Documentation example for RECURISVE WITH isn't what I would expect

Поиск
Список
Период
Сортировка
От Alex Sherwin
Тема Documentation example for RECURISVE WITH isn't what I would expect
Дата
Msg-id CAMqGX520K=3onSR45KQb=7iwk9V2Xt43V267K+Lx+XZcA=K0aw@mail.gmail.com
обсуждение исходный текст
Ответы Re: Documentation example for RECURISVE WITH isn't what I would expect  (Tom Lane <tgl@sss.pgh.pa.us>)
Список pgsql-docs
Дерево обсуждения
The 9.1 docs for RECURSIVE WITH: http://www.postgresql.org/docs/9.1/static/queries-with.html eventually builds up to this example query:

WITH RECURSIVE search_graph(id, link, data, depth, path, cycle) AS (       SELECT g.id, g.link, g.data, 1,         ARRAY[g.id],         false       FROM graph g     UNION ALL       SELECT g.id, g.link, g.data, sg.depth + 1,         path || g.id,         g.id = ANY(path)       FROM graph g, search_graph sg       WHERE g.id = sg.link AND NOT cycle
)
SELECT * FROM search_graph;

From reading the docs and this query, it would be my understanding that the recursive portion (after UNION ALL) can refer to itself, which is what it appears this query does with the definition of cycle as "g.id = ANY(path)" then later referring to it in the WHERE clause "ANT NOT cycle", meaning that I would expect this to exclude duplicate traversals.

But, in my test this is not the case, I have to modify the query (complete working example below) as follows to make it work as the docs (I believe) intended:

drop table graph;
create table graph (
id varchar not null primary key,
link varchar null,
data varchar null);

insert into graph (id, link, data) values ('a', null, 'a');
insert into graph (id, link, data) values ('ab', 'a', 'ab');
insert into graph (id, link, data) values ('ac', 'a', 'ac');
insert into graph (id, link, data) values ('aca', 'ac', 'aca');
insert into graph (id, link, data) values ('acb', 'ac', 'acb');
insert into graph (id, link, data) values ('acba', 'acb', 'acba');
insert into graph (id, link, data) values ('acbb', 'acb', 'acbb');
insert into graph (id, link, data) values ('acc', 'ac', 'acc');
insert into graph (id, link, data) values ('ad', 'a', 'ad');

with recursive search_graph(id, link, data, depth, path, cycle) as 
(
  select 
    g.id, g.link, g.data, 1, array[g.id], false 
  from graph g
  union all
  select 
    g.id, g.link, g.data, sg.depth + 1, path || g.id, g.id=any(path || g.id)
  from graph g, search_graph sg
  where g.id=sg.link and not cycle and not g.id=any(path || g.id)
)
select * from search_graph


Without the change "g.id=any(path || g.id)", cycle is never set to true.  If this were the only change, the first level ('a') would still be re-visited, but stop any further duplicate visits from there.  In order to prevent 'a' from being re-visited altogether, the WHERE must be modified as well with "and not g.id=any(path || g.id)", which essentially negates the need for cycle to begin with and can be re-factored as:

with recursive search_graph(id, link, data, depth, path) as 
(
  select 
    g.id, g.link, g.data, 1, array[g.id]
  from graph g
  union all
  select 
    g.id, g.link, g.data, sg.depth + 1, path || g.id
  from graph g, search_graph sg
  where g.id=sg.link and not g.id=any(path || g.id)
)
select * from search_graph


Unless I'm missing something here.. but I don't think the example in the docs works correctly.

To apply the docs example to my data set, would look like:

with recursive search_graph(id, link, data, depth, path, cycle) as 
(
  select 
    g.id, g.link, g.data, 1, array[g.id], false 
  from graph g
  union all
  select 
    g.id, g.link, g.data, sg.depth + 1, path || g.id, g.id=any(path)
  from graph g, search_graph sg
  where g.id=sg.link and not cycle
)
select * from search_graph


Which re-visits many rows





--
Alexander Sherwin

В списке pgsql-docs по дате отправления:

Предыдущее
От: Daniele Varrazzo
Дата:
Сообщение: Wrong advisory locks docs in pg_locks
Следующее
От: Tom Lane
Дата:
Сообщение: Re: Documentation example for RECURISVE WITH isn't what I would expect