Re: No merge sort?

Taral <taral@taral.net> writes:
> On Thu, Mar 13, 2003 at 10:30:27PM -0500, Tom Lane wrote:
>> The idea is you look at the index to make a list of main-table tuple
>> positions you are interested in, which you represent compactly as a
>> compressed bitmap.  [snip]

> And it loses bigtime in the case of LIMIT. If the unlimited query
> returns 4,000 records and I only want 20, you're retrieving 200x too
> much data from disk.

Sure.  That's why we have a planner that distinguishes between startup
cost and total cost, and interpolates when a LIMIT is involved.  But
if this mergesort idea only helps for small-limit cases, that's another
restriction on its scope of usefulness...
        regards, tom lane