However, a record of variable length can possibly be updated to a length that no longer fits on the original page, and so it must be moved to another page.
而对于可变长度的记录，其长度可能会变得不再适合初始页，因此必须将其转移到其他页。
Each index record contains a five-byte header that may be preceded by a variable-length header.
每个索引记录包含一个五字节的头，在它之前还有一个变长的头。
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。
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