Each record must have a unique, primary key which determines where in a file that record is stored. To retrieve a record, its key is hashed and the resultant value specifies which of a set of discrete "buckets" (called "groups") to look in for the record. (Within a bucket, records are scanned sequentially.) Therefore, most records (e.g. a complete document) can be read using one single disk-read operation. This same method is used to write the record back to its correct "bucket".