Postgresql : How do I select top n percent(%) entries from each group/category

To retrieve the rows based on the percentage of the number of rows in each group you can use two window functions: one to count the rows and one to give them a unique number.

select gp,       valfrom (  select gp,          val,         count(*) over (partition by gp) as cnt,         row_number() over (partition by gp order by val desc) as rn  from temp) twhere rn / cnt <= 0.75;

SQLFiddle example: http://sqlfiddle.com/#!15/94fdd/1

Btw: using char is almost always a bad idea because it is a fixed-length data type that is padded to the defined length. I hope you only did that for setting up the example and don't use it in your real table.

Referencing the response from a_horse_with_no_name, you can achieve something similar using percent_rank()

SELECT    gp,    val,    pct_rankFROM (    SELECT        gp,        val,        percent_rank() over (order by val desc) as pct_rank    FROM variables.temp    ) tWHERE pct_rank <= 0.75;

You can then set the final WHERE clause to return data at whatever percent_rank() threshold you require.

The accepted answer did not work for me. I find this solution that works for me:

SELECT * FROM temp ORDER BY val DESC     LIMIT (SELECT (count(*) / 10) AS selnum FROM temp )

It is not optimal (performance) but it works