How to use a SQL window function to calculate a percentage of an aggregate

Question

I need to calculate percentages of various dimensions in a table. I\'d like to simplify things by using window functions to calculate the denominator, however I am having an iss

Answer 1

I think you are looking for this:

SELECT d1, d2, sum(v)/sum(sum(v)) OVER (PARTITION BY d1) AS share
FROM   test
GROUP  BY d1, d2;

Produces the requested result.

Window functions are applied after aggregate functions. The outer sum() in sum(sum(v)) OVER ... is a window function (attached OVER ... clause) while the inner sum() is an aggregate function.

Effectively the same as:

WITH x AS (
   SELECT d1, d2, sum(v) AS sv
   FROM   test
   GROUP  BY d1, d2
   )
SELECT d1, d2, sv/sum(sv) OVER (PARTITION BY d1) AS share
FROM   x;

Or (without CTE):

SELECT d1, d2, sv/sum(sv) OVER (PARTITION BY d1) AS share
FROM  (
   SELECT d1, d2, sum(v) AS sv
   FROM   test
   GROUP  BY d1, d2
   ) x;

Or @Mu's variant.

Aside: Greenplum introduced correlated subqueries with version 4.2. See release notes.