Efficiently copying a tree modelled with adjacency list in postgres - postgresql

Question

I have the following table:
CREATE TABLE tree_node (
id serial primary key,
name character varying(255),
parent_id integer references tree (id)
);
The table contains many trees with up to about 1000 nodes.
(I'm able to query a tree and its descendants efficiently with a recursive query).
However, I need to be able to copy a single tree in one operation. Say I have a tree with 3 nodes, ids 1,2,3 (this is potentially a large tree). I would like to make a copy of it i.e. creating new nodes with new ids. (Here the copied tree is ids 4,5,6):
id | name | parent_id
----+-----------------+-----------
1 | NodeA |
2 | NodeA.1 | 1
3 | NodeA.1.1 | 2
4 | NodeA(copy) |
5 | NodeA.1(copy) | 4
6 | NodeA.1.1(copy) | 5
Is there a way to copy a tree and its descendants more efficiently than inserting each tree node separately (because the new parent_id is needed)?

Answer 1

There you go:
\i tmp.sql
CREATE TABLE tree_node (
id serial primary key
, name varchar
, parent_id integer references tree_node (id)
);
INSERT INTO tree_node(name, parent_id) VALUES
( 'Node-A', NULL)
, ( 'Node-A.1', 1)
, ( 'Node-A.1.1', 2)
;
SELECT * FROM tree_node;
-- Find the top value of the sequence
-- and use it as an increment on all the copies
WITH top(val) AS
(select currval('tree_node_id_seq')
)
INSERT INTO tree_node(id, name, parent_id)
SELECT id+top.val
, name|| '(copy)'
, parent_id + top.val
FROM tree_node
CROSS JOIN top
;
SELECT * FROM tree_node;
-- bump the sequence
WITH nxt AS (
select max(id) mx from tree_node
)
SELECT setval('tree_node_id_seq', (select mx FROM nxt) )
;
Output:
DROP SCHEMA
CREATE SCHEMA
SET
CREATE TABLE
INSERT 0 3
id | name | parent_id
----+------------+-----------
1 | Node-A |
2 | Node-A.1 | 1
3 | Node-A.1.1 | 2
(3 rows)
INSERT 0 3
id | name | parent_id
----+------------------+-----------
1 | Node-A |
2 | Node-A.1 | 1
3 | Node-A.1.1 | 2
4 | Node-A(copy) |
5 | Node-A.1(copy) | 4
6 | Node-A.1.1(copy) | 5
(6 rows)
setval
--------
6
(1 row)