SQL query performance question

Question

So I had this long query that looked like this:
SELECT a.BEGIN_DATE, a.END_DATE, a.DEAL_KEY, (select name from ideal dd where a.deal_key = dd.deal_key) DEALNAME, a.deal_term_key
FROM
ideal d, ideal_term a,( select deal_key, deal_term_key, max(createdOn) maxdate    from Ideal_term B
where createdOn <= '03-OCT-12 10.03.00 AM' group by deal_key, deal_term_key ) B
WHERE  a.begin_date <= '20-MAR-09 01.01.00 AM'
*     and a.end_date >= '19-MAR-09 01.00.00 AM'*
*     and A.deal_key = b.deal_key*
*     and A.deal_term_key = b.deal_term_key*
*     and     a.createdOn = b.maxdate*
*     and d.deal_key = a.deal_key*
*     and d.name like 'MVPP1 B'*
order by
*     a.begin_date, a.deal_key, a.deal_term_key;*
This performed very poorly for a record in one of the tables that has 43,000+ revisions. It took about 1 minute and 40 seconds. I asked the database guy at my company for help with it and he re-wrote it like so:
SELECT a.BEGIN_DATE, a.END_DATE, a.DEAL_KEY, (select name from ideal dd where a.deal_key = dd.deal_key) DEALNAME, a.deal_term_key
FROM ideal d
INNER JOIN (SELECT deal_key,
deal_term_key,
MAX(createdOn) maxdate
FROM Ideal_term B2
WHERE '03-OCT-12 10.03.00 AM' >= createdOn
GROUP BY deal_key, deal_term_key) B1
ON d.deal_key = B1.deal_key
INNER JOIN ideal_term a
ON B1.deal_key = A.deal_key
AND B1.deal_term_key = A.deal_term_key
AND B1.maxdate = a.createdOn
AND d.deal_key = a.deal_key + 0
WHERE a.begin_date <= '20-MAR-09 01.01.00 AM'
AND a.end_date >= '19-MAR-09 01.00.00 AM'
AND d.name LIKE 'MVPP1 B'
ORDER BY a.begin_date, a.deal_key, a.deal_term_key
this works much better, it only takes 0.13 seconds. I've bee trying to figure out why exaclty his version performs so much better. His only epxlanation was that the "+ 0" in the WHERE clause prevented Oracle from using an index for that column which created a bad plan initially.
I think there has to be more to it than that though. Can someone give me a detailed explanation of why the second version of the query performed so much faster.
Thanks.
Edited by: su**** on Oct 10, 2012 1:31 PM

Answer 1

I used Autotrace in SQL developer. Is that sufficient? Here is the Autotrace and Explain for the slow query:
and for the fast query:
I said that I thought there was more to it because when my team members and I looked at the re-worked query the database guy sent us, our initial thoughts were that in the slow query some of the tables didn't have joins and because of that the query formed a Cartesian product and this resulted in a huge 43,000+ rows matrix.
In his version all tables had joins properly defined and in addition he had that +0 which told it to ignore the index for the attribute deal_key of table ideal_term. I spoke with the database guy today and he confirmed our theory.