http://amarok.kde.org/blog/archives/812-MySQL-in-Amarok-2-Th...At first, SQLite seemed like a good choice. Using transactions, it's decently fast. It's pretty stable (those that complain about odd MySQL bugs should talk to markey, as he, being the SQLite maintainer in 1.4, can attest that SQLite's had its fair share). However, there were a few problems that in the end knocked it out of the running. The first problem is performance. Although for people with small collections it performs fairly well, people with large collections that switched to the MySQL or PostgreSQL backends in A1 would report enormous speed gains when operations performing complex or many queries were performed, such as adding many entries to the playlist, scanning files, or filtering/searching in the collection. Since we want to accommodate users with large collections just as well as those with smaller collections, and since digital music collections aren't getting smaller, the speed increase for our users with large collections was quite important. Many of our developers, after the switch to mysqle (as we call it, though that's not the official name), have noticed huge speed increases in their day-to-day use of A2, so that speed increase is carrying through to the embedded server as well as the normal server. That was the first knock against SQLite.
The other blow for SQLite came for a totally different reason. Many users (myself included) have multiple computers sharing a single Amarok database. Assuming all the computers have access to the music at the same mount point (and a few other things are configured right), this allows you to scan once, play everywhere, update the same ratings no matter where you play it, and more. Even if your aren't sharing the database among multiple computers, many users want their database stored on a particular server for speed, security, or backup reasons. If you think either of these isn't a common use-case, you'd be quite wrong. MySQL and PostrgreSQL were quite happy with this workload. It's a total no-go for SQLite, simply because it's designed for a different purpose. So SQLite had two big knocks against it. K.O.
However, just as we can't rely on the user to set up Strigi/Nepomuk correctly, we can't rely on them to get their tables set up in MySQL or PostgreSQL. So we needed the database to be embeddable, so that it could just work for the user without any setup necessary on their part. MySQL, with libmysqld, had the seeds of this in the 4.1 series, it works decently in 5.0, and it's becoming fully supported (AFAIK) in 5.1. PostgreSQL, on the other hand, does not have any such thing. (They have an interesting and cool concept of their own of embedded SQL though. Update: apparently that is part of the SQL standard. Still pretty cool. Still totally different from what we mean when we are talking about an embedded server.)
So this leaves us with -- as you guessed -- MySQL.
