Query tuning and how to force table to use index?

Question

Dear Experts,
i have two (2) question regarding performance during DRL.
Question # 1
There is a column name co_id in every transaction table. DBA suggest me to add [co_id='KPG'] in every clause which forces query to use index, resulting immediate processing. As an index was created for each table on the co_id column.
Please note that co_id has constant value 'KPG' through out the table. is it make sense to add that column in where caluse like
select a,b,c from tab1
where a='89' and co_id='KPG'
Question # 2
if i am using a column name in where clause having index on it and that column is not in my column list does it restrict query for full table scan. like
select a,b,c,d from tabletemp
where e='ABC';
Thanks in advance
Edited by: Fiz Dosani on Mar 27, 2009 12:00 PM

Answer 1

Fiz Dosani wrote:
Dear Experts,
i have two (2) question regarding performance during DRL.
Question # 1
There is a column name co_id in every transaction table. DBA suggest me to add [co_id='KPG'] in every clause which forces query to use index, resulting immediate processing. As an index was created for each table on the co_id column.
Please note that co_id has constant value 'KPG' through out the table. is it make sense to add that column in where caluse like
select a,b,c from tab1
where a='89' and co_id='KPG'If co_id is always 'KPG' it is not needed to add this condition to the table. It would be very stupid to add an (normal) index on that column. An index is used to reduce the resultset of a query by storing the values and the rowids in a specified order. When all the values are equal and index justs makes all dml operations slower without makeing any select faster.
And of cause the CBO is clever enough not to use such a index.
>
Question # 2
if i am using a column name in where clause having index on it and that column is not in my column list does it restrict query for full table scan. like
select a,b,c,d from tabletemp
where e='ABC';
Yes this is possible. However it depends from a few things.
1) How selective this condition is. In general an index will be used when selectivity is less than 5%. This factor depends a bit on the database version. it means that when less then 5% of your rows have the value 'ABC' then an index access will be faster than the full table scan.
2) Are the statistics up to date. The cost based optimizer (CBO) needs to know how many values are in that table, in the columns, in that index to make a good decision bout using an index access or a full table scan. Often one forgets to create statistics for freshly created data as in temptables.
Edited by: Sven W. on Mar 27, 2009 8:53 AM