Documentation example for RECURISVE WITH isn't what I would expect - Mailing list pgsql-docs
| From | Alex Sherwin |
|---|---|
| Subject | Documentation example for RECURISVE WITH isn't what I would expect |
| Date | |
| Msg-id | CAMqGX520K=3onSR45KQb=7iwk9V2Xt43V267K+Lx+XZcA=K0aw@mail.gmail.com Whole thread Raw |
| Responses |
Re: Documentation example for RECURISVE WITH isn't what I would expect
(Tom Lane <tgl@sss.pgh.pa.us>)
|
| List | pgsql-docs |
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:
--
Alexander Sherwin
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
from graph g
union all
select
from graph g, search_graph sg
)
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
from graph g
union all
select
from graph g, search_graph sg
)
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
from graph g
union all
select
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
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