. So the source database will most like not be perfectly consistent and clean.

General approach

Since Grails/GORM and the DatabaseMigration-Plugin (DBM-Plugin) do a great job at preparing an empty database with a matching schema for the application to run we let the framework tools generate the schema and only migrate the data using Ora2Pg.

Sounds simple, but how it is done in detail and what else to look for?

Generating the initial dabase schema

The DBM-Plugin provides a script to create a database changelog with a schema matching the domain model of your grails application. It is integrated in gradle, so you can grails dbm-generate-gorm-changelog initialdb.groovy to create the migration scripts providing a fitting schema. You then include this script in grails-app/migrations/changelog.groovy or replace all the migrations you had before included there with this initial database changelog.

To prepare an empty database to run with your application you call the gradle task dbmUpdate.

Checking all plain SQL code

If you are only using GORM’s dynamic finders, save()/update()/delete()-methods, HQL and the criteria API you are probably fine to run your application or perform the data mirgration step.

Our application has some specific parts where we use plain SQL. Because of syntactical differences you will want to check all the plain SQL if it works with PostgreSQL. The most obvious stuff is dealing with sequences or other queries where you need the dual table in Oracle.

Migrating the data

This is probably the part where the most things can go wrong. We had quite some work with data-inconsistencies and left-overs from manual corrections that happened over the course of running and upgrading the application for so many year. For younger and simpler applications this may not present any challenges but for us it was quite time-consuming.

Now you can use Ora2Pg to import the data. After the whole data import using Ora2Pg worked as intended you should check the value of the hibernate_sequence . This sequence is used to generate the ids of all grails domain objects.

Do not let the sequences from the autoincrement columns of the tables of your domain objects confuse you! They are not used by Grails/GORM. To avoid this confusion you can remove the default value of the id columns and the accompanying sequences.

Checking the result

You should always run acceptance or manual tests to make sufficiently sure that the migration worked as intended. There is always the possibility of a configuration or software error or some oversights in checking the application code.

If possible tests the result on a dedicated system with some snapshot of the real world data before making the switch on the production system. Good luck!

Migrating from Oracle to PostgreSQL

We are maintaining several applications with a SQL-Database as our data storage. If we can decide freely, we usually opt for PostgreSQL as the database management system (DBMS). But sometimes our clients have specific requirements because they are running the services on-premises so we use our customers’ choice. SQL is SQL anyway, is it not?

No it isn’t. And this year one of our customers asked us to migrate our application from Oracle to PostgreSQL. The migration was challenging even though we are using an object-relational mapper (ORM) and the necessary changes to our application code were very limited.

In this post I want to explain the general, application-agnostic challenges of such a migration. A follow-up will cover the application- and framework-specific issues.

Why is it not easy?

Luckily, PostgreSQL supports most common SQL features of Oracle, especially sequences, PL/SQL like scripts, triggers, foreign keys etc. and all the important datatypes. So you are mostly migrating from an inferior to a more powerful solution, at least feature and capability-wise from a client perspective. Please note that I am not judging the performance, replication, clustering and other administrative features here!

Unfortunately there is no simple and powerful enough tool to simply dump the oracle database into some standard SQL text format that you could pipe into psql or use with pg_restore. In addition there is also a challenge to convert the different number-types of Oracle to sematically equivalent PostgreSQL types etc.

Another challenge is coping with the referential integrity. Especially data in complex schemas with a lot of foreign keys are harder to migrate without proper tool support as you have to figure out the correct order of tables to restore.

Nevertheless, such a migration is doable, especially if you do not have too much scripting logic in your database. And there is a free tool to help you with all this stuff called Ora2Pg.

What can Ora2Pg do for you?

It can export the full database schema including constraints, convert datatypes based on configuration provided by you and offers a basic automatic conversion of PL/SQL code to PLPGSQL. When running the migration you can interactively choose what to migrate and what to skip. That allows you to only migrate the data into a readily prepared schema, for example.

How to run Ora2Pg?

Ora2Pg is a collection of perl scripts and configuration files so you need a system capable of running these. If you do not want to mess with your whole system and install all of the dependencies I prepared a Dockerfile able to run Ora2Pg:

FROM centos:7

# Prepare the system for ora2pg 
RUN yum install -y wget
RUN wget https://yum.oracle.com/RPM-GPG-KEY-oracle-ol7 -O /etc/pki/rpm-gpg/RPM-GPG-KEY-oracle

COPY ol7-temp.repo /etc/yum.repos.d/
RUN yum install -y oraclelinux-release-el7
RUN mv /etc/yum.repos.d/ol7-temp.repo /etc/yum.repos.d/ol7-temp.repo.disabled
RUN yum install -y oracle-instantclient-release-el7
RUN yum install -y oracle-instantclient-basic
RUN yum install -y oracle-instantclient-devel
RUN yum install -y oracle-instantclient-sqlplus

