Count query taking too long vs identical query with columns returned

We're currently running Oracle 8.i and have 5 tables I'm querying with approx. 1.5 millions records in each.

When I perform the following type of query (with joins between the tables and filtering):
SELECT A.col1, B.col2, C.col3, D.col4, E.col5   FROM table1 A, table2 B, table3 C, table4 D, table5 E

 WHERE A.col = B.col
   AND A.col = C.col
   AND C.col = D.col
   AND C.col = E.col
   AND A.anotherColumn = 'SOMEARG'

the response time is in the neighorbood of 3-4 seconds.

When I try to perform the EXACT query trying to use the count function, e.g.:
SELECT count(A.primaryKey)
  FROM table1 A, table2 B, table3 C, table4 D, table5 E

 WHERE A.col = B.col
   AND A.col = C.col
   AND C.col = D.col
   AND C.col = E.col
   AND A.anotherColumn = 'SOMEARG'

the response time is well over 2 minutes. I have analyzed the tables and indexes to death and tried count(1), count(*), decode(count(*), 0, 0, 1), count(rowid), all to no avail.

Can anyone help me understand why the same query would perform so poorly when I'm only trying to retrieve the count rather then the underlying data. In my naive head I thought with all things being equal the count would perform equally if not better.

Thanks for all your help,
Andreas Received on Fri Sep 07 2001 - 17:46:16 CDT