Oracle not using correct index

Question

Hi,
I have a fact table (a big table) and a dimension table representing dates.
My query is
select fdat.*, dd.dim_date from fdat_bitmap fdat, dim_dates dd where
fdat.dim_date_id = dd.dim_date_id
and dd.dim_date > TO_DATE('2011-12-20 00:00:00' , 'YYYY-MM-DD HH24:MI:SS');
and the corresponding plan is:
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | 76M| 9173M| 709K (1)| 02:21:51 |
|* 1 | HASH JOIN | | 76M| 9173M| 709K (1)| 02:21:51 |
|* 2 | INDEX FAST FULL SCAN| UI_DD_DATES_ID | 6951 | 97314 | 8 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | FDAT_BITMAP | 198M| 20G| 708K (1)| 02:21:39 |
Predicate Information (identified by operation id):
1 - access("FDAT"."DIM_DATE_ID"="DD"."DIM_DATE_ID")
2 - filter("DD"."DIM_DATE">TO_DATE(' 2011-12-20 00:00:00', 'syyyy-mm-dd
hh24:mi:ss'))
17 rows selected
When I change the query to:
select fdat.*, dd.dim_date from fdat_bitmap fdat, dim_dates dd where
fdat.dim_date_id = dd.dim_date_id
and fdat.dim_date_id > 20111220;
Explain plan changes to:
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | 390K| 46M| 43958 (1)| 00:08:48 |
| 1 | MERGE JOIN | | 390K| 46M| 43958 (1)| 00:08:48 |
| 2 | TABLE ACCESS BY INDEX ROWID | FDAT_BITMAP | 390K| 41M| 43948 (1)| 00:08:48 |
| 3 | BITMAP CONVERSION TO ROWIDS| | | | | |
|* 4 | BITMAP INDEX RANGE SCAN | I_FDATB_DIM_DID | | | | |
|* 5 | SORT JOIN | | 6991 | 97874 | 9 (12)| 00:00:01 |
|* 6 | INDEX FAST FULL SCAN | UI_DD_DATES_ID | 6991 | 97874 | 8 (0)| 00:00:01 |
Predicate Information (identified by operation id):
4 - access("FDAT"."DIM_DATE_ID">20111220)
filter("FDAT"."DIM_DATE_ID">20111220)
5 - access("FDAT"."DIM_DATE_ID"="DD"."DIM_DATE_ID")
filter("FDAT"."DIM_DATE_ID"="DD"."DIM_DATE_ID")
6 - filter("DD"."DIM_DATE_ID">20111220)
22 rows selected
My question is why the first query not resulting in the plan similar to second one ? How can make it come with plan similar to the second one without changing the query ?
Thanks,
-Rakesh

Answer 1

user12257218 wrote:
My query is
select fdat.*, dd.dim_date from fdat_bitmap fdat, dim_dates dd where
fdat.dim_date_id = dd.dim_date_id
and dd.dim_date > TO_DATE('2011-12-20 00:00:00' , 'YYYY-MM-DD HH24:MI:SS');When I change the query to:
select fdat.*, dd.dim_date from fdat_bitmap fdat, dim_dates dd where
fdat.dim_date_id = dd.dim_date_id
and fdat.dim_date_id > 20111220;My question is why the first query not resulting in the plan similar to second one ? How can make it come with plan similar to the second one without changing the query ?
To a very large extent this is because the two queries are not logically equivalent - unless you have a constraint in place that enforces the rule that:
dd.dim_date_id > 20111220 if, and only if, dd.dim_date > TO_DATE('2011-12-20 00:00:00' , 'YYYY-MM-DD HH24:MI:SS')
A constraint like: (dim_date_id = to_number(to_char(dim_date,'yyyymmdd'))) might help - provided both columns also have NOT NULL declarations (or "is not null" constraints), and provided that that's appropriate for the way your application works.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
Author: <b><em>Oracle Core</em></b>