OR condition causing unnecessary lag ?

[John Tan]

i have a query to show all non-cancelled records that goes something like this

Select * from Parent,Child where Parent.ID = Child.ID
AND ( Parent_cancelled = 0 AND Child_cancelled = 0 )

- this query took 6 seconds

now i wanted to show ALL CANCELLED records even though there aren't any
and so i have made this query and expected it to complete much less than 6 sec.

Select * from Parent,Child where Parent.ID = Child.ID
AND ( Parent_cancelled = 1 OR Child_cancelled = 1 )

- BUT this query actually took 6 seconds STILL ..

so i did some experiments and changed the filter to AND Parent_cancelled = 1
- this query showed instantly

also AND Child_cancelled = 1 showed instantly because there are no cancelled records


why combining AND ( Parent_cancelled = 1 OR Child_cancelled = 1 )
is very slow ?