2 problem with BITMAP JOIN INDEX and access plan

Question

On my schema ("LSA") I have 3 tables:
AAB_VENDUTO (~4.200.000 record)
AAB_ARTICOLO (~8.200 record)
AAB_CLIENTE (~15.000 record)
AAB_ARTICOLO.MARCA has 1079 distinct values
AAB_CLIENTE.TITOLO_STUDIO has 22 distinct values
I create 2 different bitmap join index:
CREATE BITMAP INDEX lsa.AAB_V_ARTICOLO_MARCA_IDX ON LSA.AAB_VENDUTO(MARCA)
FROM LSA.AAB_VENDUTO vv, LSA.AAB_ARTICOLO aa WHERE vv.articolo_id=aa.articolo_id;
CREATE BITMAP INDEX lsa.AAB_V_CLIENTE_TIT_STUDIO_IDX ON LSA.AAB_VENDUTO(TITOLO_STUDIO)
FROM LSA.AAB_VENDUTO vv, LSA.AAB_CLIENTE cc WHERE vv.cliente_id=cc.cliente_id;(star_trasformation_enabled is TRUE)
I run (on SQL Developer 1.5.5) these similar queries:
SELECT DISTINCT VV.cliente_id FROM LSA.AAB_VENDUTO VV JOIN LSA.AAB_ARTICOLO AA on VV.articolo_id=AA.articolo_id
WHERE (MARCA='ALGIDA' OR MARCA='SAMMONTANTA');
SELECT DISTINCT VV.cliente_id FROM LSA.AAB_VENDUTO VV JOIN LSA.AAB_CLIENTE CC  on VV.cliente_id=CC.cliente_id  
WHERE (titolo_studio='LAUREA BREVE' OR titolo_studio='MAGISTRALE');The first one use correctly the bitmap join index, this is the access plan:
!http://bitlgs.altervista.org/_altervista_ht/marca.png!
but the second query...
!http://bitlgs.altervista.org/_altervista_ht/titolo_studio.png!
why the second query doesn't use the bitmap index?
problem #2:
I create another bitmap index:
CREATE BITMAP INDEX LSA.AAB_V_CLIENTE_SESSO_IDX ON LSA.AAB_VENDUTO(SESSO)
FROM LSA.AAB_VENDUTO vv, LSA.AAB_CLIENTE cc WHERE vv.cliente_id=cc.cliente_id;and I run this query:
SELECT VV.cliente_id, COUNT(*) FROM LSA.AAB_VENDUTO VV JOIN LSA.AAB_CLIENTE CC on VV.cliente_id=CC.cliente_id
WHERE sesso='Donna' and (titolo_studio='LAUREA BREVE' or titolo_studio='MAGISTRALE') GROUP BY VV.cliente_id;this is the access plan:
!http://bitlgs.altervista.org/_altervista_ht/terzo.png!
In my opinion, the part that I have marked as 'A' is superfluous...
why it make a join with AAB_CLIENTE table? Why it filter again the titolo_studio and sesso field?
Edited by: Nirpol on 4-set-2009 13.25

Answer 1

Nirpol wrote:
I create 2 different bitmap join index:
CREATE BITMAP INDEX lsa.AAB_V_ARTICOLO_MARCA_IDX ON LSA.AAB_VENDUTO(MARCA)
FROM LSA.AAB_VENDUTO vv, LSA.AAB_ARTICOLO aa WHERE vv.articolo_id=aa.articolo_id;
CREATE BITMAP INDEX lsa.AAB_V_CLIENTE_TIT_STUDIO_IDX ON LSA.AAB_VENDUTO(TITOLO_STUDIO)
FROM LSA.AAB_VENDUTO vv, LSA.AAB_CLIENTE cc WHERE vv.cliente_id=cc.cliente_id;
What happens if you just create two plain bitmap indexes without the join?
CREATE BITMAP INDEX lsa.AAB_V_ARTICOLO_MARCA_IDX ON LSA.AAB_VENDUTO(MARCA);
CREATE BITMAP INDEX lsa.AAB_V_CLIENTE_TIT_STUDIO_IDX ON LSA.AAB_VENDUTO(TITOLO_STUDIO) ;You can also go with normal indexes on the FK rows (cliente_id) and the optimizer can do a bitmap conversion from normal index to bitmap index if needed. Are the column values evenly distributed? If not you might need to go for some column histogram. but I don't have the impression that this is really useful in your case.