SharePoint list column added to list index to improve the column performance when filter content as it improves the performance of query operations which can include filters, sorting and joins on index columns. You can add single column index or two column indices for a list. Index column are similar to index column in database, but these are stored in SharePoint as they are Name value pair instead of SQL Server. You can not add an index column if no. of items in a list exceeds 20000 list items, so you need to add them before it crosses the 20K limit. You can add up to 20 indices for a list. Not all list
SharePoint list ID column is by default an index column. You can add single line text, number, date, Boolean and metadata fields as list item index. Whereas, you can add multiline text, Hyperlink/Picture and calculated fields to list index. Choice and Person or Group can be added to the list index if they are single value.
Below table shows the list of supported and unsupported field for list indexing
|Supported Field Types||Unsupported Field Types|
|Text (Single Line)||Text (Multiline Line)|
|Choice (Single value)||Choice (Multi value)|
|Person or Group (Single value)||Person or Group (Multi value)|
|Date & Time||Calculated|