create table qq (id serial, p geography(Point,4326), s int4);

insert into qq (p,s) select 
ST_SetSRID(ST_MakePoint(p.lat, p.long),4326 )
, (random()*1000)::int 
from (select (0.5 - random()) * 180 as lat, random()*90 as long from 
    (select generate_series(1,1000000)) as t) as p;


CREATE INDEX idx_spatial_pop_geom
  ON qq
  USING gist
  (p);

select st_distance(p,ST_SetSRID(ST_MakePoint(55.5, 45.45), 4326)) AS RNG  from qq 
 where st_distance(p,ST_SetSRID(ST_MakePoint(55.5, 45.45), 4326)) < 300000

Seq Scan on qq  (cost=0.00..357197.25 rows=333333 width=56)
  Filter: (_st_distance(p, '0101000020E61000000000000000C04B409A99999999B94640'::geography, 0::double precision, true) < 300000::double precision)

select st_distance(p,ST_SetSRID(ST_MakePoint(55.5, 45.45), 4326)) AS RNG  from qq 
 ORDER BY p <-> ST_SetSRID(ST_MakePoint(55.5, 45.45), 4326)::geography  asc limit 10

ERROR:  operator does not exist: geography <-> geography
LINE 22:  ORDER BY p <-> ST_SetSRID(ST_MakePoint(55.5, 45.45), 4326):...
                     ^
HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
********** Error **********

ERROR: operator does not exist: geography <-> geography
SQL state: 42883
Hint: No operator matches the given name and argument type(s). You might need to add explicit type casts.
Character: 645

select st_distance(p,ST_SetSRID(ST_MakePoint(55.5, 45.45), 4326)) from qq 
where ST_DWithin(p, ST_SetSRID(ST_MakePoint(55.5, 45.45), 4326), 300000, true)