Xml generation for large site

Hi.
I have a site with 800k post. Some times a day when I go to the admin console one task run in background (the site map generation I guess). The problem is that run a lot of queries to de DB and as many processes as php allows (150 in my case). Those queries only bring 100 post at a time, which means that it can run more than 8000 times.
Is there any parameter to get more records in each query? Or which would be the way to optimize this process (memory is not a big problem)?

The query executed :

SELECT l.ID, post_title, post_content, post_name, post_parent, post_author, post_modified_gmt, post_date, post_date_gmt
			FROM (
				SELECT wp_posts.ID
				FROM wp_posts
		WHERE wp_posts.post_status = 'publish'
			AND wp_posts.post_type = 'post'
			AND wp_posts.post_password = ''
			AND wp_posts.post_date != '0000-00-00 00:00:00'
				ORDER BY wp_posts.post_modified ASC LIMIT 100 OFFSET 254900
			)
			o JOIN wp_posts l ON l.ID = o.ID