前言

MySQL支援的單列索引長度是3072bytes，有的欄位以“位元組byte”為單位，有的欄位以“字元character”為單位，又因不同的字符集一個字元佔bytes長度不一樣，導致建立索引時比較頭疼。

索引長度計算：

• 索引欄位如果沒有設定NOT NULL，則需要新增一個位元組。
• 定長欄位，如INT佔4個位元組、DATE佔3個位元組、CHAR(N)佔N個字元。（注意字元與位元組的區別，字元的長度取決於當前的字符集）
• 變長欄位，如VARCHAR(N)佔用N個字元+2個位元組。

字符集長度

(root@localhost) [information_schema]> select * from information_schema.CHARACTER_SETS;
+--------------------+----------------------+---------------------------------+--------+
| CHARACTER_SET_NAME | DEFAULT_COLLATE_NAME | DESCRIPTION                     | MAXLEN |
+--------------------+----------------------+---------------------------------+--------+
| big5               | big5_chinese_ci      | Big5 Traditional Chinese        |      2 |
| dec8               | dec8_swedish_ci      | DEC West European               |      1 |
| cp850              | cp850_general_ci     | DOS West European               |      1 |
| hp8                | hp8_english_ci       | HP West European                |      1 |
| koi8r              | koi8r_general_ci     | KOI8-R Relcom Russian           |      1 |
| latin1             | latin1_swedish_ci    | cp1252 West European            |      1 |
| latin2             | latin2_general_ci    | ISO 8859-2 Central European     |      1 |
| swe7               | swe7_swedish_ci      | 7bit Swedish                    |      1 |
| ascii              | ascii_general_ci     | US ASCII                        |      1 |
| ujis               | ujis_japanese_ci     | EUC-JP Japanese                 |      3 |
| sjis               | sjis_japanese_ci     | Shift-JIS Japanese              |      2 |
| hebrew             | hebrew_general_ci    | ISO 8859-8 Hebrew               |      1 |
| tis620             | tis620_thai_ci       | TIS620 Thai                     |      1 |
| euckr              | euckr_korean_ci      | EUC-KR Korean                   |      2 |
| koi8u              | koi8u_general_ci     | KOI8-U Ukrainian                |      1 |
| gb2312             | gb2312_chinese_ci    | GB2312 Simplified Chinese       |      2 |
| greek              | greek_general_ci     | ISO 8859-7 Greek                |      1 |
| cp1250             | cp1250_general_ci    | Windows Central European        |      1 |
| gbk                | gbk_chinese_ci       | GBK Simplified Chinese          |      2 |
| latin5             | latin5_turkish_ci    | ISO 8859-9 Turkish              |      1 |
| armscii8           | armscii8_general_ci  | ARMSCII-8 Armenian              |      1 |
| utf8               | utf8_general_ci      | UTF-8 Unicode                   |      3 |
| ucs2               | ucs2_general_ci      | UCS-2 Unicode                   |      2 |
| cp866              | cp866_general_ci     | DOS Russian                     |      1 |
| keybcs2            | keybcs2_general_ci   | DOS Kamenicky Czech-Slovak      |      1 |
| macce              | macce_general_ci     | Mac Central European            |      1 |
| macroman           | macroman_general_ci  | Mac West European               |      1 |
| cp852              | cp852_general_ci     | DOS Central European            |      1 |
| latin7             | latin7_general_ci    | ISO 8859-13 Baltic              |      1 |
| utf8mb4            | utf8mb4_general_ci   | UTF-8 Unicode                   |      4 |
| cp1251             | cp1251_general_ci    | Windows Cyrillic                |      1 |
| utf16              | utf16_general_ci     | UTF-16 Unicode                  |      4 |
| utf16le            | utf16le_general_ci   | UTF-16LE Unicode                |      4 |
| cp1256             | cp1256_general_ci    | Windows Arabic                  |      1 |
| cp1257             | cp1257_general_ci    | Windows Baltic                  |      1 |
| utf32              | utf32_general_ci     | UTF-32 Unicode                  |      4 |
| binary             | binary               | Binary pseudo charset           |      1 |
| geostd8            | geostd8_general_ci   | GEOSTD8 Georgian                |      1 |
| cp932              | cp932_japanese_ci    | SJIS for Windows Japanese       |      2 |
| eucjpms            | eucjpms_japanese_ci  | UJIS for Windows Japanese       |      3 |
| gb18030            | gb18030_chinese_ci   | China National Standard GB18030 |      4 |
+--------------------+----------------------+---------------------------------+--------+