I am working with pgRouting, following this nice guide：A Beginner’s Guide to pgRouting
I am now in this step：Calculating shortest routes, with these codes below
SELECT * FROM shortest_path(' SELECT gid AS id, start_id::int4 AS source, end_id::int4 AS target, shape_leng::float8 AS cost FROM network', 1, 5110, false, false);
and my network table looks like this
apparently I have no column "shape_leng", so I need to create a new column in table network, and it is the length of every row.
But I don't know how I should type the ST_length code.
Can someone give me some advice？