another optimizer question

hi,
can anyone explain me why there are different query plans for "select ...
from ... where y!=x"  and "select ... from ... where y<x or y>x" for
integers, please?
see the details below...

thanks,
kuba

db_cen7=# analyze;
ANALYZE

db_cen7=# \d ts19                                        Table "ts19" Column   |           Type           |
         Modifiers
 
-----------+--------------------------+--------------------------------------------------------ts19pk___ | integer
           | not null default
 
nextval('"ts19_ts19pk____seq"'::text)ts19datum | timestamp with time zone | not nullts19zavaz | integer
| not nullts19cislo | integer                  | not nullts19text_ | character varying(65536) | not nullts19idpri |
integer                 | not null
 
Indexes: ts19_ts19zavaz_idx
Primary key: ts19_pkey

db_cen7=# explain analyze select * from ts19 where ts19zavaz != 7 order by
ts19pk___ desc limit 10;
NOTICE:  QUERY PLAN:

Limit  (cost=89635.63..89635.63 rows=1 width=38) (actual
time=50868.17..50868.18 rows=10 loops=1) ->  Sort  (cost=89635.63..89635.63 rows=1 width=38) (actual
time=50868.16..50868.17 rows=11 loops=1)       ->  Seq Scan on ts19  (cost=0.00..89635.62 rows=1 width=38)
(actual time=95.99..50852.34 rows=300 loops=1)
Total runtime: 50868.27 msec

db_cen7=# explain analyze select * from ts19 where ts19zavaz < 7 or
ts19zavaz > 7 order by ts19pk___ desc limit 10;
NOTICE:  QUERY PLAN:

Limit  (cost=4.04..4.04 rows=1 width=38) (actual time=1118.28..1118.29
rows=10 loops=1) ->  Sort  (cost=4.04..4.04 rows=1 width=38) (actual
time=1118.27..1118.28 rows=11 loops=1)       ->  Index Scan using ts19_ts19zavaz_idx, ts19_ts19zavaz_idx on
ts19  (cost=0.00..4.03 rows=1 width=38) (actual time=0.03..1117.58
rows=300 loops=1)
Total runtime: 1118.40 msec

the runtime times depends on the machine load but generally the second
query is much faster...

more info:

db_cen7=# select count(*) from ts19; count
---------4190527
(1 row)

db_cen7=# select distinct(ts19zavaz) from ts19;ts19zavaz
-----------        3        7
(2 rows)

db_cen7=# select count(*) from ts19 where ts19zavaz = 3;count
-------  300
(1 row)

db_cen7=# select version();                           version
---------------------------------------------------------------PostgreSQL 7.2.1 on i686-pc-linux-gnu, compiled by GCC
2.95.4
(1 row)