Improving a simple Query

Question

Following is a simple query, what i want to know, i have added upper select to get d.name which is the description of region_code. Can i get whole result with single select
SELECT t.region_code,d.name, t.emp_contr
FROM
(SELECT c.br_region_fo_code as Region_Code,
SUM(c.employer_contribution) AS emp_contr
FROM core_business.cb_contr_emp_pmt_slip c
GROUP BY c.br_region_fo_code ) t,
general_information.cb_region_fo d
WHERE t.br_region_fo_code = d.region_fo_code;

Answer 1

Boneist wrote:
malhi wrote:
Following is a simple query, what i want to know, i have added upper select to get d.name which is the description of region_code. Can i get whole result with single select
SELECT t.region_code,d.name, t.emp_contr
FROM
(SELECT c.br_region_fo_code as Region_Code,
SUM(c.employer_contribution) AS emp_contr
FROM core_business.cb_contr_emp_pmt_slip c
GROUP BY c.br_region_fo_code ) t,
general_information.cb_region_fo d
WHERE t.br_region_fo_code = d.region_fo_code;To be honest, I wouldn't bother rewriting the above query - it looks like it's filtering early (although Oracle could choose to rewrite it so that it does the join first, I guess), so that you're reducing the number of rows that the outer query has to join to. That means less work. If it is rewriting, I'd stick a no_merge hint on the subquery, to tell Oracle to do the grouping first before joining.
You could rewrite the above query as:
SELECT c.br_region_fo_code AS region_code,
d.name,
SUM(c.employer_contribution) AS emp_contr
FROM   core_business.cb_contr_emp_pmt_slip c,
general_information.cb_region_fo d
WHERE  t.br_region_fo_code = d.region_fo_code
GROUP BY c.br_region_fo_code, d.name;but whether Oracle will filter early or not is another matter. You would have to test both runs.I believe that Jonathan Lewis had a demonstration of execution plans that showed Oracle transforming queries to "push" GROUP BY clause prior to a join when sufficient contraints were in place to allow that and there was a performance benefit in doing so. I'd certainly be interested in seeing whether this was being done. The optimisation was really aimed at reducing the size of the group by key columns.