The same old doubt with joins

Question

Hi,
cant get this thing off from me :-)....so please dont get annoyed...
Still the same childish doubt got into me after discussion with a friend of mine....
(and ghosh i dont have the tkprof or such settings with me to test the sequence)...
select a.col1, b.col1,c.col1
from a,b,c
where a.col1 = b.col2
and a.col1 = c.col3
and a.col1 < 10;
how will the parser execute this??
one opinion
A)
1.select all the columns from a,b,c and create complete cross joins between them
2. apply the filter a.col1<10 first as it comes last in where clause ( dont think this is no longer valid for CBO )
3. apply second join condition and filter
4. apply first join condition
5. release the result sets
second opinin
B)
1. select table c and make it driving table (valid for CBO???!!!)
2. select table b and make a cross join with b
3. select table a and first filter a.col1<10 then join with the above cross-joined-resultset based on the join conditions
4. pass the final res
confused!!!??
(Michels has already cleared the doubt in one of the posts , but still I got 2 different opinion from people who are experienced ....)
so just wanted to take a final confirmed opinion thats all
s
Message was edited by:
user571638

Answer 1

First off:
2. apply the filter a.col1<10 first as it comes last in where clause ( dont think this is no longer valid for CBO )
This was never valid. Even the RBO had more sense than to just work blindly through the WHERE predicates in a fixed order.
Also,
> 5. release the result sets
is not really right - if possible it will begin to stream rows back to you before it has fetched them all, whereas the above description makes it look as though it gathers all the results up in some temporary store before passing them all back at once.
Regarding the optimizer's joining strategy, any permutation and ordering is possible including cross joins, although it has various strategies that mean it does not have to work through every conceivable permutation. It will try various approaches, calculate their costs, and pick the one it thinks is going to be most efficient.