Query running slow when using case inside when

Question

Hi All,
I am in the process of creating a query as per the following conditions:
select count(*) from Alpha A, Beta B
where
(case
when
A.col1='Pete' and substr(A.col2,1,12)=substr(B.col2,1,12)
then 1
else
-- for all other cases when A.col1 is not equal to 'Pete', substr 1-15 needs to be checked on A and B
when A.col1 != 'Pete' and substr(A.col2,1,15)=substr(B.col2,1,15)
then 1
else 0
end)=1
When i run the whole query together, it continues to run forever and i have to eventually kill it. However, when i run the 2 WHEN clauses seperately, it runs within a second and fetches me the correct data. What goes wrong while merging these two inside 2 WHEN clauses that it causes the query to drag?
Please advise.
Thanks in advance.

Answer 1

Not sure,
Are you saying that you need both the counts seperately? or a combined count?
how are you joining alpha and beta tables? what is the join condition?
you need to do something like this,
SELECT COUNT(CASE WHEN
                    (A.col1 = 'Pete' AND SUBSTR(A.col2,,1,12)=SUBSTR(B.col2,,1,13))
                 OR ( A.col1 != 'Pete' AND SUBSTR(A.col2,,1,15)=SUBSTR(B.col2,,1,15))
                THEN 1 ELSE 0
             END)
FROM alpha A, beta b
WHERE alpha.join_cloumn= beta.join_columnG.