Select TOP N and BOTTOM N

Question

Trying to fetch top n bottom n rows. Though it gives me result but, it takes lot of time. I believe it scans table twice.

Code used:
WITH TI AS
(SELECT * FROM
(S         


        

Answer 1

You can combine into a single query and a single pass over the table using analytic functions, generating two pseudocolumns in this case:

select column1, column2, column3,
  row_number() over (order by column1 desc) rn_desc,
  row_number() over (order by column1 asc) rn_asc
from your_table;

and then filtering using that query as an inline view (or CTE):

select column1, column2, column3
from (
  select column1, column2, column3,
    row_number() over (order by column1 desc) as rn_desc,
    row_number() over (order by column1 asc) as rn_asc
  from your_table
)
where rn_desc <=5
or rn_asc <= 5;

I've assumed your ordering is on column1, and picked your_table as a table name as you didn't include that either, so change as appropriate. Depending on how you want to handle ties, you might want to use the rank() or dense_rank() functions instead.

---

From @mathguy's comment, this may well perform better:

select column1, column2, column3
from (
  select column1, column2, column3,
    row_number() over (order by column1 desc) as rn,
    count(*) over () as cnt
  from your_table
)
where rn <=5
or cnt - rn < 5;

Answer 2

The best way to solve this problem depends in part on your Oracle version. Here is a very simple (and, I suspect, very efficient) solution using the match_recognize clause, added in version 12.1.

I illustrate it using the EMPLOYEES table in the standard HR schema, ordering by SALARY. The only trick here is to select the top and bottom five rows, and to ignore everything in between; that (the "ignoring") is what the {- ... -} operator does in the pattern sub-clause.

select employee_id, first_name, last_name, salary
from   hr.employees
match_recognize(
  order by salary desc
  all rows per match
  pattern ( a{5} {- a* -} a{5} )
  define a as 0 = 0             -- For reasons known only to Oracle, DEFINE is required.
);

EMPLOYEE_ID FIRST_NAME           LAST_NAME                     SALARY
----------- -------------------- ------------------------- ----------
        100 Steven               King                           24000
        101 Neena                Kochhar                        17000
        102 Lex                  De Haan                        17000
        145 John                 Russell                        14000
        146 Karen                Partners                       13500
        135 Ki                   Gee                             2400
        127 James                Landry                          2400
        136 Hazel                Philtanker                      2200
        128 Steven               Markle                          2200
        132 TJ                   Olson                           2100