Question

I have following SQL query that selects some results from my table:

select  avg(c3), count(c3), std  
from ssims where obraz = 'lena' group by std order by std

But I have various number of tests performed for different values of std, so it returns me something like that:

0.906176136363636;44;5
0.881669302325582;43;10
0.855873409090909;44;15
0.829195813953488;43;20
0.802071590909091;44;25
0.774523720930233;43;30
0.747213636363636;44;35
0.720115581395349;43;40
0.694712954545455;44;45
0.668683255813953;43;50

What I would like to do is to select average of constant (i.e. 20) number of results for every std value. So after such query, the second column would be 20 for each row.

How to do it? I tried limits and top, but without success

Answer 1

In PostgreSQL 8.3:

SELECT  a[1] AS avg_std, a[2] AS cnt_std, std
FROM    (
        SELECT  (
                SELECT  ARRAY[AVG(c3) , COUNT(*)]
                FROM    (
                        SELECT  c3
                        FROM    ssims si
                        WHERE   obraz = 'lena'
                                AND si.std = so.std
                        ORDER BY
                                id
                        LIMIT 20
                        ) q
                ) a
        FROM    (
                SELECT  DISTINCT std
                FROM    ssims
                WHERE   obraz = 'lena'
                ) so
        ) q

This will count both AVG and COUNT in a single index scan for each std.

Create a composite index on (obraz, std, id) for this to work fast.

In PostgreSQL 8.4:

SELECT  AVG(c3), COUNT(*), std
FROM    (
        SELECT  std, c3, ROW_NUMBER() OVER (PARTITION BY std ORDER BY id) AS rn
        FROM    ssims
        WHERE   obraz = 'lena'
        ) q
WHERE   rn <= 20
GROUP BY
        std