SQL & PL/SQL

Here is my situation. I had a query which I use to run in Production (oracle 9.2.0.5) and Reporting database (9.2.0.3). The time taken to run in both databases was almost the same until 2 months ago which was about 2 minutes. Now in production the query does not run at all where as in Reporting it continues to run in about 2 minutes. Some of the things I obsevred in P are 1) the optimizer_index_cost_adj parameter was changed to 20 from 100 in order to improve the performance of a paycalc program about 3 months ago. Even with this parameter being set to 20, the query use to run in 2 minutes until 2 months ago. in the last two months the GL table increased in size from 25 million rows to 27 million rows. With optimizer_index_cost_adj of 20 and Gl table of 25 million rows it runs fine, but with 27 million rows it does not run at all. If I change the value of optimizer_index_cost_adj to 100 then the query runs with 27 million rows in 2 minutes and I found that it uses full table scan. In Reporting database it always used full table sacn as found thru explain plan. CBO determines which scan is best and it uses that. So my question is that by making optimizer_index_cost_adj = 20, does oracle forces it to use index scan when the table size is 27 million rows? Isn't the index scan is not faster than full table scan? In what situation the full table scan is faster than index scan? If I drop all the indexes on the GL table then the query runs faster in production as it uses full table scan. What is the real benefit of changing optimizer_index_cost_adj values? Any input is most welcome.