why the execution plan can't be changed?

Hi,all:

There is a sql can't use a correct execution plan:

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select id, splorderid, agtorderid, resid, resroomstayid, resroomtypeid, oldresid, vendorcode, vendorname, hotelcode, hotelname, province, citycode, cityname, channelcode, officecode, officename, rateplancode, rateplanname, roomtypecode, roomtypename, roomrateplanname, payment, status, bookdate, checkindate, checkoutdate, roomnumber, roomnight, roomrate, totalrate, totalbalrate, currency, exchangerate, roomratecny, totalratecny, totalbalratecny, guests, roomnos, confirmnos, comments, opname, updatetime, audittype

from chk_roomstay c

where c.resid=:resid;

User_B running the sql via synonym 'chk_roomstay',and the base table is owned by User_A.on the column "resid" has a unique index,but the execution plan is  "table access full",i try to change the execution plan via "grant" command  on the base table but it can't be changed,also because this i can't generate 10053 trace.why the execution plan can't be changed?