limit clause breaks query planner?

Hi,

I have a single table with about 10 million rows, and two indexes. Index A
is on a column A with 95% null values. Index B is on a column B with about
10 values, ie. About a million rows of each value.

When I do a simple query on the table (no joins) with the following
condition:

A is null AND

B = '21'

it uses the correct index, index B. However, when I add a limit clause of
15, postgres decides to do a sequential scan :s. Looking at the results
from explain:

"Limit (cost=0.00..3.69 rows=15 width=128)"

" -> Seq Scan on my_table this_ (cost=0.00..252424.24 rows=1025157
width=128)"

" Filter: ((A IS NULL) AND ((B)::text = '21'::text))"

It appears that postgres is (very incorrectly) assuming that it will only
have to retrieve 15 rows on a sequential scan, and gives a total cost of
3.69. In reality, it has to scan many rows forward until it finds the
correct value, yielding very poor performance for my table.

If I disable sequential scan (set enable_seqscan=false) it then incorrectly
uses the index A that has 95% null values: it seems to incorrectly apply the
same logic again that it will only have to retrieve 15 rows with the limit
clause, and thinks that the index scan using A is faster than index scan B.

Only by deleting the index on A and disabling sequential scan will it use
the correct index, which is of course by far the fastest.

Is there an assumption in the planner that a limit of 15 will mean that
postgres will only have to read 15 rows? If so is this a bad assumption?
If a particular query is faster without a limit, then surely it will also be
faster with the limit.

Any workarounds for this?

Thanks

David