postgresql ignores index for recursive query - postgresql

Question

I have a table representing graph of hierarchy links (parent_id, child_id)
The table has indexes on parent, child and both.
The graph may contain loops and i need to check them (or, maybe i need find all loops to eliminate them).
And i need to query all parents of a node recustively.
For this i use this query (it is supposed to be saved in view):
WITH RECURSIVE recursion(parent_id, child_id, node_id, path) AS (
SELECT h.parent_id,
h.child_id,
h.child_id AS node_id,
ARRAY[h.parent_id, h.child_id] AS path
FROM hierarchy h
UNION ALL
SELECT h.parent_id,
h.child_id,
r.node_id,
ARRAY[h.parent_id] || r.path
FROM hierarchy h JOIN recursion r ON h.child_id = r.parent_id
WHERE NOT r.path #> ARRAY[h.parent_id]
)
SELECT parent_id,
child_id,
node_id,
path
FROM recursion
where node_id = 883
For this query postgres is going to use very terrific plan:
"CTE Scan on recursion (cost=2703799682.88..4162807558.70 rows=324223972 width=56)"
" Filter: (node_id = 883)"
" CTE recursion"
" -> Recursive Union (cost=0.00..2703799682.88 rows=64844794481 width=56)"
" -> Seq Scan on hierarchy h (cost=0.00..74728.61 rows=4210061 width=56)"
" -> Merge Join (cost=10058756.99..140682906.47 rows=6484058442 width=56)"
" Merge Cond: (h_1.child_id = r.parent_id)"
" Join Filter: (NOT (r.path #> ARRAY[h_1.parent_id]))"
" -> Index Scan using hierarchy_idx_child on hierarchy h_1 (cost=0.43..256998.25 rows=4210061 width=16)"
" -> Materialize (cost=10058756.56..10269259.61 rows=42100610 width=48)"
" -> Sort (cost=10058756.56..10164008.08 rows=42100610 width=48)"
" Sort Key: r.parent_id"
" -> WorkTable Scan on recursion r (cost=0.00..842012.20 rows=42100610 width=48)"
It seems like postgres does not understand that external filter on node_id is applied to child_id in first recursive subquery.
I suppose i'm doing very wrong thing. But where exactly?

Answer 1

Looks like you just need to move WHERE node_id = 883 to first part of union:
WITH RECURSIVE recursion(parent_id, child_id, node_id, path) AS (
SELECT h.parent_id,
h.child_id,
h.child_id AS node_id,
ARRAY[h.parent_id, h.child_id] AS path
FROM hierarchy h
WHERE node_id = 883
UNION ALL
SELECT h.parent_id,
h.child_id,
r.node_id,
ARRAY[h.parent_id] || r.path
FROM hierarchy h JOIN recursion r ON h.child_id = r.parent_id
WHERE NOT r.path #> ARRAY[h.parent_id]
)
SELECT parent_id,
child_id,
node_id,
path
FROM recursion

Answer 2

Here is much more effective way to solve the graph traversing tasks.
CREATE OR REPLACE FUNCTION public.terr_ancestors(IN bigint)
RETURNS TABLE(node_id bigint, depth integer, path bigint[]) AS
$BODY$
WITH RECURSIVE recursion(node_id, depth, path) AS (
SELECT $1 as node_id, 0, ARRAY[$1] AS path
UNION ALL
SELECT h.parent_id as node_id, r.depth + 1, h.parent_id || r.path
FROM hierarchy h JOIN recursion r ON h.child_id = r.node_id
WHERE h.parent_id != ANY(path)
)
SELECT * FROM recursion
$BODY$
And similar way for descendants.