Is there any chance to get some kind of a result set sifting mechanism in Postgres?

Hello Everyone!

Is there any chance to get some kind of a result set sifting mechanism in
Postgres?

What I am looking for is a way to get for example: "nulls last" in a result
set, without having to call "order by" or having to use UNION ALL, and if
possible to get this in a single result set pass.

Something on this line: SELECT a, b, c FROM my_table WHERE a nulls last
OFFSET 0 LIMIT 25

I don't want to use order by or union all because these are time consuming
operations, especially on  large data sets and when comparations are done
on dynamic values (eg: geolocation distances in between a mobile and a
static location)

What I would expect from such a feature, will be speeds comparable with non
sorted selects, while getting a very rudimentary ordering.

A use case for such a mechanism will be the implementation of QUICK
relevant search results for a search engine.

I'm not familiar with how Postgres logic handles simple select queries, but
the way I would envision a result set sifting logic, would be to collect
the result set, in 2 separate lists, based on the sifting condition, and
then concatenate these 2 lists and return the result, when the pagination
requests conditions are met.

Any idea if such a functionality is feasible ?

Thank you.

  PS: if ever implemented, the  sifting mechanism could be extended to
accommodate any type of thresholds, not just null values.