Maxim Sloyko Maxim Sloyko - 1 month ago 15
SQL Question

Filtering by window function result in Postgresql

Ok, initially this was just a joke we had with a friend of mine, but it turned into interesting technical question :)

I have the following

stuff
table:

CREATE TABLE stuff
(
id serial PRIMARY KEY,
volume integer NOT NULL DEFAULT 0,
priority smallint NOT NULL DEFAULT 0,
);


The table contains the records for all of my stuff, with respective volume and priority (how much I need it).

I have a bag with specified volume, say
1000
. I want to select from the table all stuff I can put into a bag, packing the most important stuff first.

This seems like the case for using window functions, so here is the query I came up with:

select s.*, sum(volume) OVER previous_rows as total
from stuff s
where total < 1000
WINDOW previous_rows as
(ORDER BY priority desc ROWS between UNBOUNDED PRECEDING and CURRENT ROW)
order by priority desc


The problem with it, however, is that Postgres complains:

ERROR: column "total" does not exist
LINE 3: where total < 1000


If I remove this filter, total column gets properly calculated, results properly sorted but all stuff gets selected, which is not what I want.

So, how do I do this? How do I select only items that can fit into the bag?

Answer

I haven't worked with PostgreSQL. However, my best guess would be using an inline view.

SELECT a.*
FROM (
    SELECT s.*, sum(volume) OVER previous_rows AS total
    FROM stuff AS s
    WINDOW previous_rows AS (
         ORDER BY priority desc
         ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
    )
    ORDER BY priority DESC
) AS a
WHERE a.total < 1000;