Postgresql – Help with Recursive CTE query with INSERT, ON CONFLICT and RETURNING

postgresqlpostgresql-9.5

I'm trying to write a query to insert filenames to the following table table path

CREATE SEQUENCE path_id_seq;
CREATE TABLE path (
    id INTEGER PRIMARY KEY DEFAULT NEXTVAL('path_id_seq'),
    parent_id INTEGER,
    name TEXT NOT NULL);

CREATE UNIQUE INDEX path_parent_id_name_ix
    ON path (COALESCE(parent_id, 0), name);

ALTER TABLE path
    ADD CONSTRAINT ios_path_parent_id_fk
    FOREIGN KEY (parent_id) REFERENCES path (id)
    ON DELETE CASCADE
    DEFERRABLE INITIALLY DEFERRED;

For example, given filenames Assets/images/a.png and Assets/images/b.png I'd like to insert the following rows.

id | parent_id |       name        
----+-----------+-------------------
  1 |         ¤ | Assets
  2 |         1 | images
  3 |         2 | a.png
  4 |         2 | b.png

Inserting the parent is easy;

INSERT INTO path (name)
VALUES ('Assets')
ON CONFLICT (COALESCE(parent_id, 0), name)
DO UPDATE SET name = EXCLUDED.name -- this strange update is so that we get rows back
RETURNING id, parent_id, name;

which yields; 

 id | parent_id |  name  
----+-----------+--------
  1 |         ¤ | Assets

But I can't seem to put together a recursive CTE to use the parent's id as the parent_id in the next INSERT statement.

It will probably look somethings like this invalid query below. 

WITH RECURSIVE names AS (
    SELECT name, lag(name, 1) OVER () as previous_name
    FROM (SELECT unnest(string_to_array('Assets/images/232.png', '/')) as name) names),

paths(id, name) AS (
    INSERT INTO path (name)
    SELECT name
    FROM names
    WHERE previous_name IS NULL
    ON CONFLICT (COALESCE(parent_id, 0), name)
    DO UPDATE SET name = EXCLUDED.name
    RETURNING id, name
    UNION
    INSERT INTO path (parent_id, name)
    SELECT paths.id, names.name
    FROM paths
    JOIN names ON names.previous_name = paths.name
    ON CONFLICT (COALESCE(parent_id, 0), name)
    DO UPDATE SET name = EXCLUDED.name
    RETURNING id, name)

SELECT *
FROM paths;

But I just can't seem to get it to work. If anyone sees a simpler way to do this it would be much appreciated.

Best Answer

You don't need a recursive CTE, you can use nextval() in the result from the unnest() directly and then use lag() to get the value from the previous row. To deal with existing rows, I would do this using a join between the result of the unnest() and the path table, detecting new and existing rows.

For the new rows, a new ID will be generated and only those will be inserted:

insert into path (id, name, parent_id)
select id, name, parent_id
from (
  select *,
         case 
           when is_new
             then lag(id) over (order by level) 
             else existing_parent
         end as parent_id
  from (
    SELECT case 
              when p.id is null then nextval('path_id_seq') 
              else p.id
           end as id, 
           x.name, x.level, p.parent_id as existing_parent, 
           p.id is null as is_new        
    FROM unnest(string_to_array('Assets/images/232.png', '/')) with ordinality as x(name, level)
      LEFT JOIN path p on p.name = x.name
    order by level
  ) t1
) t2
where is_new;

Live example: http://rextester.com/YGMT19565

The only "drawback" if you will is that the sequence values are not necessarily assigned in the "order" you'd expect. But as the only purpose of a generated primary key is to be unique (and nothing else) this shouldn't be a problem.

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