今天更新redmine的时候，需要安装mysql gem包。
结果一直报错：
Building native extensions. This could take a while... ERROR: Error installing mysql: ERROR: Failed to build gem native extension. /usr/bin/ruby1.8 extconf.rb --with-mysql-config=/usr/local/mysql/bin/mysql_config extconf.rb:10: command not found: /usr/local/mysql/bin/mysql_config --cflags *** extconf.rb failed *** Could not create Makefile due to some reason, probably lack of necessary libraries and/or headers. Check the mkmf.log file for more details. You may need configuration options. Provided configuration options: --with-opt-dir --without-opt-dir --with-opt-include --without-opt-include=${opt-dir}/include --with-opt-lib --without-opt-lib=${opt-dir}/lib --with-make-prog --without-make-prog --srcdir=. --curdir --ruby=/usr/bin/ruby1.8 --with-mysql-config
在用尽我对ruby可怜的已知知识之后，开始google，尝试了第一种方法：
http://blog.bmn.name/2008/02/rails-gem-install-mysql-throws-error-extconfrb-failed/
但是我痛苦地发现我使用deb包安装的mysql没有mysql_config这个文件夹。
接着在万能的stackoverflow上找到了解决办法：
http://stackoverflow.com/questions/5795309/gem-install-mysql-fail
安装了mysql客户端的dev库包：
sudo apt-get install libmysqlclient-dev
# 该包的描述是：This package includes development libraries and header files.
然后就欢快地通过了gem install mysql~
PS:同理，如果安装Postgresql的pg gem包时，如果缺少libpq-dev包，也会导致同样的错误。