Sparse dense difference index index
Each code value intensive search index file corresponds to an index value
Sparse index file index entries for only some of the value of the index code
MySQL's InnoDB
If a primary key is defined as the primary key index intensive
If no primary key definition, a unique non-null the first index of the table is a dense index
If the above conditions are satisfied, it generates a hidden internal innodb primary key (dense Index)
Primary key value related to non-primary key index is stored bit and its corresponding key, comprising two lookup.
InnoDB index and data are stored together (a file)
And MyISAM index and data storage are separated (two files)