Strange select query

Hellow,

I notice wired behavior of backend,

for instance I've 3 tables TA, TB, TC with about 1k of records each,

and I do something like that:

select * from TA as a, TB as b, TC as c where
position('some text' in a.textfield)>0 or
(position('some text' in b.textfield)>0 and a.index=b.referencefield)
or
(position('some text' in c.textfield)>0 and a.index=b.referencefield
and b.other_referencefield=c.index);

the backend start to execute query, but it seems to be 'never
ending story' because after 15 minutes of work I still haven't result,
and backend still was eating my RAM. I know that this query is
ugly and bad, but I think backend will reject such query at parsing.

regards
Robert Partyka