CREATE OR REPLACE FUNCTION add_edge(parent_id uuid, child_id uuid) /* Adds an edge between ``parent`` and ``child``. Designed to work with Directed Acyclic Graphs (DAG) (or said in another way, trees with multiple parents without cycles). This method will raise an exception if: - Parent is the same as child. - Child contains the parent. - Edge parent - child already exists. Influenced by: - https://www.codeproject.com/Articles/22824/A-Model-to-Represent-Directed-Acyclic-Graphs-DAG - http://patshaughnessy.net/2017/12/12/installing-the-postgres-ltree-extension - https://en.wikipedia.org/wiki/Directed_acyclic_graph */ RETURNS void AS $$ DECLARE parent text := replace(CAST(parent_id as text), '-', '_'); child text := replace(CAST(child_id as text), '-', '_'); BEGIN if parent = child then raise exception 'Cannot create edge: the parent is the same as the child.'; end if; if EXISTS( SELECT 1 FROM path where path.path ~ CAST('*.' || child || '.*.' || parent || '.*' as lquery) ) then raise exception 'Cannot create edge: child already contains parent.'; end if; -- We have two subgraphs: the parent subgraph that goes from the parent to the root, -- and the child subgraph, going from the child (which is the root of this subgraph) -- to all the leafs. -- We do the cartesian product from all the paths of the parent subgraph that end in the parent -- WITH all the paths that start from the child that end to its leafs. insert into path (lot_id, path) ( select distinct lot_id, fp.path || subpath(path.path, index(path.path, text2ltree(child))) from path, (select path.path from path where path.path ~ CAST('*.' || parent AS lquery)) as fp where path.path ~ CAST('*.' || child || '.*' AS lquery) ); -- Cleanup: old paths that start with the child (that where used above in the cartesian product) -- have became a subset of the result of the cartesian product, thus being redundant. delete from path where path.path ~ CAST(child || '.*' AS lquery); END $$ LANGUAGE plpgsql; CREATE OR REPLACE FUNCTION delete_edge(parent_id uuid, child_id uuid) /* Deletes an edge between ``parent`` and ``child``. Designed to work with DAG (See ``add_edge`` function). This method will raise an exception if the relationship does not exist. */ RETURNS void AS $$ DECLARE parent text := replace(CAST(parent_id as text), '-', '_'); child text := replace(CAST(child_id as text), '-', '_'); number int; BEGIN -- to delete we remove from the path of the descendants of the child -- (and the child) any ancestor coming from this edge. -- When we added the edge we did a cartesian product. When removing -- this part of the path we will have duplicate paths. -- don't check uniqueness for path key until we delete duplicates SET CONSTRAINTS path_unique DEFERRED; -- remove everything above the child lot_id in the path -- this creates duplicates on path and lot_id update path set path = subpath(path, index(path, text2ltree(child))) where path ~ CAST('*.' || parent || '.' || child || '.*' AS lquery); -- remove duplicates -- we need an id field exclusively for this operation -- from https://wiki.postgresql.org/wiki/Deleting_duplicates DELETE FROM path WHERE id IN (SELECT id FROM (SELECT id, ROW_NUMBER() OVER (partition BY lot_id, path) AS rnum FROM path) t WHERE t.rnum > 1); -- re-activate uniqueness check and perform check -- todo we should put this in a kind-of finally clause SET CONSTRAINTS path_unique IMMEDIATE; -- After the update the one of the paths of the child will be -- containing only the child. -- This can only be when the child has no parent at all. -- In case the child has more than one parent, remove this path -- (note that we want it to remove it too from descendants of this -- child, ex. 'child_id'.'desc1') select COUNT(1) into number from path where lot_id = child_id; IF number > 1 THEN delete from path where path <@ text2ltree(child); end if; END $$ LANGUAGE plpgsql;