PostgreSQL 9.4 prepared statement behavior

gistfile1.txt

marmotta=# -- prepare statement
marmotta=# prepare boom as SELECT id FROM triples WHERE subject = $1 AND predicate = $2 AND object = $3 AND context =
$4 AND deleted = false;
PREPARE

marmotta=# -- run 1
marmotta=# explain execute boom('718127302357929984', '564051914678095872', '564051914636152835', '564051914489352192');
                                                                  QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_triples_spo on triples  (cost=0.57..3.60 rows=1 width=8)
   Index Cond: ((subject = 718127302357929984::bigint) AND (predicate = 564051914678095872::bigint) AND (object = 5640
51914636152835::bigint))
   Filter: (context = 564051914489352192::bigint)
(3 rows)

marmotta=# -- run 2
explain execute boom('718127302357929984', '564051914678095872', '564051914636152835', '564051914489352192');
                                                                  QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_triples_spo on triples  (cost=0.57..3.60 rows=1 width=8)
   Index Cond: ((subject = 718127302357929984::bigint) AND (predicate = 564051914678095872::bigint) AND (object = 5640
51914636152835::bigint))
   Filter: (context = 564051914489352192::bigint)
(3 rows)

marmotta=# -- run 3
explain execute boom('718127302357929984', '564051914678095872', '564051914636152835', '564051914489352192');
                                                                  QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_triples_spo on triples  (cost=0.57..3.60 rows=1 width=8)
   Index Cond: ((subject = 718127302357929984::bigint) AND (predicate = 564051914678095872::bigint) AND (object = 5640
51914636152835::bigint))
   Filter: (context = 564051914489352192::bigint)
(3 rows)

marmotta=# -- run 4
explain execute boom('718127302357929984', '564051914678095872', '564051914636152835', '564051914489352192');
                                                                  QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_triples_spo on triples  (cost=0.57..3.60 rows=1 width=8)
   Index Cond: ((subject = 718127302357929984::bigint) AND (predicate = 564051914678095872::bigint) AND (object = 564051914636152835::bigint))
   Filter: (context = 564051914489352192::bigint)
(3 rows)

marmotta=# -- run 5
explain execute boom('718127302357929984', '564051914678095872', '564051914636152835', '564051914489352192');
                                                                  QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_triples_spo on triples  (cost=0.57..3.60 rows=1 width=8)
   Index Cond: ((subject = 718127302357929984::bigint) AND (predicate = 564051914678095872::bigint) AND (object = 564051914636152835::bigint))
   Filter: (context = 564051914489352192::bigint)
(3 rows)

marmotta=# -- run 6
explain execute boom('718127302357929984', '564051914678095872', '564051914636152835', '564051914489352192');
                                  QUERY PLAN
-------------------------------------------------------------------------------
 Index Scan using idx_triples_cop on triples  (cost=0.57..3.60 rows=1 width=8)
   Index Cond: ((context = $4) AND (object = $3) AND (predicate = $2))
   Filter: (subject = $1)
(3 rows)