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Displaying posts with tag: Oracle (reset)
MySQL InnoDB Cluster Disaster Recovery contingency via a Group Replication Replica

Just recently, I have been asked to look into what a Disaster Recovery site for InnoDB Cluster would look like.

If you’re reading this, then I assume you’re familiar with what MySQL InnoDB Cluster is, and how it is configured, components, etc.

Reminder: InnoDB Cluster (Group Replication, Shell & Router) in version 8.0 has had serious improvements from 5.7. Please try it out.

So, given that, and given that we want to consider how best to fulfill the need, i.e. create a DR site for our InnoDB Cluster, let’s get started.

Basically I’ll be looking at the following scenario:

InnoDB Cluster Source site with a Group Replication Disaster Recovery Site.

Now, just before we get into the nitty-gritty, here’s the scope.

Life is already hard enough, so we want as much automated as possible, so, yes, InnoDB Cluster gets some of that done, but there are other parts we will still have …

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MySQL Linux to Windows

My students want to transfer their course setup MySQL files from the Linux VM that I provide to Microsoft Windows 10. This post explains how because I found a couple small errors in the Google’d posts they would most likely see.

The first step is required because when I didn’t assign a name or domain to the the Fedora VM, which allows it to run as localhost on any student machine. In tandem, I didn’t assign a static IP address but opted for dynamic IP assignment. That means, the first step to securely copy the files requires you to find the assigned IP address. You can do that with the following Linux command:

ifconfig -a | grep 'inet[[:blank:]]' | head -1 | cut -c 14-30

It would return something like:

192.168.147.198

After you have discovered the IP address, you need to download PuTTy from their web site because includes the pscp (PuTTy …

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SQL EXISTS and NOT EXISTS

Introduction In this article, we are going to see how the SQL EXISTS operator works and when you should use it. Although the EXISTS operator has been available since SQL:86, the very first edition of the SQL Standard, I found that there are still many application developers who don’t realize how powerful SQL subquery expressions really are when it comes to filtering a given table based on a condition evaluated on a different table. Database table model Let’s assume we have the following two tables in our database, that form a one-to-many... Read More

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A beginner’s guide to SQL CROSS JOIN

Introduction In this article, we are going to see how a CROSS JOIN works, and we will also make use of this SQL join type to build a poker card game. Database table model For our poker card game application, we have created the ranks and suits database tables: The ranks table defines the ranking of cards, as well as the name and symbol used for each card rank: The suits table describes the four possible categories used by the French playing cards: Cartesian product In the set theory, the Cartesian product... Read More

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A beginner’s guide to database deadlock

Introduction In this article, we are going to see how a deadlock can occur in a relational database system, and how Oracle, SQL Server, PostgreSQL, or MySQL recover from a deadlock situation. Database locking Relational database systems use various locks to guarantee transaction ACID properties. For instance, no matter what relational database system you are using, locks will always be acquired when modifying (e.g., UPDATE or DELETE) a certain table record. Without locking a row that was modified by a currently running transaction, Atomicity would be compromised. Using locking for controlling access... Read More

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JDBC Driver Maven dependency list

Introduction Ever wanted to connect to a relational database using Java and didn’t know which JDBC Driver Maven dependency to use? If so, this article is surely going to help you from now on. Oracle Since September 2019, the Oracle JDBC Driver is available on Maven Central. For JDK 10 and 11, use the following Maven dependency: For JDK 8, use the ojdbc8 artifact instead: For more details about the proper version to use, check out the following Maven Central link. MySQL The MySQL Driver is available on Maven Central, so just... Read More

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Session Variables

In MySQL and Oracle, you set a session variable quite differently. That means you should expect there differences between setting a session variable in Postgres. This blog post lets you see how to set them in all three databases. I’m always curious what people think but I’m willing to bet that MySQL is the simplest approach. Postgres is a bit more complex because you must use a function call, but Oracle is the most complex.

The difference between MySQL and Postgres is an “@” symbol versus a current_setting() function call. Oracle is more complex because it involves the mechanics in Oracle’s sqlplus shell, SQL dialect, and PL/SQL language (required to assign a value to a variable).

MySQL

MySQL lets you declare a session variable in one step and use it one way in a SQL statement or stored procedure.

  1. You set a session variable on a single line with the following …
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What’s Faster? COUNT(*) or COUNT(1)?

One of the biggest and undead myths in SQL is that COUNT(*) is faster than COUNT(1). Or was it that COUNT(1) is faster than COUNT(*)? Impossible to remember, because there’s really no reason at all why one should be faster than the other. But is the myth justified?

Let’s measure!

How does COUNT(…) work?

But first, let’s look into some theory. The two ways to count things are not exactly the same thing. Why?

  • COUNT(*) counts all the tuples in a group
  • COUNT(<expr>) counts all the tuples in a group for which <expr> evaluates to something that IS NOT NULL

This distinction can be quite useful. Most of the time, we’ll simply COUNT(*) for convenience, but there are (at least) two cases where we don’t want that, for example:

When outer joining

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Entity Framework 6.3 and .NET Core 3 Support

.NET Core 3 .NET Core was presented by Microsoft in 2016, but its 1.x versions had limited set of features comparing to Full .NET Framework. Since then .NET Core has been drastically improved. .NET Core 2.0 has a significant part of Full .NET Framework features and includes new functionality and significant performance optimizations. This year, […]

How to limit the SQL query result set to Top-N rows only

Introduction In this article, we are going to see how we can limit the SQL query result set to the Top-N rows only. Limiting the SQL result set is very important when the underlying query could end up fetching a very large number of records, which can have a significant impact on application performance. Why limit the number of rows of a SQL query? Fetching more data than necessary is the number one cause of data access performance issues. When a given business use case is developed, the amount of data available... Read More

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