Is this planner choice easily explained?

This is a summary from a thread on the dbmail mailing list.

I am trying to understand some apparently odd behaviour with 7.2.1 and
7.2.3. Adding a LIMIT statement causes what seems like a very bad choice of
strategy. The first statement is fine:

     explain SELECT messageblk FROM messageblks WHERE message_idnr =
100::bigint
     ORDER BY messageblk_idnr ;

gives:

     Sort  (cost=5793.33..5793.33 rows=1453 width=40)
       ->  Index Scan using messageblks_msg_idx on messageblks
     (cost=0.00..5716.99 rows=1453 width=40)

and returns almost instantly, whereas, just adding a limit:

     explain SELECT messageblk FROM messageblks WHERE message_idnr =
100::bigint
     ORDER BY messageblk_idnr
     limit 1;

gives:

     Limit  (cost=0.00..777.50 rows=1 width=40)
       ->  Index Scan using messageblks_id_idx on messageblks
     (cost=0.00..1129984.15 rows=1453 width=40)


which takes several minutes to run.

The relevant metadata is:

                                Table "messageblks"
      Column      |  Type  |                       Modifiers
-----------------+--------
+-------------------------------------------------------
  messageblk_idnr | bigint | not null default
nextval('messageblk_idnr_seq'::text)
  message_idnr    | bigint | not null default '0'
  messageblk      | text   | not null
  blocksize       | bigint | not null default '0'
Indexes: messageblks_msg_idx
Primary key: messageblks_pkey
Unique keys: messageblks_id_idx


Index "messageblks_id_idx"
      Column      |  Type
-----------------+--------
  messageblk_idnr | bigint
unique btree


Index "messageblks_msg_idx"
     Column    |  Type
--------------+--------
  message_idnr | bigint
btree


If anyone could explain the likely reasons for the choice, I would be very
interested. Even given the planner estimates, they don't look like sensible
choices.




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