As of 2012-10-26, Percona (a fork of the MySQL database server) isn't in the FreeBSD ports system. Unfortunatelly, Percona doesn't build out of the box on FreeBSD 9.0 amd64. Instead, you have to follow these steps:
sudo make -C /usr/ports/devel/cmake install clean
sudo make -C /usr/ports/devel/bison install clean
tar xzf Percona-Server-5.5.27-rel29.0.tar.gz
cd Percona-Server-5.5.27-rel29.0 patch -p1 <../Percona-Server-5.5.27-rel29.0.FreeBSD_9_0.patch
cmake -D WITH_ARCHIVE_STORAGE_ENGINE:BOOL=y -D WITH_FEDERATED_STORAGE_ENGINE:BOOL=y -D WITH_READLINE:BOOL=y -D WITH_SSL:STRING=system -D CMAKE_INSTALL_PREFIX:STRING=/usr/local/percona .
make
sudo make install
export PATH=$PATH:/usr/local/percona/bin
sudo pw groupadd mysql -g 88 sudo pw useradd mysql -u 88 -g mysql -c "MySQL Daemon" -d /var/db/mysql -s /usr/sbin/nologin sudo mkdir -m 0750 /var/db/mysql sudo chown mysql:mysql /var/db/mysql
sudo cp usr/ports/databases/mysql55-server/files/mysql-server.in /usr/local/etc/rc.d/mysql-server sudo chmod 0755 /usr/local/etc/rc.d/mysql-server
cd /usr/local/etc/rc.d sudo patch -p0 < ~/mysql-server.percona.patch sudo rm mysql-server.orig
mysql_enable="YES"
[mysqld] innodb_file_per_table = 1 innodb_log_files_in_group = 2 innodb_log_file_size = 256M
/usr/local/etc/rc.d/mysql-server start