Oracle 11g on Linux : Query Optimization issue

Question

Hi guru,
I am facing one query optimization related problem in group by query
Table (10 million Records)
Product(ProductId number,ProductName varchar(100),CategoryId VARCHAR2(38),SubCategoryId VARCHAR2(38))
Index
create index idxCategory on Product (CategoryId,SubCategoryId)
Query1:To find product count for all CategoryId and SubCategoryId
select CategoryId,SubCategoryId,count(*) from Product group by CategoryId,SubCategoryId
Above query is not using index idxCategory and doing table scan which is very costly.
When I fire Query2: select count(*) from Product group by CategoryId,SubCategoryId
then it is properly using index idxCategory and very fast.
Even I specified hint in Query1 but it is not using hint.
Can anybody suggest why oracle is not using index in Query1 and what should I do so that Query1 will use Index.
Thanks in advance.

Answer 1

user644199 wrote:
I am facing one query optimization related problem in group by query
Query1:To find product count for all CategoryId and SubCategoryId
select CategoryId,SubCategoryId,count(*) from Product group by CategoryId,SubCategoryId
Above query is not using index idxCategory and doing table scan which is very costly.
When I fire Query2: select count(*) from Product group by CategoryId,SubCategoryId
then it is properly using index idxCategory and very fast.
Even I specified hint in Query1 but it is not using hint.
Can anybody suggest why oracle is not using index in Query1 and what should I do so that Query1 will use Index.The most obvious reason that the table needs to be visited would be that the columns "CategoryId" / "SubCategoryId" can be NULL but then this should apply to both queries. You could try the following to check the NULL issue:
select CategoryId,SubCategoryId,count(*) from Product where CategoryId is not null and SubCategoryId is not null group by CategoryId,SubCategoryId
Does this query use the index?
Can you show us the hint you've used to force the index usage and the EXPLAIN PLAN output of the two queries including the "Predicate Information" section? Use DBMS_XPLAN.DISPLAY to get a proper output, and use the \ tag before and after when posting here to format it using fixed font. Use the "Quote" button in the message editor to see how I used the \ tag here.
Are above queries representing the actual queries used or did you omit some predicates etc. for simplicity?
By the way, VARCHAR2(38) and ...ID as name, are these columns storing number values?
Regards,
Randolf
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