Bitmap indexes and group by

Question

I'm trying to understand why Oracle 8.1.6 sometimes uses bitmap indexes sometimes not.
Of course I have all the statistics, my bitmap indexes are valid and so on.
The problem is this:
- I have a customer table very very large
- I have many columns with bitmap indexes
- I run this statement:
select education_key, count(*)
from customer
group by education_key
and obtain a correct execution plan:
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=235 Card=5 Bytes=10)
1 0 SORT (GROUP BY NOSORT) (Cost=235 Card=5 Bytes=10)
2 1 BITMAP CONVERSION (COUNT)
3 2 BITMAP INDEX (FULL SCAN) OF 'CL_EDU'
with the use of the bitmap indexes (only five different values)
- now I want to put a condition on a column
which has a bitmap index.
The new statement is:
select education_key, count(*)
from customer
where age_key = 30
group by education_key
No join, only a scan of a sort of
fact table. In this case Oracle
doesn't use the bitmap index on
education_key:
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=3458 Card=5 Bytes=20
1 0 SORT (GROUP BY) (Cost=3458 Card=5 Bytes=20)
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'CUSTOMER' (Cost=3395 C
ard=18296 Bytes=73184)
3 2 BITMAP CONVERSION (TO ROWIDS)
4 3 BITMAP INDEX (SINGLE VALUE) OF 'CL_AGE'
With this execution plan Oracle has to
read some blocks from the table when I
think that using both the bitmap indexes
the query is faster.
Someone knows why Oracle has this
behaviour? I'm trying to change
many things (also altering statistics
value), but it seems that with a
condition Oracle doesn'use the bitmap
index on the group by column.
null

Answer 1

Hi Michael, what is the address for the lasagne? ....
In init.ora.parms I had 32Mb for bitmap index merge, I suppose it's enough, hovewer I have tried to increase it and nothing changes.
I already use STAR_TRASFORMATION_ENABLED=TRUE.
I have tried with different level of statistics (estimate, compute, also with very precise histograms), always the same
behaviour. I have also tried to change, in a manual way, the statistics in order to change the Oracle behaviour but nothing ...
For this particular statement even if I use the INDEX_COMBINE hint, I mantain the same execution plan.
I use CHOOSE, FIRST_ROWS, ALL_ROWS: always the same behaviour.
I think that Oracle is not able to use a bitmap index on a column involved in a group by when there is another condition in the select.
Hovewer with bitmap indexes, I have many other problems (to read 1% of the table Oracle decides to make a full table scan even if there are the correct bitmap indexes ...).
Many of them are solved using BITMAP_INDEX, but it's difficult to use it in a query tool like Discover or Business Objects because I have to fix the use of INDEX_COMBINE also in situation in which this use is not correct from a performance point of view.
The life with the query optimizer is very hard!
Thank you for your guesses