create table tbl(x);
create index tbl_x_idx on tbl(x);
insert into tbl(x) values ( 1 );
insert into tbl(x) values ( 2 );
insert into tbl(x) values ( 3 );
insert into tbl(x) values ( 5 );
insert into tbl(x) values ( 6 );
insert into tbl(x) values ( 7 );

explain  query plan
select x,
  (select max(prev.x) from tbl as prev where prev.x<tbl.x) as prev,
  (select min(next.x) from tbl as next where next.x>tbl.x) as next
from tbl order by x asc;

x|prev|next
 1 || 2 
 2 | 1 | 3 
 3 | 2 | 5 
 5 | 3 | 6 
 6 | 5 | 7 
 7 | 6 |