inet <<= and indexes

I have a table with about 60,000 ips, and postgres will use the index to
find the ips in a subnet if I type in a literal cidr block
(e.g. address <<= '99.88.5.0'), but it will not use the index if the cidr
block is in a variable (e.g. address <<= block). I have tried setting
ENABLE_SEQSCAN to false and using different values for ALTER TABLE...SET
STATISTICS, but it still uses sequential scans.

Here are several example EXPLAIN results. At the bottom, I have included a
simple python script which I used to set up the test environment.

-Edwin Grubbs

# psql egrubbs
egrubbs=> \d ip
         Table "ip"
 Column  | Type | Modifiers
---------+------+-----------
 address | inet |
Unique keys: ip_address_index

egrubbs=> \d network
      Table "network"
 Column | Type | Modifiers
--------+------+-----------
 block  | cidr |

egrubbs=> explain select * from ip where address <<= '99.88.5.0/24';
NOTICE:  QUERY PLAN:

Index Scan using ip_address_index on ip  (cost=0.00..150.39 rows=32768
width=12)
EXPLAIN
egrubbs=> explain select * from ip where address <<= (select inet
'99.88.5.0/24');
NOTICE:  QUERY PLAN:

Seq Scan on ip  (cost=0.00..1205.20 rows=32768 width=12)
  InitPlan
    ->  Result  (cost=0.00..0.01 rows=1 width=0)

EXPLAIN
egrubbs=> explain select * from ip where address <<= (select cidr
'99.88.5.0/24');
NOTICE:  QUERY PLAN:

Seq Scan on ip  (cost=0.00..1205.20 rows=32768 width=12)
  InitPlan
    ->  Result  (cost=0.00..0.01 rows=1 width=0)

EXPLAIN
egrubbs=> explain select * from ip join network on address <<= block;
NOTICE:  QUERY PLAN:

Nested Loop  (cost=0.00..3722.14 rows=65536 width=24)
  ->  Seq Scan on network  (cost=0.00..1.02 rows=2 width=12)
  ->  Seq Scan on ip  (cost=0.00..1041.36 rows=65536 width=12)

EXPLAIN
egrubbs=> explain select * from ip, network where address <<= block;
NOTICE:  QUERY PLAN:

Nested Loop  (cost=0.00..3722.14 rows=65536 width=24)
  ->  Seq Scan on network  (cost=0.00..1.02 rows=2 width=12)
  ->  Seq Scan on ip  (cost=0.00..1041.36 rows=65536 width=12)

EXPLAIN
egrubbs=> \q

------------- python script for setting up test data ---------------
#!/usr/bin/python2.2

import psycopg

db = psycopg.connect('dbname=egrubbs')
db.autocommit()
cursor = db.cursor()

cursor.execute('''CREATE TABLE ip (address inet)''')

for i in range(0, 256):
    print i
    for j in range(0, 256):
        cursor.execute('''
            INSERT INTO ip (address)
            VALUES ('99.88.%d.%d')
            ''' % (i, j))

cursor.execute('''CREATE UNIQUE INDEX ip_address_index ON ip (address)''')

cursor.execute('''CREATE TABLE network (block cidr)''')
cursor.execute('''
    INSERT INTO network (block)
    VALUES ('99.88.5.0/24')
    ''')
cursor.execute('''
    INSERT INTO network (block)
    VALUES ('99.88.12.0/24')
    ''')

cursor.execute('''VACUUM ANALYZE ip''')
cursor.execute('''VACUUM ANALYZE network''')
------------------- end of script ----------------------------