Indexing in Databases

Advantages:
Searching (Retrieval) is faster as the data is sorted in that column so internally a Binary Search can 
be used which reduces the time from O(N) i.e. linear learch to O(lg N) i.e. binary search.
Hence searching/retrieving becomes faster(i.e. database is optimized)

Disadvantage:
1) Takes extra space
2) Insertion, Updation and Deletion is slow since if tuples are inserted, updated or deleted after 
creating indexes then the indexes are also modified and reassigned every time for every insert/update/delete
operation.

Please watch the below video of 10 minutes carefully:
https://www.youtube.com/watch?v=ow6eSj-dNKg