Find cluster given node in PostgreSQL

Question

I am representing a graph in Postgres 9.1 (happens to be bidirectional and cyclic):

CREATE TABLE nodes (
  id          SERIAL PRIMARY KEY,
  name        text
);

CREATE TABLE edges (
  id          SERIAL PRIMARY KEY,
  node1_id    int REFERENCES nodes(id),
  node2_id    int REFERENCES nodes(id)
);

Given a particular node ID, want to retrieve all other nodes in that cluster. I started with the "Paths from a single node" example here, and this is where I got:

WITH RECURSIVE search_graph(id, path) AS (
    SELECT id, ARRAY[id]
    FROM nodes
  UNION
    SELECT e.node2_id, sg.path || e.node2_id
    FROM search_graph sg
    JOIN edges e
    ON e.node1_id = sg.id
)
-- find all nodes connected to node 3
SELECT DISTINCT id FROM search_graph WHERE path @> ARRAY[3];

I can't figure out a) if there is a simpler way to write this since I don't care about collecting the full path, and b) how to make it traverse in both directions (node1->node2 and node2->node1 for each edge). Shedding any light on a good approach would be appreciated. Thanks!

Answer 1

A couple points.

First, you really want to make sure your path traversal is not going to go into a loop. Secondly handling both sides is not too bad. Finally depending on what you are doing, you may want to push the where clause into the CTE somehow to reduce generating every possible graph network and then picking the one you want.

Traversing itself both directions is not too hard. I haven't tested this but it should be possible with something like:

WITH RECURSIVE search_graph(path, last_node1, last_node2) AS (
     SELECT ARRAY[id], id, id
     FROM nodes WHERE id = 3 -- start where we want to start!
     UNION ALL
     SELECT sg.path || e.node2_id || e.node1_id, e.node1_id, e.node2_id
     FROM search_graph sg
     JOIN edges e
     ON (e.node1_id = sg.last_node2 AND NOT path @> array[e.node2_id]) 
        OR (e.node2_id = sg.last_node1 AND NOT path @> array[e.node1_id])
 )
-- Moved where clause to start of graph search
SELECT distinct unnest(path) FROM search_graph;  -- duplicates possible

Hope this helps.