title: Index in postgresql
body: <p>Firstly, I have a table in database <strong>USERS</strong> with almost 30 Million records in it. I have different indices for each column. But some of the column have only 2 to 3 non null values while others are Null but still their index size is 847 MB a little less than the one index that contain unique value for each row. </p>
  
  <p>Can anyone know why is it like this?</p>
  
  <p>Secondly, in PostgreSQL we have a index for primary key index for each column by default what if we delete that index what will be the consequences?
  What that index is really use for? 
  As i'm searching based on values in other columns only will it be safe to delete index for primary key?</p>