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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