A table is built on the top of the file. The queries and frequency of queries will decide the need for making index on one or more fields. There will be an space overhead for storing the table but this need must be justified.
When the size of index table is too big, only a small portion of this table can be fetched into main memory and keeping the keys in sorted order becomes impossible. Hence this entire thing is maintained using B Tree. The same information of index table with essential organization can be represented using B Tree.
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