Why oh why, weird performance on joining large tables

Question

Hello.
I have a large table cotaining dates and customer data. Organised as:
DATE CUSTOMER_ID INFOCOLUMN1 INFOCOLUMN2 etc...
Rows per date are a couple of million.
What I'm trying to do is to make a comparison between date a and date b and track changes in the database.
When I do a:
SELECT stuff
FROM table t1
INNER JOIN table t2
  ON t1.CUSTOMER_ID = t2.CUSTOMER_ID
WHERE t1.date = TO_DATE(SOME_DATE)
AND t2.date = TO_DATE(SOME_OTHER_DATE)I get a result in about 40 seconds which is acceptable.
Then I try doing:
SELECT stuff
FROM (SELECT TO_DATE(LAST_DAY(ADD_MONTHS(SYSDATE, 0 - r.l))) AS DATE FROM dual INNER JOIN (SELECT level l FROM dual CONNECT BY LEVEL <= 1) r ON 1 = 1) time
INNER JOIN table t1
  ON t1.date = time.date
INNER JOIN table t2
  ON t1.CUSTOMER_ID = t2.CUSTOMER_ID
WHERE t2.date = ADD_MONTHS(time.date, -1)Ie i generate a datefield from a subselect which I then use to join the tables with.
When I try that the query takes an hour or two to complete with the same resultset as the first example.
THe only difference is that in the first case I give the dates literally but in the other case I generate them in the subselect. It's the same dates and they are formatted as dates in both cases.
Any ideas?
Thanks
Edited by: user1970293 on 2010-apr-29 00:52
Edited by: user1970293 on 2010-apr-29 00:59

Answer 1

When I try that the query takes an hour or two to complete with the same resultset as the first example.If you get the same results, than why change the query to the second one?
THe only difference is that in the first case I give the dates literally but in the other case I generate them in the subselect. It's the same dates and they are formatted as dates in both cases.Dates are dates,... the formatting is just "pretty"
This
select to_date(last_day(add_months(sysdate
                                  ,0 - r.l)))
  from dual
inner join (select level l from dual connect by level <= 1) r on 1 = 1doesn't make much sense... what is it supposed to do?
(by the way: you are doing a TO_DATE on a DATE...)