Re: where in (select array)

Marcus Engene wrote:
> Hi List,
>
> I have the might_like table that contains products a user might like if
> he likes the present one (item).
>
> CREATE TABLE might_like
> (
> item INTEGER NOT NULL
> ,created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
> ,child INTEGER NOT NULL
> )
> WITHOUT OIDS;
>
> CREATE INDEX might_like_x1 ON might_like(item);
>
> Since there are (will be) houndreds of thousands of items, and 20+ might
> like items, i thought it would be nice to reduce the set to 1/20th by
> using a vector.
>
> CREATE TABLE might_like_vector
> (
> item INTEGER NOT NULL
> ,created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
> ,child_arr INTEGER[]
> )
> WITHOUT OIDS;

You haven't reduced the set at all, you've just turned part of it
sideways. You might gain something on your search, but I'm guessing
you've not tested it.

Hmm - the attached script generates 100,000 items and 10 liked ones for
each (well for the first 99,990 it says you like the next 10 items).
They're all given different timestamps at day intervals which means
you'll end up with 6 or seven matches for you sample query.

> But then this don't work:
>
> select
> ...
> from
> item pic
> where
> pic.objectid in (
> select mlv.child_arr
> from might_like_vector mlv
> where mlv.item = 125 AND
> mlv.created_at > now() - interval '1 week'
> )
> limit 16

Without messing around with arrays you get this query (which seems
readable enough to me)

SELECT
objectid, objname
FROM
items i
JOIN might_like m ON (i.objectid = m.child)
WHERE
m.created_at > (now() - '1 week'::interval)
AND m.item = 125
ORDER BY
objectid
LIMIT
16
;

I'm getting times less than a millisecond for this - are you sure it's
worth fiddling with arrays?

--
Richard Huxton
Archonet Ltd