RUN yum install -y perl perl-CPAN perl-DBI perl-Time-HiRes perl-YAML perl-local-lib make gcc
RUN yum install -y perl-App-cpanminus

RUN cpanm CPAN::Config
RUN cpanm CPAN::FirstTime

ENV LD_LIBRARY_PATH=/usr/lib/oracle/21/client64/lib
ENV ORACLE_HOME=/usr/lib/oracle/21/client64

RUN perl -MCPAN -e 'install DBD::Oracle'

COPY ora2pg-21.1.tar.gz /tmp

WORKDIR /tmp
RUN tar zxf ora2pg-21.1.tar.gz && cd ora2pg-21.1 && perl Makefile.PL && make && make install

RUN mkdir -p /migration
RUN ora2pg --project_base /migration --init_project my_project
WORKDIR /migration/my_project

# uncomment this if you have a customized ora2pg.conf
#COPY ora2pg.conf /migration/my_project/config/

CMD ora2pg -t SHOW_VERSION -c config/ora2pg.conf && ora2pg -t SHOW_TABLE -c config/ora2pg.conf\
 && ora2pg -t SHOW_REPORT --estimate_cost -c config/ora2pg.conf\
 && ./export_schema.sh && ora2pg -t INSERT -o data.sql -b ./data -c ./config/ora2pg.conf

Here are the commands and the workflow to export the oracle database using the above docker image:

docker build -t o2pg .
# this will fail initially but create the project structure and generate a default configuration file
docker run --name oracle-export o2pg
# copy the project structure to the host system
docker cp oracle-export:/migration/my_project ./my_project_migration/

Now you can edit the configuration in my_project_migration/config and copy it to the directory you have built and run the docker commands. Most importantly you have to change the connection parameters at the top of the ora2pg.conf file. When you are ready to make the first go you need to enable configuration copying in the Dockerfile and rebuild the image. Now you should get your first somehow usable export.

The most import config options we changed for our projects are:

  • Connection parameters
  • Excluded tables that you do not want to migrate
  • Deletion of the contents of the target tables
  • Conversion of some datatypes like NUMBER(*,0) to bigint and NUMBER:1 to boolean for some columns

Most of the defaults are sensible to begin with but you can tailor the export specifically to your needs. If you feel ready to try the import you can run the import using a second docker image based on the following Dockerfile-import:

FROM centos:7

# Prepare the system for ora2pg 
RUN yum install -y wget
RUN wget https://yum.oracle.com/RPM-GPG-KEY-oracle-ol7 -O /etc/pki/rpm-gpg/RPM-GPG-KEY-oracle

COPY ol7-temp.repo /etc/yum.repos.d/
RUN yum install -y oraclelinux-release-el7
RUN mv /etc/yum.repos.d/ol7-temp.repo /etc/yum.repos.d/ol7-temp.repo.disabled
RUN yum install -y oracle-instantclient-release-el7
RUN yum install -y oracle-instantclient-basic
RUN yum install -y oracle-instantclient-devel
RUN yum install -y oracle-instantclient-sqlplus
RUN yum install -y postgresql-server

RUN yum install -y perl perl-CPAN perl-DBI perl-Time-HiRes perl-YAML perl-local-lib make gcc
RUN yum install -y perl-App-cpanminus

RUN cpanm CPAN::Config
RUN cpanm CPAN::FirstTime

ENV LD_LIBRARY_PATH=/usr/lib/oracle/21/client64/lib
ENV ORACLE_HOME=/usr/lib/oracle/21/client64

RUN perl -MCPAN -e 'install DBD::Oracle'

COPY ora2pg-21.1.tar.gz /tmp

WORKDIR /tmp
RUN tar zxf ora2pg-21.1.tar.gz && cd ora2pg-21.1 && perl Makefile.PL && make && make install

# you need to mount the project volume to /my_project
WORKDIR /my_project

ENV pg_port=5432

CMD ./import_all.sh -d $pg_db -h $pg_host -p $pg_port -U $pg_user -o $pg_user

To run the import with your exported project run build and run the import container as follows:

docker build -t postgres-import -f Dockerfile-import .
docker run -it --rm -e pg_host=target-db.intranet -e pg_db=my_project_db -e pg_user=my_db_user -v ./my_project_migration:/my_project postgres-import

Then you can interactively provide the database password and decide which migration steps to perform.

Caveat

Depending on your schema, data and privileges in the target database it may be necessary to disable all triggers before importing and reenable them after a successful import. This can done by replacing all occurences of TRIGGER USER by TRIGGER ALL in the file data/data.sql. You may need appropriate privileges for this to work.

