Cover Index :
It contains all the columns which query use That means we have to use all the columns which are available in select statement , as well as in WHERE statement to create our cover index
Create Cover Index :
Your indexes can go slower with the time if you have very heavy OLTP workload. if you have lots of insert , update or deletes operation on your table, it's quite possible your data has moved around a lot in your table, and it has created a lot of empty spaces in your B-tree indexes. if your table is not changed much, you do not need to reindex it, but if it is changed quite a lot, then it is very, important that you
remove all of the inefficiency of index by reindex it periodically. Reindexing it will improve performance of your index very much. It is absolutely a safe operation , and with syntax concurrently, You should execute this command only on those tables where there is heavy OLTP workload and you should run it periodically.
Example: I reindex all of my table where there is significant change every week. That means I have a job which executes once a week on the table where I have slow performance. Let's understand the syntax of reindex
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