SQL Explain differs with same query

Question

DB 1: Oracle EE 11gR1 64-bit
DB 2: Oracle XE 11gR2 64-bit
Both on OEL 64-bit
DB 1 does not do a SORT related to the subquery whereas DB 2 does do a SORT. Otherwise EXPLAN PLAN identical. But performance on DB 2 noticeably faster.
Here is the SQL:
select col4
from tab1 join tab2 on tab1.col1 = tab2.col1
where tab2.col3 = (select max(col3) from tab3);
Explain plan (brief)
DB 2:
FILTER-
HASH JOIN
SORT
DB 1:
FILTER
HASH JOIN
Any hints (no pun) on what might cause this to occur? Is the optimizer different from XE 11gR4 and EE 11gR1 (wouldn't think so)? Or is it a parameter setting, memory or temp space (default settings apply for both DBs).
Thanks

Answer 1

Yes, the optimizer is different in 11gR2 than 11gR1. Also the optimizer in Enterprise Edition will have more choices available than in XE (partitioning, for example; parallel processing, since XE uses only 1 CPU), but in this case I doubt the difference is that deep.
Besides optimizer differences, you need to consider these questions first when you see different plans:
Is the volume of data the same in both databases?
Are statistics up to date in both databases?
Are statistics gathered the same way in each database?
For anything more specific we need to see the 4-digit oracle version for each database, exact explain plans (copy-paste), and possibly some CREATE TABLE and INSERT statements if someone wants to reproduce what you are seeing in their database.