Oracle 9i ignores INDEX hint

Question

Hello,
I try to convince Oracle to do a index scan rather then a full table scan in a query. To
do this I created a view on top of the table which simply adds this hint:
CREATE OR REPLACE VIEW V_FHDDSC3_FACT_DATA_4WEEK AS
SELECT /*+ INDEX(T P_FHDDSC3_FACT_DATA_4WEEK) */
T.*
FROM T_FHDDSC3_FACT_DATA_4WEEK T
The whole query is here:
SELECT v_fhddsc3_geography_dim.geog_tag, v_fhddsc3_product_dim.prod_tag,
v_fhddsc3_time_4week_dim.time_tag, v_fhddsc3_fact_data_4week.m001,
v_fhddsc3_fact_data_4week.m002, v_fhddsc3_fact_data_4week.m003,
v_fhddsc3_fact_data_4week.m004, v_fhddsc3_fact_data_4week.m005,
v_fhddsc3_fact_data_4week.m006, v_fhddsc3_fact_data_4week.m007,
v_fhddsc3_fact_data_4week.m008, v_fhddsc3_fact_data_4week.m009,
v_fhddsc3_fact_data_4week.m010
FROM v_fhddsc3_geography_dim,
v_fhddsc3_time_4week_dim,
v_fhddsc3_fact_data_4week,
v_fhddsc3_product_dim
WHERE ( v_fhddsc3_geography_dim.geog_key =
v_fhddsc3_fact_data_4week.geog_key
AND v_fhddsc3_product_dim.prod_key =
v_fhddsc3_fact_data_4week.prod_key
AND v_fhddsc3_time_4week_dim.time_key =
v_fhddsc3_fact_data_4week.time_key
AND ( v_fhddsc3_geography_dim.geog_tag IN
(:"SYS_B_00",
:"SYS_B_01",
:"SYS_B_02",
:"SYS_B_03",
:"SYS_B_04",
:"SYS_B_05",
:"SYS_B_06",
:"SYS_B_07",
:"SYS_B_08"
AND v_fhddsc3_product_dim.hierarchy_level IN (:"SYS_B_09")
AND v_fhddsc3_time_4week_dim.time_tag IN
(:"SYS_B_10",
:"SYS_B_11",
:"SYS_B_12",
:"SYS_B_13",
:"SYS_B_14",
:"SYS_B_15",
:"SYS_B_16",
:"SYS_B_17",
:"SYS_B_18",
:"SYS_B_19",
:"SYS_B_20",
:"SYS_B_21",
:"SYS_B_22",
:"SYS_B_23",
:"SYS_B_24",
:"SYS_B_25",
:"SYS_B_26",
:"SYS_B_27",
:"SYS_B_28",
:"SYS_B_29",
:"SYS_B_30",
:"SYS_B_31",
:"SYS_B_32",
:"SYS_B_33",
:"SYS_B_34",
:"SYS_B_35",
:"SYS_B_36"
The execution plan shows a full table scan on T_FHDDSC3_FACT_DATA_4WEEK.
The index is the primary key of the table containing PROD_KEY, GEOG_KEY and TIME_KEY.
Why is Oracle 9i ignoring index hints?
I came across the same problem some months ago, when we migrated one database from 8i to 9i. After that an INDEX_ASC hint was suddenly ignored. We used an index + and INDEX_ASC hint to sort a hierarchical query.
We had to change the algorithmn and use a Database function to get it sorted right. This is painfull and slow.
Any ideas?
Kai

Answer 1

Kai,
Remember a hint is just that, a hint to the optimizer, it does not override the execution plan created by the optimizer. Possibilities are that the statistics in the database are out of date, which will affect the Cost based optimizer. Indexes on views are tricky as they would rely on the indexes on the underlying tables.
Few AskTom links which may help:
http://asktom.oracle.com/pls/ask/f?p=4950:8:1705043::NO::F4950_P8_DISPLAYID,F4950_P8_CRITERIA:313416745628,%7Bhint%7D%20and%20%7Bindex%7D
http://asktom.oracle.com/pls/ask/f?p=4950:8:1705043::NO::F4950_P8_DISPLAYID,F4950_P8_CRITERIA:641623836427,%7Bhint%7D%20and%20%7Bindex%7D
http://asktom.oracle.com/pls/ask/f?p=4950:8:1705043::NO::F4950_P8_DISPLAYID,F4950_P8_CRITERIA:1197786003246,%7Bhint%7D%20and%20%7Bindex%7D
Sometimes the full scan can be quicker, than the index depending on the query.
Regds
Dave