Source(oracle 11g) with oracle 8i using db link with full table scan

Question

Hi,
I'm using oracle 8i version and i'm facing some issue  while using DBlinks.
SourceDB1 I'm using oracle 8i(Source)
select *  from tab1
tabl where id in
(select id from stab1@SourceDB1  where updt_seq_num > 167720 and work_grp_id in (2900,2901))
For this, I could able to retrive the data.
but, today we have migrated one of our source from oracle 8i to oracle 11G
when I'm executing
select *  from tab2
tabl where id in
(select id from stab2@SourceDB2  where updt_seq_num > 167720 and work_grp_id in (2900,2901))
we couldn't able to retrive data as it's scaning full table scan.
when oracle 8i source it's scanning using INDEX scan but if the source oracle 11 h it's scanning full table scan.
Could you please advise how to resolve this issue for source oracle 11i ?
Please let us know, if you need any information.

Answer 1

Blocks that are read via full table scans are stored in the buffer cache, but putting them at the MRU end ensures that they don't push the rest of the useful blocks out of the buffer cache. In your example, if you're full scanning a 2 GB table (T) with a 500 MB buffer cache, the first block that is read from T is put at the MRU end of the buffer cache, displacing the previous block that was at the MRU end of the cache. The next block that is read from T is also put at the MRU end of the buffer cache, displacing the previous block that was at the MRU end of the cache, so block #2 from T displaces block #1 from T. So, you're reading 2 GB of data, but you're constantly purging the older blocks, so you're only really using that last block of the buffer cache.
Justin