innodb change buffer记录格式

/** Structure for an SQL data tuple of fields (logical record) */
struct dtuple_t {
	ulint		info_bits;	/*!< info bits of an index record:
					the default is 0; this field is used
					if an index record is built from
					a data tuple */
	ulint		n_fields;	/*!< number of fields in dtuple */
	ulint		n_fields_cmp;	/*!< number of fields which should
					be used in comparison services
					of rem0cmp.*; the index search
					is performed by comparing only these
					fields, others are ignored; the
					default value in dtuple creation is
					the same value as n_fields */
	dfield_t*	fields;		/*!< fields */
	UT_LIST_NODE_T(dtuple_t) tuple_list;
					/*!< data tuples can be linked into a
					list using this field */
};
struct dfield_t{
	void*		data;	/*!< pointer to data */
	unsigned	ext:1;	/*!< TRUE=externally stored, FALSE=local */
	unsigned	len:32;	/*!< data length; UNIV_SQL_NULL if SQL null */
	dtype_t		type;	/*!< type of data