Indexing options to avoid contention -- 10gR2

  • From: David Aldridge <david@xxxxxxxxxxxxxxxxxx>
  • To: oracle-l@xxxxxxxxxxxxx
  • Date: Tue, 8 Apr 2008 18:29:13 -0700 (PDT)

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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