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JOIN two tables

SELECT 
	count(gid) AS links, 
	sum(a.cost) AS cost, 
	sum(length) AS length

FROM shortest_path(
	'SELECT gid AS id, 
		source::integer, 
		target::integer, 
		length::double precision AS cost 
	FROM <TABLE A> 
	WHERE <...>, 
	false, 
	false
) AS a LEFT JOIN <TABLE B> ON (a.gid = gid)