Indexing options to avoid contention -- 10gR2

I have a large and busy OLTP table, 100GB or so, against which there is a need to capture changes. Until an asynchronous CDC solution is in place we have to rely on two columns: create_date and update_date (null until the first update), both being of DATE type of course.

These are currently unindexed, but there is a desire to index them to improve change capture performance for queries such as:

select ...
from ...

where     (create_dt >= trunc(sysdate)-1 and create_dt < trunc(sysdate))
       or (update_dt >= trunc(sysdate)-1 and update_dt < trunc(sysdate))

The desire is obviously to provide the maximum performance benefit while reducing the impact on the OLTP system.

I thought of four different indexing options:

i) Indexing the columns separately, leading in the best case to an unusual execution plan where the indexes are range scanned and the results merged before accessing the table. ii) A single composite index (create_date,update_date),leading to a fast full index scan. iii) A single composite index (update_date,create_date), rewriting the query predicate as ...

   (create_dt >= trunc(sysdate)-1 and create_dt < trunc(sysdate) and update_date is null) or (update_dt >= trunc(sysdate)-1 and update_dt < trunc(sysdate)) ... and leading to two index range scans. (not sure about this) iv) A single-column function based index on (Nvl(update_dt,create_dt)) and rewriting the predicate appropriately.

Whichever of these is chosen the growth pattern for the data naturally tends towards index contention as all the new values are equal to sysdate.

So the nub of my problem comes down to these questions:

Is it possible to mitigate the index contention problem with a suitably high initrans values? Does it seem likely that implementing a reverse index would be worthwhile in reducing contention (I could probably take the pain of the fast full scan if it was, although it would lead to a larger index due to the 50/50 block splits).

Would you regard implementing a function-based index as risky in any significant way on major tables of a busy OLTP system? How about a reverse function based index?

Basically, "what would you do"?

Maybe I'm overthinking it, but I'd like to go to the sytem developers with a fully thought out set of options (even if they do tell me to take a running jump anyway).

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