The Index Scan icon indicates that the entire temporary index will be scanned. All of the entries contained within the index will be processed, unless the icon label is Index Scan Distinct or Index Scan Distinct, Preload, in which case, for any duplicate key entries that would be selected, only one of the entries will be processed. Any selection criteria, whose predicates match the key columns of the index, can then be applied against the index entries. Only those key entries that match the specified key selection will be used to select rows from the corresponding table data.
Also referred to as: Index Scan, Preload; Index Scan Distinct; Index Scan Distinct, Preload
See Tips for index scan for more details.