在Postgres的數組字段上應用聚合函數？

Question

是否有可能對整數[]字段（或其他數組數組）中的所有值應用聚合（如avg（），stddev（））？

CREATE TABLE widget 
(
    measurement integer[] 
); 

insert into widget (measurement) values ('{1, 2, 3}'); 

select avg(measurement::integer[]) from widget; 

ERROR: function avg(integer[]) does not exist 
LINE 4: select avg(measurement::integer[]) from widget; 
      ^
HINT: No function matches the given name and argument types. You might need to add explicit type casts. 

********** Error ********** 

ERROR: function avg(integer[]) does not exist 
SQL state: 42883 
Hint: No function matches the given name and argument types. You might need to add explicit type casts. 
Character: 71 

我可以解決通過分割陣列爲多行像

select avg(m)::float from (select unnest(measurement) m from widget) q; 

但它是那麼優雅。

謝謝。

Answer 1

你可以這樣創建簡單的功能：

create function array_avg(_data anyarray) 
returns numeric 
as 
$$ 
    select avg(a) 
    from unnest(_data) as a 
$$ language sql; 

和查詢像這樣

select avg(array_avg(measurement)) 
from widget; 

，或者你可以簡單地做

select avg((select avg(a) from unnest(measurement) as a)) 
from widget; 

sql fiddle demo