See the following results:

Code:
select count(*) from table1
--9746801

select count(*) from table2
--3845

SELECT count(*)
FROM table1 
WHERE table1.col3<=SYSDATE
AND (
(
table1.col4='a' 
AND 
table1.col5<100
)
OR
(
table1.col4='b' 
AND 
(
table1.col5=10 
OR
table1.col5=20
)
)
) 
AND table1.col6<>'abc' 
AND table1.col6<>'def' 
AND table1.col6<>'ghi'
--857747 in 138.389 seconds

SELECT count(1)
FROM table2 WHERE table2.colc='xy' 
AND table2.cold='z'
--77 in 0.441 seconds