This will only return one row, so the ORDER part of the FB2 plan is
superfluous, and makes the query slower than the FB 1.5 plan.

SELECT
pbi.QTY, COALESCE(SUM(cbi.QTY_SHIP),0)
FROM
RPL_POBILL_ITM pbi
LEFT JOIN RPL_CLM_POBILL_ITM cbi
ON cbi.POBILL_ITM = pbi.ID
WHERE
pbi.ID = ?
GROUP BY
pbi.QTY

FB 2
PLAN JOIN (PBI ORDER I_RPL_POBILL_ITM_QTY INDEX (PK_RPL_POBILL_ITM), CBI
INDEX (FK_RPLCLMPOBILLITM_POBILLITM))
FB 1.5
PLAN SORT (JOIN (PBI INDEX (PK_RPL_POBILL_ITM),CBI INDEX
(FK_RPLCLMPOBILLITM_POBILLITM)))

How do I keep FB2 from trying to sort on an index when I'm guaranteed to
have only one row?