Different execution plans for the same sql

Question

Hi,
Im testing our new 10gR2 database on Linux and I can't understand why the same query use different plan.
Here are the details.
Table name: invoice_detail
Records: About 10,670,900
Columns:
No
seq (the primary key is No+Seq). Each invoices contains +/- 10 invoice_details.
category ( <- 10 different values )
State ( <- 3 different values )
Basically, I have an index on the primary key and another index on Category + State.
My request:
select *
from invoice_detail
where no=123456 <- Best index to use
and state <> 'CANCEL'
and category = 'INVOICE'
If i run this query from Toad or sql+, that's fine.
The same query (i'm watching it from EM) executed via Forms use the category+state index.
When I first import the database, the last thing I do is to run DBMS_STATS.GATHER_DATABASE_STATS.
At this point, Forms use the right index.
The day after (after the database has been analyzed with the predefined job via EM) Forms use the wrong index.
I re-analyzed everything with exec DBMS_STATS.GATHER_DATABASE_STATS but the problem is still there.
Thanks in advance

Answer 1

I'm already using bind variables.
I changed the "Estimated Percentage" to 100% in "Gather Optimizer Statistics Default Options" and now it seems to use the correct index. I'm stressed because I dont understand why it chooses different plan for the same sql.
Actually, my users test the migration 1 day after I load all the data (drop schema-create schema-load data-analyze database) and at this point everythings go fine. After the second analyze of the database, the DB choose the wrong indexes.
I really cannot migrate until I understand why it happens.
Any ideas?
TIA