Re: Simple Join

Kevin Brown wrote:

>
>
> I only had two explicit indexes. One was on to_ship.ordered_product_id and
> the other was on ordered_products.paid. ordered_products.id is a primary
> key. This is on your query with an index added on suspended_sub:
>
> dli=# explain analyze SELECT ordered_products.product_id
> dli-# FROM to_ship INNER JOIN ordered_products
> dli-# ON (to_ship.ordered_product_id = ordered_products.id
> dli(# AND ordered_products.paid = TRUE AND
> dli(# ordered_products.suspended_sub = FALSE);
> QUERY PLAN
> ---------------------------------------------------------------------------------------------------------------------------------------------------------
> Hash Join (cost=5126.19..31528.40 rows=20591 width=8) (actual
> time=4554.190..23519.618 rows=14367 loops=1)
> Hash Cond: ("outer".ordered_product_id = "inner".id)
> -> Seq Scan on to_ship (cost=0.00..11529.12 rows=611612 width=8) (actual
> time=11.254..15192.042 rows=611612 loops=1)
> -> Hash (cost=4954.79..4954.79 rows=21759 width=16) (actual
> time=4494.900..4494.900 rows=18042 loops=1)
> -> Index Scan using paid_index on ordered_products
> (cost=0.00..4954.79 rows=21759 width=16) (actual time=72.431..4414.697
> rows=18042 loops=1)
> Index Cond: (paid = true)
> Filter: (paid AND (NOT suspended_sub))
> Total runtime: 23532.785 ms
> (8 rows)
>

Well - that had no effect at all :-) You don't have and index on
to_ship.ordered_product_id do you? - try adding one (ANALYZE again), and
let use know what happens (you may want to play with SET
enable_seqscan=off as well).

And also, if you are only ever interested in paid = true and
suspended_sub = false, then you can recreate these indexes as partials -
e.g:

CREATE INDEX paid_index ON ordered_products (paid) WHERE paid = true;
CREATE INDEX suspended_sub_index ON ordered_products (suspended_sub)
WHERE suspended_sub = false;

> So what's the best way to performance wiggle this info out of the db? The
> list of values is only about 30 tuples long out of this query, so I was
> figuring I could trigger on insert to to_ship to place the value into another
> table if it didn't already exist. I'd rather the writing be slow than the
> reading.

Yeah - all sort of horrible denormalizations are possible :-), hopefully
we can get the original query to work ok, and avoid the need to add code
or triggers to you app.