Skip to content

Define Overlap Column

Overlap columns were designed to allow developers to create multi-segment (multi-column) relationships before full support for multi-segment relationships was added to DataFlex (introduced in revision 11.0). Overlap columns are now considered a legacy technique and are supported only as such.

Columns (other than Overlap) in the embedded database tables occupy contiguous lengths of bytes. Overlap columns can overlap these columns starting and ending at any byte position.

Start Offset

Start offset is the byte-offset marking the first byte of the overlap column. This is counted from the beginning position of the table's first column.

End Offset

End offset is the byte-offset marking the last byte of the overlap column. This is counted from the beginning position of the table's first column.

  • If you have already entered a length for the column, the defaults will be:
  • Start Offset: 1

  • End Offset: 1 plus the length

  • If you had previously entered offsets for the column:

  • The default start position will remain as before.
  • The default end position will be calculated to yield the length implied by your entries.

You may not specify an end offset that is less than the start offset.

Start Column

You can calculate the start offset by selecting a column in the Start Column form. When you select a start column, its start offset is placed into the Start Offset for the overlap.

End Column

You can calculate the end offset by selecting a column in the End Column form. When you select an end column, its end offset is placed into the End Offset for the overlap.

OK

Click OK to apply the entered offset definition.

Cancel

Click Cancel to close this dialog without applying the entered offset definition.

Legacy Support

Overlap columns are provided to support legacy applications. It is recommended that you do not use overlap-type columns in your database. SQL databases may not support overlap columns.

Underlap columns are columns that have a smaller byte range than a single overlapped column; underlap columns are a legacy technique that is no longer supported.

See Also