如 SQL 标准中所述，此函数返回输入字符串的八位字节长度或字节长度。
This function, as mentioned in SQL standard, returns the length of the input string in octets or bytes.
第一个字节定义后面紧跟的八位元数，如果是7f或更小，这就是等价的ascii值。
The first byte defines the number of octets to follow, or if it is 7f or less, it is the value of an ASCII equivalent.
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