Tuning Queries

Question

1) While using toad for sql tuning,it's adding +0 to the where clause columns ?
What's the significance of adding zero?
2) One more thing I observed it's rewriting joins :
For ex :
select *
FROM tablea a,
tableb b,
tablec c,
tabled d
WHERE a.CREATED = b.USER
AND a.UPDATED = c.USER
AND a.BOOK_ID = d.BOOK_ID(+)
toad rewriting that query as follows :
select *
FROM tablea a1,
tableb b1,
tablec c1,
tabled d1
WHERE a1.CREATED = b1.USER
AND a1.UPDATED = c1.USER
AND a1.BOOK_ID = d1.BOOK_ID
UNION ALL
FROM tablea a3,
tableb b3,
tablec c3
WHERE a3.CREATED = b3.USER
AND a3.UPDATED = c3.USER
AND (NOT EXISTS (SELECT 'X'
FROM tabled d3
WHERE a3.BOOK_ID = d3.BOOK_ID))
3) Can I join main view with another view for reporting purposes or joining view with other view's tables directly.In My requirement, using main view is mandatory and using other view is optional?
Thanks,
Kiran

Answer 1

1&2 seem to be related to using rules of thumb and tuning techniques that are at least 10 years old.
I wouldn't trust any such tool to provide reliable "advice".
Adding +0 to a numberic column or concatenating a null to a varchar2 column will prevent an index from being considered.
The rewrite of the query to use a UNION ALL is to prevent using an OUTER JOIN which people used to advise avoiding at all costs.
Tuning by heuristics is rarely effective and there's a reason that the optimizer is no longer rule-based.
3) Can I join main view with another view for reporting purposes or joining view with other view's tables directly.In My requirement, using main view is mandatory and using other view is optional?You could join main view with another view.
Or with the other views tables directly.
Whether you should do one or the other depends on the specific circumstances.