Performance EXPERT between Oracle and SQLServer!!

Question

Hi all,
I have this query:
select t.status, count(*) from brcapdb2.titulo t
group by t.status
order by t.status;
The hardware is the same and when i run this on Oracle it takes about 20 seconds and the same query on SQLServer takes 2 seconds.
Both databases are using parallelism but the main difference is that SQLServer use an index that Oracle didn´t use. This index is created on column "status".
Even if i try to force this index with HINT on Oracle i can´t use it.
Any advice will be apreciated.
Tks,
Paulo.

Answer 1

I don't know anything about SQL Server, but Oracle can use an index for your query but only if all possible values of status can be retrieved from the index.
Oracle doesn't store an index entry if all indexed columns are null. So in order to give you the count where the status is null, Oracle needs to access the table.
In this example, I have a not null constraint on status:
SQL> select t.status, count(*) from titulo t
  2  group by t.status
  3  order by t.status;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=19 Card=13 Bytes=104)
   1    0   SORT (GROUP BY) (Cost=19 Card=13 Bytes=104)
   2    1     INDEX (FAST FULL SCAN) OF 'T_IDX' (NON-UNIQUE) (Cost=5 Card=7995 Bytes=63960)As you can see, it never accesses the table.
If I remove the not null constraint on the status column, it uses a full table scan:
SQL> select t.status, count(*) from titulo t
  2  group by t.status
  3  order by t.status;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=19 Card=13 Bytes=104)
   1    0   SORT (GROUP BY) (Cost=19 Card=13 Bytes=104)
   2    1     TABLE ACCESS (FULL) OF 'TITULO' (Cost=5 Card=7995 Bytes=63960)Now if I add a where clause to eliminate null values, it goes back to an index scan:
SQL> select t.status, count(*) from titulo t
  2  where status is not null
  3  group by t.status
  4  order by t.status;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=19 Card=13 Bytes=104)
   1    0   SORT (GROUP BY) (Cost=19 Card=13 Bytes=104)
   2    1     INDEX (FAST FULL SCAN) OF 'T_IDX' (NON-UNIQUE) (Cost=5 Card=7995 Bytes=63960)Message was edited by:
Eric H