
EXPLAIN
SELECT COUNT(*) FROM intersects_order t1, intersects_order t2 
WHERE ST_Intersects(t1.geom, t2.geom);
/* result in 8.4:
 Aggregate  (cost=383.62..383.63 rows=1 width=0)
   ->  Nested Loop  (cost=0.00..381.54 rows=831 width=0)
         Join Filter: _st_intersects(t1.geom, t2.geom)
         ->  Seq Scan on intersects_order t1  (cost=0.00..11.23 rows=623 width=28667)
         ->  Index Scan using idx_i_o_geom on intersects_order t2  (cost=0.00..0.33 rows=1 width=28667)
               Index Cond: (t1.geom && t2.geom)
(6 rows)


and in 9.1

 Aggregate  (cost=2521.62..2521.63 rows=1 width=0)
   ->  Nested Loop  (cost=0.00..2519.54 rows=831 width=0)
         Join Filter: _st_intersects(t1.geom, t2.geom)
         ->  Seq Scan on intersects_order t1  (cost=0.00..565.23 rows=623 width=28667)
         ->  Index Scan using idx_i_o_geom on intersects_order t2  (cost=0.00..2.87 rows=1 width=28667)
               Index Cond: (t1.geom && geom)
(6 rows)

*/

EXPLAIN
SELECT COUNT(*) FROM intersects_order t1, intersects_order t2 
WHERE ST_Intersects(t2.geom, t1.geom);

/* result in 8.4:

 Aggregate  (cost=383.62..383.63 rows=1 width=0)
   ->  Nested Loop  (cost=0.00..381.54 rows=831 width=0)
         Join Filter: _st_intersects(t2.geom, t1.geom)
         ->  Seq Scan on intersects_order t1  (cost=0.00..11.23 rows=623 width=28667)
         ->  Index Scan using idx_i_o_geom on intersects_order t2  (cost=0.00..0.33 rows=1 width=28667)
               Index Cond: (t2.geom && t1.geom)


and in 9.1

 Aggregate  (cost=2521.62..2521.63 rows=1 width=0)
   ->  Nested Loop  (cost=0.00..2519.54 rows=831 width=0)
         Join Filter: _st_intersects(t2.geom, t1.geom)
         ->  Seq Scan on intersects_order t1  (cost=0.00..565.23 rows=623 width=28667)
         ->  Index Scan using idx_i_o_geom on intersects_order t2  (cost=0.00..2.87 rows=1 width=28667)
               Index Cond: (geom && t1.geom)
(6 rows)


*/