These entry numbers are not unique and they change if the current index is changed, or if the selection of entries is changed. Samples allow pointers to selected records to be saved in a file for later recall. The record numbers that make up a sample are stored in a file which is ordered on the value of a particular parameter. The pointers contained in the file are then used to access entries in that order. The index file is used both to search rapidly for all entries corresponding to a range of values, and to list the selected sample in the order of a particular parameter value.
All indexes are in descending order.
The number of disk reads required to perform the search determines the speed at which the records are found. To optimize the search, an index file also contains the ordered parameter values. Only one index can be used at a time. Standard indexes are provided for commonly used parameters, such as coordinates , time , and name . If a search parameter is not indexed a sequential search is made.