Re: Simple Join

Mark Kirkwood wrote:
> Kevin Brown wrote:
>
>> I'll just start by warning that I'm new-ish to postgresql.
>>
>> I'm running 8.1 installed from source on a Debian Sarge server. I
>> have a simple query that I believe I've placed the indexes correctly
>> for, and I still end up with a seq scan. It makes sense, kinda, but
>> it should be able to use the index to gather the right values. I do
>> have a production set of data inserted into the tables, so this is
>> running realistically:
>>
>> dli=# explain analyze SELECT ordered_products.product_id
>> dli-# FROM to_ship, ordered_products
>> dli-# WHERE to_ship.ordered_product_id = ordered_products.id AND
>> dli-# ordered_products.paid = TRUE AND
>> dli-# ordered_products.suspended_sub = FALSE;
>
>
> You scan 600000 rows from to_ship to get about 25000 - so some way to
> cut this down would help.
>
> Try out an explicit INNER JOIN which includes the filter info for paid
> and suspended_sub in the join condition (you may need indexes on each of
> id, paid and suspended_sub, so that the 8.1 optimizer can use a bitmap
> scan):
>
>
> SELECT ordered_products.product_id
> FROM to_ship INNER JOIN ordered_products
> ON (to_ship.ordered_product_id = ordered_products.id
> AND ordered_products.paid = TRUE AND
> ordered_products.suspended_sub = FALSE);

It has been a quiet day today, so I took another look at this. If the
selectivity of clauses :

paid = TRUE
suspended_sub = FALSE

is fairly high, then rewriting as a subquery might help:

SELECT o.product_id
FROM ordered_products o
WHERE o.paid = TRUE
AND o.suspended_sub = FALSE
AND EXISTS (
SELECT 1
FROM to_ship s
WHERE s.ordered_product_id = o.id
);

However it depends on you not needing anything from to_ship in the
SELECT list...

Cheers

Mark