Oracle paging statements and Performance

The Oracle paging statement and the olts_trade.trd_item table have a total of 1665 pieces of data. SQL developer is used for performance analysis (explain Plan ). ------------------------------------------ SQL code select * from (select rownum rn, t. * from (select itm. * from OLTS_TRADE.trd_item itm) t where rownum <= 100) t1 where t1.rn> = 90; cost = 11315 go through the full table scan explain SQL code select itm1. * from (select rownum rn, t. n_id from (select itm. n_id from OLTS_TRADE.trd_item itm) t where rownum <= 100) t1 inner join OLTS_TRADE.trd_item itm1 on t1.n _ id = itm1.n _ id where t1.rn> = 90; the above n_id is the primary key cost = 171 using the primary key index explain SQL code select * from olts_trade.trd_item where n_id in (select n_id from (select rownum _, n_id from (select n_ID from olts_trade.trd_item) where rownum <= 100) where rownum _> = 90) cost = 172 used in --------------------------------------------