The following table shows a simplified example of data files for a table that includes customer IDs:

Data File	Customer ID	Range of Values
1	1000, 1005, 1010, 1015, 1020, 1025	1000-1025
2	1001, 1006, 1011, 1016, 1021, 1026	1001-1026
3	1002, 1007, 1012, 1017, 1022, 1027	1002-1027
4	1003, 1008, 1013, 1018, 1023, 1028	1003-1028
5	1004, 1009, 1014, 1019, 1024, 1029	1004-1029

You run a query to search for customer ID 1011. Without the index, the query processes all data files because 1011 is in the value range of all the data files. You create an index on the customer ID column and run the query again. The query performance improves because the query only processes one data file. Data file 2 is the only data file that includes the value 1011.