這篇文章是我對explain出來之後index的長度產生的一個疑問，然後刨根問底之後總結下來的。
具體可見：http://topic.csdn.net/u/20100727/14/9a63689e-7fdc-4b33-b215-7d08b2aef569.html
總結如下：
一、關於utf8
UTF-8 is the 8-bit encoding of Unicode. It is a variable-width encoding and also a
strict superset of 7-bit ASCII. A strict superset means
that each and every character in 7-bit ASCII is
available in UTF-8 with the same corresponding codepoint
value.
One Unicode character can be 1 byte, 2 bytes, 3 bytes or 4 bytes in this encoding.
Characters from the European scripts are represented in either 1 or 2 bytes;
characters from most Asian scripts are represented in 3 bytes, while supplementary
characters are represented in 4 bytes.
二、When a nullable column is indexed, it requires an extra byte per entry
因此，當一個char(4)的索引，它的index length是3*4+1=13