Query performance on same table with many DML operations

Question

Hi all,
I am having one table with 100 rows of data. After that, i inserted, deleted, modified data so many times.
The select statement after DML operations is taking so much of time compare with before DML operations (There is no much difference in data).
If i created same table again newly with same data and fire the same select statement, it is taking less time.
My question is, is there any command like compress or re-indexing or something like that to improve the performance without creating new table again.
Thanks in advance,
Pal

Answer 1

Try searching "rebuilding indexes" on http://asktom.oracle.com. You will get lots of hits and many lively discussions. Certainly Tom's opinion is that re-build are very rarley required.
As far as I know, Oracle has always re-used deleted rows in indexes as long as the new row belongs in that place in the index. The only situation I am aware of where deleted rows do not get re-used is where you have a monotonically increasing key (e.g one generated by a seqence), and most, but not all, of the older rows are deleted over time.
For example if you had a table like this where seq_no is populated by a sequence and indexed
seq_no         NUMBER
processed_flag VARCHAR2(1)
trans_date     DATEand then did deletes like:
DELETE FROM t
WHERE processed_flag = 'Y' and
      trans_date <= ADD_MONTHS(sysdate, -24);that deleted the 99% of the rows in the time period that were processed, leaving only a few. Then, the index leaf blocks would be very sparsely populated (i.e. lots of deleted rows in them), but since the current seq_no values are much larger than those old ones remaining, the space could not be re-used. Any leaf block that had all of its rows deleted would be reused in another part of the index.
HTH
John