Performance issue on query. Help needed.

Question

This is mainly a performance issue. I hope someone can help me on this.
Basically I have four tables Master (150000 records), Child1 (100000+ records), Child2 (50 million records !), Child 3 (10000+ records)
(please pardon the aliases).
Now every record in master has more than one corresponding record in each of the child tables (one to many).
Also there may not be any record in any or all of the tables for a particular master record.
Now, I need to fetch the max of last_updated_date for every master record in each of the 3 child tables and then find the maximum of
the three last_active_dates obtained from the 3 tables.
eg: for Master ID 100, I need to query Child1 for all the records of Master ID 100 and get the max last_updated_date.
Same for the other 2 tables and then get the maximum of these three values.
(I also need to take care of cases where no record may be found in a child table for a Master ID)
Writing a procedure that uses cursors that fetches the value from each of the child table hits performance
badly. And thing is I need to find out the last_updated_date for every Master record (all 150000 of them). It'll probably take days to do this.
SELECT MAX (C1.LAST_UPDATED_DATE)
,MAX (C2.LAST_UPDATED_DATE)
,MAX (C3.LAST_UPDATED_DATE)
FROM CHILD1 C1
,CHILD2 C2
,CHILD3 C3
WHERE C1.MASTER_ID = 100
OR C2.MASTER_ID = 100
OR C3.MASTER_ID = 100
I tried the above but I got a temp tablespace error. I don't think the query is good enough at all.
(The OR clause is to take care of no records in any child table. If there's an AND, then the join and hence select will
fail even if there is no record in one child table but valid values in the other 2 tables).
Thanks a lot.
Edited by: user773489 on Dec 16, 2008 11:49 AM

Answer 1

Not sure I understand the problem. The max you are getting from the above is already the greatest out of the three - that's why we do the UNION ALL.
Here's sample code without output, maybe this will clear it up:
with a as (
select 10 MASTER_ID, to_date('12/15/2008', 'MM/DD/YYYY') LAST_DTE from dual UNION ALL
select 20 MASTER_ID, to_date('12/01/2008', 'MM/DD/YYYY') LAST_DTE from dual UNION ALL
select 30 MASTER_ID, to_date('12/02/2008', 'MM/DD/YYYY') LAST_DTE from dual
b as (
select 10 MASTER_ID, to_date('12/14/2008', 'MM/DD/YYYY') LAST_DTE from dual UNION ALL
select 20 MASTER_ID, to_date('12/02/2008', 'MM/DD/YYYY') LAST_DTE from dual UNION ALL
select 40 MASTER_ID, to_date('11/15/2008', 'MM/DD/YYYY') LAST_DTE from dual
c as (
select 10 MASTER_ID, to_date('12/07/2008', 'MM/DD/YYYY') LAST_DTE from dual UNION ALL
select 30 MASTER_ID, to_date('11/29/2008', 'MM/DD/YYYY') LAST_DTE from dual UNION ALL
select 40 MASTER_ID, to_date('12/13/2008', 'MM/DD/YYYY') LAST_DTE from dual
select MASTER_ID, MAX(LAST_DTE)
FROM
(select MASTER_ID, LAST_DTE from a UNION ALL
select MASTER_ID, LAST_DTE from b UNION ALL
select MASTER_ID, LAST_DTE from c)
group by MASTER_ID;
MASTER_ID              MAX(LAST_DTE)            
30                     02-DEC-08                
40                     13-DEC-08                
20                     02-DEC-08                
10                     15-DEC-08                
4 rows selectedEdited by: tk-7381344 on Dec 16, 2008 12:38 PM