Very Slow Query due to Bitmap Conversion

Question

I have a strange problem with the performance of a spatial query. If I perform a 'SELECT non_geom_column FROM my_table WHERE complicated_join_query' the result comes back sub-second. However, when I replace the column selected with geometry and perform 'SELECT geom_column FROM my_table WHERE same_complicated_join_query' the response takes over a minute.
The issue is that in the second case, despite the identical where clause, the explain plan is significantly different. In the 'select geom_column' query there is a BITMAP CONVERSION (TO ROWIDS) which accounts for all of the extra time, where as in the 'select other_column' query that conversion is replaced with TABLE ACCESS (BY INDEX ROWID) which is near instant.
I have tried putting in some hints, although I do not have much experience with hints, and have also tried nesting the query in various sub-selects. Whatever I try I can not persuade the explain plan to drop the bitmap conversion when I select the geometry column. The full query and an explanation of that query are below. I have run out of things to try, so any help or suggestions at all would be much appreciated.
Regards,
Chris
Explanation and query
My application allows users to select geometries from a map image through clicking, dragging a box and various other means. The image is then refreshed - highlighting geometries based on the query with which I am having trouble. The user is then able to deselect any of those highlighted geometries, or append others with additional clicks or dragged selections.
If there are 2 (or any even number of) clicks within the same geometry then that geometry is deselected. Alternatively the geometry could have been selected through an intersection with a dragged box, and then clicked in to deselect - again an even number of selections. Any odd number of selections (i.e. selecting, deselecting, then selecting again) would result in the geometry being selected.
The application can not know if the multiple user clicks are in the same geometry, as it simply has an image to work with, so all it does is pass all the clicks so far to the database to deal with.
My query therefore does each spatial point or rectangle query in turn and then appends the unique key for the rows each returned to a list. After performing all of the queries it groups the list by the key and the groups with an odd total are 'selected'. To do this logic in a single where clause I have ended up with nested select statements that are joined with union all commands.
The query is therefore..
SELECT
--the below column (geometry) makes it very slow...replacing it with any non-spatial column takes less than 1/100 of the time - that is my problem!
geometry
FROM
my_table
WHERE
primary_key IN
SELECT primary_key FROM
SELECT primary_key FROM my_table WHERE
sdo_relate(geometry, mdsys.sdo_geometry(2003, 81989, NULL, sdo_elem_info_array(1, 1003, 3), sdo_ordinate_array( rectangle co-ords )), 'mask=anyinteract') = 'TRUE'
UNION ALL SELECT primary_key FROM my_table WHERE
sdo_relate(geometry, mdsys.sdo_geometry(2001, 81989, sdo_point_type( point co-ords , NULL), NULL, NULL), 'mask=anyinteract') = 'TRUE'
--potentially more 'union all select...' here
GROUP BY primary_key HAVING mod(count(*),2) = 1     
AND
--the below is the bounding rectangle of the whole image to be returned
sdo_filter(geometry, mdsys.sdo_geometry(2003, 81989, NULL, sdo_elem_info_array(1, 1003, 3), sdo_ordinate_array( outer rectangle co-ords )), 'mask=anyinteract') = 'TRUE'

Answer 1

Hi
Thanks for the reply. After a lot more googling- it turns out this is a general Oracle problem and is not solely related to use of the GEOMETRY column. It seems that sometimes, the Oracle optimiser makes an arbitrary decision to do bitmap conversion. No amount of hints will get it to change its mind !
One person reported a similarly negative change after table statistic collection had run.
Why changing the columns being retrieved should change the execution path, I do not know.
We have a numeric primary key which is always set to a positive value. When I added "AND primary_key_column > 0" (a pretty pointless clause) the optimiser changed the way it works and we got it working fast again.
Chris