Date: Wed, 21 Dec 2005 19:55:20 +1300
From: Mark Kirkwood <markir@paradise.net.nz>
Subject: Re: Simple Join
In-reply-to: <43A0AB0A.3000403@paradise.net.nz>
To: Kevin Brown <blargity@gmail.com>
Cc: pgsql-performance@postgresql.org
Message-id: <43A8FC58.1020609@paradise.net.nz>
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References: <200512141603.52583.blargity@gmail.com>
	<43A0AB0A.3000403@paradise.net.nz>

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