Final words

Such a migration is not an easy task but may be worth it in total cost of ownership and maybe developer satisfaction as Oracle has some oddities and limitations for backend developers.

I will tackle some application specific issues with such a migration in a follow-up article that we encountered when migrating our system from Oracle to PostgreSQL using the above approach and tools.

Pagination in SQL

Pagination is the task of dividing a data set into subsequent parts of the whole data set. For example, a search engine initially only shows the first 15 results for a search query. The user can then step through the rest of the results the by clicking a “Next” button.

Ideally this feature is also supported by the underlying database system. Otherwise, the application would have to load all matching data records from the database, just to filter out the major part of of them, because the user only wanted to see page 3 of 50. A pagination request has two components: a limit and an offset. If a page contains a maximum of 15 items and page 3 is requested, then the limit would be 15 and the offset would be 30 = (page-1) × limit.

PostgreSQL, MySQL, MariaDB

The database systems PostgreSQL, MySQL and MariaDB have a straight forward syntax for pagination: LIMIT {number} OFFSET {number} . So a simple SQL query with pagination might look like this:

SELECT * FROM users ORDER BY name LIMIT 15 OFFSET 30;

Oracle DB

Oracle DB didn’t have a dedicated syntax for pagination before Oracle 12c, but it was still possible to achieve the same result with other means. With Oracle 12c a new syntax for pagination was introduced under the name “Row limiting clause”. First I’ll show the old method, then the new syntax.

The old method is based on ROWNUM . If you wanted to specify both an offset and a limit, you had to nest multiple queries:

SELECT *
FROM (SELECT *, rownum AS rnum
      FROM (SELECT *
            FROM users
            ORDER BY name)
      WHERE rownum < 45)
WHERE rnum >= 30;

The newer row limiting clause syntax is shorter and looks as follows:

SELECT * FROM users ORDER BY name
  OFFSET 30 ROWS FETCH NEXT 15 ROWS ONLY;

This syntax also allows the option to specify a percentage of rows instead of a fixed number of rows:

SELECT * FROM users ORDER BY name
  FETCH FIRST 20 PERCENT ROWS ONLY;

MS SQL Server

Microsoft’s SQL Server also supports the Oracle-like syntax with OFFSET and FETCH clauses and recommends the usage of this syntax for pagination.

The World of SQL Dialects

For software projects I work with various relational database management systems (RDBMs), mainly PostgreSQL, MySQL/MariaDB, Oracle Database and Microsoft SQL Server. All of these use SQL as a query language, but the dialects of this language vary wildly, especially when it comes to non-standardized features. One such feature I often use is the aggregation of a list to a string. It does the following.

LEGS    ANIMAL
-----------------
2       Ostrich
2       Human
4       Cat
4       Dog
4       Capybara
6       Ant
8       Spider

Given a table like the one above it groups the elements of a column that have the same value in another column together in a string, concatenated by a separator like a comma: 

LEGS    ANIMALS
----------------------------
2       Human, Ostrich
4       Capybara, Cat, Dog
6       Ant
8       Spider

This simple operation has four different syntaxes in the four mentioned database systems, which I want to demonstrate.

PostgreSQL

In PostgreSQL the function is called STRING_AGG:

SELECT legs,
  STRING_AGG(animal, ', ' ORDER BY animal) AS animals
FROM fauna
GROUP BY legs
ORDER BY legs;
MySQL / MariaDB

In MySQL and its fork MariaDB the function is called GROUP_CONCAT, and it has a special syntax to specify the separator:

SELECT legs,
  GROUP_CONCAT(animal ORDER BY animal SEPARATOR ', ') AS animals
FROM fauna
GROUP BY legs
ORDER BY legs;
Oracle

Oracle calls it LISTAGG and specifies the grouping via WITHIN GROUP.

SELECT legs,
  LISTAGG(animal, ', ') WITHIN GROUP (ORDER BY animal) AS animals
FROM fauna
GROUP BY legs
ORDER BY legs;
Microsoft SQL Server

SQL Server calls it STRING_AGG like PostgreSQL, but specifies the grouping via WITHIN GROUP like Oracle:

SELECT legs,
  STRING_AGG(animal, ', ') WITHIN GROUP (ORDER BY animal) AS animals
FROM fauna
GROUP BY legs
ORDER BY legs;

Unfortunately, as developers we have to live with all these dialects of SQL. Even though there is an ISO standards committee for SQL, database creators love to build non-standard extensions into their products. The situation is worse than the browser-specific extensions and differences of JavaScript, HTML and CSS in modern web browsers. One thing that can paper over these differences are OR-Mappers like Hibernate or query languages like Hibernate’s HQL that abstract over SQL, but they come with their own set of problems.