Indexing in AEM 6.1

Hi,

I am trying to understand more about indexing in AEM 6.1. I have gone through Adobe forums and understood below things related to indexing

     What needs to be indexed - Basically long running queries needs to be indexed to improve the performance.

     How do we know which is running longer time - Long running queries can be found from "Query performance" console. Also, if a query doesn't have indexing and if it needs to be indexed then, warnings will be logged in "error.log" like below *WARN* Traversed 1000 nodes with filter Filter(query=select …) consider creating an index or changing the query.

I am looking to understand the below things:

     1. What is indexing and exactly what is happening internally within the repository during indexing?

     2. How are we getting the results faster for the queries which is indexed? (or) After indexing are the results being stored in some location and the results are picked up from that location instead of traversing through the complete repository?

Note: I have also created indexing for long running queries in my application and have seen statements related o re-indexing in error.log which states that re-indexing completed successfully. Also I ran re-indexing for custom index and noticed 2 properties that added to the custom index

     reindex - false --> which means re-indexing was completed

    reindexCount - 2 --> re-indexing ran twice

Thanks & Regards,

Dharanya Manokaran