Difference between Index column and simple column in SharePoint?
What is the need of creating index column ?
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An indexed column in SharePoint makes sense to improve performances.
If your list contain a lot (~1000s) of records, and you often query (filter or sort) by a given column, you can index that column. That way, when displaying a filtered/sorted view (based on this column), SharePoint does not have to loop throuh all the items to decide whitch ones to display (firts): thanks to the index it can quickly get the elements it needs.
A lot of articles on the Web can be found, that discuss the topic deeply:
As you can see, it's only a matter of googling something like "sharepoint indexed column"...