Does the entire query always have to execute?

Question

We have a large (100 line) query that is used for detecting bad data. The query looks like
select * from table where
clause1 is true -- for example, start_date is null
OR clause2 is true -- end_date is null
OR clause3 is true -- start_year < 1970
. 100 more clauses
It looks to me like every clause is being evaluated for every row even though the first clause that evaluates to true is enough to select the row. I believe every clause is being evaluated because we often get failures from the toDate() function for columns that are null and there are null checks before the toDate() call.
Is it possible to make the query evaluation end "early"?
Thanks for any help or advice,
-=beeky

Answer 1

Hi,
As Justin said, Oracle will quit evaluating your WHERE clause as soon as it finds a condition that is TRUE. If you have evidence that it is evaluating some condition even though an earlier condition is TRUE, then it must be evaluating the conditions in a different order. Make sure you have up-to-date statistics and appropriate indexes, so the optimizer can make an intelligent choice about which conditions to evaluate first.
You can force the order by putting the conditions in a CASE statement.
For example, say you're now doing this:
select  *
from    table_x
where   start_date is null
   OR   end_date is null
   OR   start_year < 1970
;If you know that the third condition (start_year < 1970) is the most restrictive, and therefore you want to make sure it is done first, then you can re-write the query like this:
select  *
from    table_x
where   CASE
            WHEN  start_year < 1970   THEN 1
            WHEN  start_date is null  THEN 1
            WHEN  end_date is null    THEN 1
                                      ELSE 0
        END = 1
;