Highly obscure and erratic

hi,
 I cannot get what causes the difference in the execution of these 2 commands
1. SELECT * FROM dbmedia WHERE ID IN ((SELECT id FROM wdmedia WHERE word = 'whatever') )  LIMIT 20;
2. SELECT * FROM dbmedia WHERE ID IN (1234,2345,1242,1256,1245,1567,2222,22,345,234,567,456,35,56) ;

  where ID is the primary key.
  First let me tell that i have done all vacuum's ..analyze,full et all.
The explain output should make it clear that for the former a "sequential"
scan is done and for the later an "index" scan is done.Also the time
take indicates that a sequential scan(atleast not index scan) is done
on the former.
-------------------------------------------------------------------------
explain SELECT * FROM dbmedia WHERE ID IN ((SELECT id FROM wdmedia WHERE word = 'whatever') )  LIMIT 20;
NOTICE:  QUERY PLAN:

Limit  (cost=0.00..100544.53 rows=20 width=76)
  ->  Seq Scan on dbmedia  (cost=0.00..507161673.46 rows=100883 width=76)
      ^^^^^^^                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  GAWD!!
      SubPlan
                    ->  Materialize  (cost=5027.19..5027.19 rows=2575 width=4)
                                    ->  Index Scan using wdkmedia on wdmedia  (cost=0.00..5027.19 rows=2575 width=4)


----------------------------------------------------------------------------

 explain SELECT * FROM dbmedia WHERE ID IN (1234,2345,1242,1256,1245,1567,2222,22,345,234,567,456,35,56) ;
 NOTICE:  QUERY PLAN:

 Index Scan using dbmedia_pkey, dbmedia_pkey, dbmedia_pkey, dbmedia_pkey, dbmedia_pkey, dbmedia_pkey, dbmedia_pkey,
dbmedia_pkey,dbmedia_pkey, dbmedia_pkey, dbmedia_pkey, dbmedia_pkey, dbmedia_pkey, dbmedia_pkey on dbmedia
(cost=0.00..59.40rows=1 width=76) 
 --------------------------------------------------------------------------

 observe the index scan..

OK , for those ppl who might be feeling that the o/p of the nested query in the
former case might be a significant portion (and so the db does a sq scan)
let me say that the result set of sub-query  (SELECT id FROM wdmedia WHERE word = 'whatever') is of length 1,000 while
thetable dbmedia is of length 
100,000 and if a db selects to do a seq scan due to this then ... i can
say no more.
 postgresql developers/gurus please help.
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Varun
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