Improve PostgreSQL query plan when using NX_ACCESS_ALLOWED and NX_IN_TREE

PostgreSQL use a default cost for nx_access_allowed function, the query planner choose to filter first on access instead of checking for other clause which is most of the time a wrong choice.

For instance
SELECT hierarchy.id
FROM hierarchy
LEFT JOIN dublincore ON hierarchy.id = dublincore.id
LEFT JOIN versions ON hierarchy.id = versions.id
LEFT JOIN misc ON hierarchy.id = misc.id
WHERE
hierarchy.primarytype IN ('MailMessage', 'Thread', 'Note', 'AdvancedSearch', 'Document', 'search_results',
'Picture', 'QueryNav', 'File', 'ContextualLink')
AND (EXISTS (
SELECT 1
FROM dc_contributors
WHERE hierarchy.id = dc_contributors.id
AND (dc_contributors.item = 'Administrator')))
AND (versions.id IS NULL)
AND (misc.lifecyclestate <> 'deleted')
AND NX_ACCESS_ALLOWED(hierarchy.id, '

{administrators,Administrator,Everyone}',
'{Browse,Read,ReadProperties,ReadRemove,ReadWrite,Everything}')
ORDER BY dublincore.modified DESC;

The default query plan will check nx_access_allowed before the sub query:
Filter: (nx_access_allowed(id, '{administrators,Administrator,Everyone}

'::character varying[], '

{Browse,Read,ReadProperties,ReadRemove,ReadWrite,Everything}

'::character varying[]) AND (subplan))

this won't happen with a cost of 1000

ALTER FUNCTION nx_access_allowed(id character varying, users character varying[], permissions character varying[]) COST 10000;