CREATE TABLE dbmail_messages
(
  message_idnr int8 NOT NULL DEFAULT nextval('dbmail_message_idnr_seq'::text),
...
);

CREATE INDEX dbmail_messages_idx
  ON dbmail_messages
  USING btree
  (message_idnr);

explain 
select COUNT(*)
from dbmail_messages
WHERE message_idnr =  1109589 

"Aggregate  (cost=4119.34..4119.34 rows=1 width=0)"
"  ->  Index Scan using dbmail_messages_idx on dbmail_messages  (cost=0.00..4116.43 rows=1160 width=0)"
"        Index Cond: (message_idnr = 1109589)"

explain 
select COUNT(*)
from dbmail_messages
WHERE message_idnr =  1109589  OR  message_idnr =  1109243 

"Aggregate  (cost=7860.32..7860.32 rows=1 width=0)"
"  ->  Seq Scan on dbmail_messages  (cost=0.00..7854.54 rows=2313 width=0)"
"        Filter: ((message_idnr = 1109589) OR (message_idnr = 1109243))"

...
WHERE message_idnr IN ( 1109589 , 1109243 , 1108982 , 1108678 , ...порядка  90  констант)