handling of variable length field [计] 可变长字段处理
For each non-NULL variable-length field, the record header contains the length of the column in one or two bytes.
对于每个非空的变长字段，记录头使用一或两个字节包含了列的长度。
A field in which the number of the characters can be changed. A variable-length field must contain a data area to indicate its end and length.
字符数量可变的一种字段，它必须包含一个数据域以指示字段的终止及长度。
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