geometry poligons performance and index

 Hi
 I need to set a query which performs a intersection check between two set
of
 polygons which are stored into two tables.
 I wrote the where instruction so:
 ...
 where area(intersection(t1.the_geom,t2.the_geom))>0
 ...

 The results are ok, but the query is too slow. I tried to applay a GIST and
 gtree index but performance did not improve.
 By explaining the query I see that the where condition gets the query slow.
 Is there a more simple way to check if two poligons have a not null
 intersection (they overlap)?

 Is there a kind of index I can use ?

 THanks

 Giu

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