1  with t(z,d) as (
  2  select 1,'t1' from dual union all
  3  select 2,'t2' from dual union all
  4  select 2,'t3' from dual union all
  5  select 1,'t4' from dual union all
  6  select 4,'t5' from dual union all
  7  select 2,'t6' from dual union all
  8  select 4,'t7' from dual union all
  9  select 5,'t8' from dual )
 10  ,tt as (
 11  select
 12    t.*
 13   ,lag(z,1,-z) over (order by d) la
 14  from t
 15  )
 16* select z,d from tt where z<>la order by d
SQL> /

         Z D
---------- --
         1 t1
         2 t2
         1 t4
         4 t5
         2 t6
         4 t7
         5 t8

7 rows selected.

  1  with t(z,d) as (
  2    select 'a','t1' from dual union all
  3    select 'b','t2' from dual union all
  4    select 'b','t3' from dual union all
  5    select 'b','t4' from dual union all
  6    select 'c','t5' from dual union all
  7    select 'a','t6' from dual union all
  8    select 'a','t7' from dual union all
  9    select 'b','t8' from dual )
 10    ,tt as (
 11    select
 12      t.*
 13     ,lag(z,1,z||'*') over (order by d) la
 14    from t
 15    )
 16* select z,d from tt where z<>la order by d
SQL> /

Z D
- --
a t1
b t2
c t5
a t6
b t8

  1  with t(z,d) as (
  2    select '','t1' from dual union all
  3    select '','t2' from dual union all
  4    select 'c','t3' from dual union all
  5    select '','t4' from dual union all
  6    select 'c','t5' from dual union all
  7    select 'a','t6' from dual union all
  8    select 'a','t7' from dual union all
  9    select '','t8' from dual )
 10    ,tt as (
 11    select
 12      t.*
 13     ,lag(z,1,z||'*') over (order by d) la
 14    from t
 15    )
 16  select z,d
 17   from tt
 18* where (z is not null and z<>nvl(la,z||'*')) or (z is null and la is not null) order by d
SQL> /

Z D
- --
  t1
c t3
  t4
c t5
a t6
  t8

6 rows selected.

SQL> select * from t order by d;

Z          D
---------- ----------
null       t1
null       t2
c          t3
null       t4
c          t5
a          t6
a          t7
null       t8

8 rows selected.

SQL> ed
Wrote file afiedt.buf

  1  with
  2    tt as (
  3    select
  4      t.*
  5     ,lag(z,1,z||'*') over (order by d) la
  6    from t --Ваша таблічка на 100 000
  7    )
  8  select z,d
  9   from tt
 10* where (z is not null and z<>nvl(la,z||'*')) or (z is null and la is not null) order by d
SQL> /

Z          D
---------- ----------
null       t1
c          t3
null       t4
c          t5
a          t6
null       t8

6 rows selected.