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
