Some operations in Perl (i.e. foreach, assigning arrays) need to know the size of the whole array. When Perl needs to know the size of an array it call the method FETCHSIZE. Since not all DBD drivers/DBMS returns the number of selected rows after an SQL SELECT, the only way to really determine the number of selected rows would be to fetch them all from the DBMS. Since this could cause a lot of work, it may be very inefficent. Therefore DBIx::Recordset by default calls die() when Perl calls FETCHSIZE. If you know your DBD drivers returns the correct value in $sth -> rows after the execution of an SELECT, you can set $FetchsizeWarn to zero to let FETCHSIZE return the value from $sth -> rows. Setting it to 1 will cause DBIx::Recordset to only issue a warning, but perform the operation.

NOTE: Since I don't have enough experience with the behaviour of this feature with different DBMS, this is considered experimental.