ORDER BY and LIMIT not propagated on inherited tables / UNIONs

Hi,

I'm using inherited tables to partition some data which can grow very
large. Recently I discovered that a simple query that on a regular table
would use an index was instead using seq scans (70s vs a guessed 2s).
The well known query is:

SELECT foo FROM bar ORDER BY foo DESC LIMIT 1

(The same applies for SELECT MIN(foo) FROM bar using 8.1)

The query plan generated when running the query on a table which has
inheritance forces the planner to choose a seq_scan for each table.
Wouldn't be a good thing to also promote ORDER BYs and LIMITs to each
subscan (like WHERE does)?

I needed a quick solution, so I wrote a function which looks each
inherited table separately and my problem is partially solved, but I
think that a (hopefully) little change in the optimizer could be advisable.

Attached are some EXPLAIN ANALYZE outputs of my suggestion.

Best regards
--
Matteo Beccati
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