A great choice of database for a very simple and small application, such as a blog or forum. A very bad database for any enterprise application, or even for educational purposes, since it is highly unstandard and will teach you bad habits (unstandard casting, implicit nonsensical casting such as strings to numbers, weak subqueries, too simple "explain" output, no schemas, case sensitivity problems, annoying but manageable unicode support, no timezone support, etc.). Instead, choose PostgreSQL for professional or other large tasks.
MySQL is the fastest free open source database when you are NOT using transactions (using myisam engine) (which is why it is so great for blogs). But when you are using transactions (using innodb engine) (which any enterprise application must always use) it is VERY slow.
A properly optimized query written by a master or that you spent days writing might be fastest in Oracle, but in all other cases, PostgreSQL beats both Oracle and MySQL (innodb).
"My blog server goes so fast since I started using MySQL's myisam engine."
"MySQL's lack of time zone support is such a pain in the ass."
"MySQL is still running crash recovery, since 2 weeks ago (due to a known bug.... uh 'feature request' according to their developers, where it uses a primitive and ultra slow bubble sort to sort the transactions) . I wonder if my business will ever be up and running again."
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