Extra cost of "lossy mode" Bitmap Scan plan

Hi.

I found the current planner doesn't care about "lossy mode" on Bitmap Scan.
I think costs of following two plans are equal now.

(a) Having enough work_mem => normal Bitmap Scan.

(b) Having less work_mem than estimated rows => Bitmap Scan with "lossy mode".
Slower than (a).

So, sometimes the planner makes a wrong choice.
For example, on some queries the planner doesn't choose a faster Index Scan plan
but a much slower Bitmap Scan (in actually lossy).

My understanding is that we can know whether the plan is lossy or not
like following.

int tbm_maxentries = work_mem * 1024L;
if (estimatedNumLows < tbm_maxentries) {
/* not lossy */
} else {
/* lossy : we may add some extra costs to total costs */
}

Any ideas how to do this?

Best regards,

-----
MINOWA Taro (Higepon)

Cybozu Labs, Inc.

http://www.monaos.org/
http://code.google.com/p/mosh-scheme/