Explain plan is different for same query

Question

Hi all,
I have a query, which basically selects some columns from a remote database view. The query is as follows:
select * from tab1@remotedb, tab2@remotedb
where tab1.cash_id = tab2.id
and tab1.date = '01-JAN-2003'
and tab2.country_code = 'GB';
Now, i am working on two environments, one is production and other is development. Production environment has following specification:
1. Remotedb = Oracle9i, Linux OS
2. Database on which query is running = Oracle10g, Linux OS
Development environment has following specification:
1. Remotedb = Oracle10g, Windows OS
2. Database on which query is running = Oracle10g, Linux OS
Both databases in development and production environments are on different machines.
when i execute the above query on production, i see full table scans on both tables in execution plan(TOAD), but when i execute the query in development, i see that both remote database tables are using index.
Why am i getting different execution plans on both databases? is there is any difference of user rights/priviliges or there is a difference of statistics on both databases. I have checked the statistics for both tables on Production and Development databases, they are updated.
This issue is creating a performance disaster in our Production system. Any kind of help or knowledge sharing is appreciated.
Thank you and Best Regards.

Answer 1

select * from tab1@remotedb, tab2@remotedb
where tab1.cash_id = tab2.id
and tab1.date = '01-JAN-2003'
and tab2.country_code = 'GB';
I assume that tab1.date is a date column. You are doing an implizit type conversion here. I think the way how those conversions are done, was changed from 9i to 10g. So that in 10g index usage is possible, while in 9i is not (not very sure about this).
Change your query to this:
select * from tab1@remotedb, tab2@remotedb
where tab1.cash_id = tab2.id
and tab1.date = to_date('01-JAN-2003','DD-MON-YYYY')
and tab2.country_code = 'GB';
But compare and consider the results. Especially if the column tab1.date holds time values too.
and tab1.date = to_date('01-JAN-2003','DD-MON-YYYY')is not the same as
and to_char(tab1.date) = '01-JAN-2003'maybe you must change to
and tab1.date >= to_date('01-JAN-2003','DD-MON-YYYY')
      and tab1.date < to_date('01-JAN-2003','DD-MON-YYYY') + 1Depends on your data.