pgr_dijkstra invalid memory request size

Question

I am trying to create a network of paths using pgr_dijkstra. I keep getting an invalid memory error:

ERROR:  invalid memory alloc request size 1080000000
CONTEXT:  SQL function "pgr_dijkstra" statement 1
SQL statement "SELECT * 
FROM pgr_dijkstra('SELECT id, source, target, cost FROM p797.cost_lines', source_arr, target_arr, false)"
PL/pgSQL function inline_code_block line 10 at SQL statement
SQL state: XX000

I have adjusted work_mem up to 1.2GB in order to try and handle this query (I am sole user of the database). I thought that this would solve the memory error but it has not. Any ideas as to why I am still getting this error? The query works fine on small areas. The area I am trying to process now is very large.

DO
$$
DECLARE
source_arr text[];
target_arr text[];
BEGIN
SELECT array_agg(source) FROM source_locations INTO source_arr ;                                                                                                                                  
SELECT array_agg(target) FROM target_locations INTO target_arr ;        
DROP TABLE IF EXISTS dijkstra;
CREATE TABLE dijkstra AS 
SELECT * 
FROM pgr_dijkstra('SELECT id, source, target, cost FROM cost_lines', source_arr, target_arr, false);
END $$;

Version 2.6.2 of pgRouting