What is Indexing?

Storing <key="user's node address", value="user's node address"> into seperate database for faster reterival is indexing.
Example:
  consider 100M entries in facebook social Graph. if user wants to see his news feed, we cannot search all nodes Rather start at node(for user), and this is achieved by storing node's address. 

Hash Table: (Key: user_id, Value: user_address)

key(userid)       Value=Address in Memory
123456            0x40      //user1
42                0x66      //user2
Indexing

Reverse Indexing

Reverse indexing is used for efficient reverse lookups.
See Indexing.
Reverse Indexing is reverse mapping. <key=user's node address, value=user_id's>, it will help to map all userId's associated with a particular address.

Reverse Hash Table: 

(Key: user_address, Value: List of user_ids associated with that address)

When Reverse Lookup is useful

when we need to perform lookups based on attributes other than the primary key (user ID)