An index for a table is data organization that enables certain quires to access one or more records of that table fast. Proper using of indexes is therefore essential to high performance. Improper selection of indexes can lead to the following mishaps.

- Indexes that are maintained but never used

- Fields that are scanned in order to return a single record

- Multitable joins that run on for hours because the wrong indexes are present

When a query is initiated without an index, 4D starts at the beginning of the table being queried and examines each record sequentially, one by one. When querying with an index, the index structure is typically traversed first - potentially skipping many records - then 4D returns the matching records.