Connected to Oracle Database 10g Enterprise Edition Release  10 . 1 . 0 . 5 . 0  
Connected as test

SQL> create table space_acc
   2   (
   3   DAT DATE not null,
   4   GBYTES NUMBER not null
   5   );

Table created

SQL> alter table space_acc add primary key (DAT) using index;

Table altered

SQL> INSERT INTO space_acc
   2       SELECT SYSDATE - round( 10000  /  2 ) + rownum +  0 . 5  * dbms_random.VALUE() AS dat,
   3              round( 500  +  10  * rownum +  20  * dbms_random.VALUE()) AS GBytes
   4         FROM (SELECT * FROM dual CONNECT BY  1  =  1 )
   5        WHERE rownum <  10000 
   6   ;

 9999  rows inserted

SQL> exec dbms_stats.gather_schema_stats (ownname => user);

PL/SQL procedure successfully completed

SQL> set serveroutput on;
SQL> 
SQL> declare
   2     type record_result is record (d_begin date, d_end date, s_begin number);
   3     type record_list is table of record_result;
   4     type record_cur is ref cursor return record_result;
   5     cr record_cur;
   6     r record_list;
   7     t1 timestamp;
   8   begin
   9     -- Мой вариант
  10     t1 := systimestamp;
  11     open cr for select
  12                 s.dat d_begin,
  13                 lead (s.dat) over (order by s.dat) d_end,
  14                 s.gbytes s_begin
  15                 from
  16                 space_acc s;
  17     fetch cr bulk collect into r;
  18     close cr;
  19     dbms_output.put_line ('Мой вариант: ' || (systimestamp - t1));
  20     -- Ваш вариант
  21     t1 := systimestamp;
  22     open cr for SELECT space_acc.dat AS d_begin,
  23                  MIN(space_acc1.dat) AS d_end,
  24                  space_acc.gbytes AS s_begin
  25                  FROM space_acc
  26                  JOIN space_acc space_acc1 ON space_acc.dat < space_acc1.dat
  27                  GROUP BY space_acc.dat, space_acc.gbytes;
  28     fetch cr bulk collect into r;
  29     close cr;
  30     dbms_output.put_line ('Ваш вариант: ' || (systimestamp - t1));
  31   end;
  32   /

Мой вариант: + 000000000   00 : 00 : 00 . 047000000 
Ваш вариант: + 000000000   00 : 01 : 13 . 313000000 

PL/SQL procedure successfully completed