Indexes for 'greater than' queries - sql

Question

I have several queries, most of them being:
select * from Blah where col > 0
and
select * from Blah where date > current_date
Since they're both kind of a range, would an unclustered b+ tree index on col and date be a good idea to speed up the queries? Or a hash index? Or would no index be better?

Answer 1

Creating an INDEX on the column used in the filter predicate as a date range condition should be useful as it would do a INDEX RANGE SCAN.
Here is a demonstration about How to create, display and read EXPLAIN PLAN in Oracle.
Let's see the test cases for both scenarios:
Test# 1 : Without index
SQL> EXPLAIN PLAN FOR
2 SELECT * FROM emp WHERE hiredate > to_date('01/04/1981','mm/dd/yyyy');
Explained.
SQL>
SQL> SELECT * FROM TABLE(dbms_xplan.display);
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------
Plan hash value: 3956160932
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 518 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| EMP | 14 | 518 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------
1 - filter("HIREDATE">TO_DATE(' 1981-01-04 00:00:00', 'syyyy-mm-dd
hh24:mi:ss'))
14 rows selected.
SQL>
Test# 1 : With index
SQL> CREATE INDEX emp_idx ON emp(hiredate);
Index created.
SQL>
SQL> EXPLAIN PLAN FOR
2 SELECT * FROM emp WHERE hiredate > to_date('01/04/1981','mm/dd/yyyy');
Explained.
SQL>
SQL> SELECT * FROM TABLE(dbms_xplan.display);
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------
Plan hash value: 3589413211
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 518 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID BATCHED| EMP | 14 | 518 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | EMP_IDX | 14 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------
---------------------------------------------------
2 - access("HIREDATE">TO_DATE(' 1981-01-04 00:00:00', 'syyyy-mm-dd hh24:mi:ss'))
14 rows selected.
SQL>
So, in the second test case, you see an index range scan. I would suggest you to do a similar test on your environment too.

Answer 2

Yes. If you index on any column and are only filtering on that column the index will be used.
The question is pretty vague. Maybe with more code or a bigger example, more help could be given around the performance benefits on a specific query.
In your case an index on col (first query) and date (second query) would speed up those specific queries.