Hello,
I recently saw a query with a wrong execution plan and looks to be caused
by an overestimate of a particular where clause. Ex:
select
count(*)
from
orders
where
partition_date = sysdate-1 and
order_procesing_date > = business_procesing_date
the count itself is = 42K
the count itself without second condition ~ 24M
the cardinality estimate ~ 24M
the cardinality estimate if I add parallel hint: 1M (this is an interesting
one and looks to be related with the fact that parallel queries get DS
level 8)
the cardinality estimate if I rewrite the condition : order_procesing_date
- business_procesing_date >= interval '0' minute :1.4M
I haven't tried yet but I suspect that if I create extended stats on
expression ( order_procesing_date - business_procesing_date) can get me
better results
The question is what is the logic behind beter cardinality estimates if I
rewrite the where clause?
What will be the best approach for this query?
Thank you.
