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.

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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;