Descending scans may deadlock with queries performing update operations using ascending scans and with queries performing page splits and shrinks, except when the backward scans are performed at transaction isolation level 0.
The allow backward scans configuration parameter controls whether the optimizer uses the backward scan strategy. The default value of 1 allows descending scans.
See the System Administration Guide for more information on this parameter.
Also, see “Index scans” for information on the number of ascending and descending scans performed and “Deadlocks by lock type” for information on detecting deadlocks.