Indexes enhance database perfrmance. For example, when you find yourself frequently searching or sorting the table using first_name and last_name fields, it makes sense to create a compound index with these two fields.
Name - Name of the index. It should be unique in the database.
Unique - Requires that the set of values in the fields of the index was unique in the table. Unlike UNIQUE constraint, Unique index can apply to multiple columns.
Columns - The list of columns in an index. At least one column should be selected.
After saving the table, the index fields list cannot be saved. PostgreSQL database dictates this limitation.