Dameng database_ LENGTH_ IN_ Influence of char and charset

Concept ：LENGTH_IN_CHAR:VARCHAR Whether the length of the type object is in characters .

1： yes , When set to in characters , The definition length is not really adjusted according to the character length , Instead, the stored length value is enlarged according to the theoretical character length . Therefore, the actual number of characters that can be inserted exceeds the defined length , This is also allowed . meanwhile , Length of bytes stored 8188 The upper limit remains unchanged , in other words , Even if the column length is defined as 8188 character , The total byte length of the string that can actually be inserted still cannot exceed 8188;0： no , all VARCHAR The length of the type object is in bytes .

Concept ：CHARSET/UNICODE_FLAG： Character set options .0 representative GB18030,1 representative UTF-8,2 Stands for Korean character set EUC-KR.

GB18030 Code down compatible GBK and GB2312, Compatibility means not only character compatibility , And the encoding of the same characters is the same .

UTF-8 Is used to solve the international character of a multi byte encoding , It uses English 8 position （ A byte ）, Use of Chinese 24 by （ Three bytes ） Encoding .

GB18030 Numbers and letters account for 1 Bytes , Ordinary Chinese characters account for 2 Bytes , Some traditional and minority languages account for 4 byte ,Unicode Use... In daydream UTF-8 Coding format , European alphabetic characters account for 1 To 2 Bytes , Most of the characters in Asia account for 3 Bytes , The additional characters are 4 Bytes . If only Chinese and alphanumeric characters are stored , Generally speaking GB18030 Save more space .

Note these two parameters are generally used together ,CHARSET/UNICODE_FLAG Once it is determined that it cannot be modified , It needs to be set carefully .

such as ：UNICODE_FLAG=1、LENGTH_IN_CHAR=0 A Chinese character occupies three bytes , One English occupies one byte, as shown in the figure ：

UNICODE_FLAG=1、LENGTH_IN_CHAR=1 One Chinese character takes up three bytes , A letter takes up a byte , One character four bytes （ That is, a varchar The unit occupies four bytes ）, You can save four English letters

UNICODE_FLAG=0、LENGTH_IN_CHAR=1 A Chinese character takes up two bytes and one character , A letter takes up a byte , One character can store one Chinese character or two English letters .

UNICODE_FLAG=0、LENGTH_IN_CHAR=0 A Chinese character takes up two bytes , An English character takes up one byte . A Chinese character takes up two bytes .

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