Once a logical record is formed, field setting on the logical record is done. Field setting is a process in which SQL*Loader uses control-file field specifications to determine which parts of logical record data correspond to which control-file fields. It is possible for two or more field specifications to claim the same data. Also, it is possible for a logical record to contain data that is not claimed by any control-file field specification.
Most control-file field specifications claim a particular part of the logical record. This mapping takes the following forms:
The byte position of the data field's beginning, end, or both, can be specified. This specification form is not the most flexible, but it provides high field-setting performance.
The strings delimiting (enclosing, terminating, or both) a particular data field can be specified. A delimited data field is assumed to start where the last data field ended, unless the byte position of the start of the data field is specified.
You can specify the byte offset, the length of the data field, or both. This way each field starts a specified number of bytes from where the last one ended and continues for a specified length.
Length-value data types can be used. In this case, the first n number of bytes of the data field contain information about how long the rest of the data field is.