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Adjust cycle detection examples and tests

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Adjust the existing cycle detection example and test queries to put
the cycle column before the path column.  This is mainly because the
SQL-standard CYCLE clause puts them in that order, and so if we added
that feature that would make the sequence of examples more consistent
and easier to follow.

Discussion: https://www.postgresql.org/message-id/c5603982-0088-7f14-0caa-fdcd0c837b57@2ndquadrant.com
This commit is contained in:
Peter Eisentraut
2020-10-12 07:46:20 +02:00
parent 88ea7a1188
commit 3fb676504d
3 changed files with 83 additions and 83 deletions
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26
doc/src/sgml/queries.sgml
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@ -2144,20 +2144,20 @@ SELECT * FROM search_graph;
<literal>UNION ALL</literal> to <literal>UNION</literal> would not eliminate the looping.
Instead we need to recognize whether we have reached the same row again
while following a particular path of links. We add two columns
<structfield>path</structfield> and <structfield>cycle</structfield> to the loop-prone query:
<structfield>is_cycle</structfield> and <structfield>path</structfield> to the loop-prone query:
<programlisting>
WITH RECURSIVE search_graph(id, link, data, depth, path, cycle) AS (
WITH RECURSIVE search_graph(id, link, data, depth, is_cycle, path) AS (
SELECT g.id, g.link, g.data, 1,
ARRAY[g.id],
false
false,
ARRAY[g.id]
FROM graph g
UNION ALL
SELECT g.id, g.link, g.data, sg.depth + 1,
path || g.id,
g.id = ANY(path)
g.id = ANY(path),
path || g.id
FROM graph g, search_graph sg
WHERE g.id = sg.link AND NOT cycle
WHERE g.id = sg.link AND NOT is_cycle
)
SELECT * FROM search_graph;
</programlisting>
@ -2172,17 +2172,17 @@ SELECT * FROM search_graph;
compare fields <structfield>f1</structfield> and <structfield>f2</structfield>:
<programlisting>
WITH RECURSIVE search_graph(id, link, data, depth, path, cycle) AS (
WITH RECURSIVE search_graph(id, link, data, depth, is_cycle, path) AS (
SELECT g.id, g.link, g.data, 1,
ARRAY[ROW(g.f1, g.f2)],
false
false,
ARRAY[ROW(g.f1, g.f2)]
FROM graph g
UNION ALL
SELECT g.id, g.link, g.data, sg.depth + 1,
path || ROW(g.f1, g.f2),
ROW(g.f1, g.f2) = ANY(path)
ROW(g.f1, g.f2) = ANY(path),
path || ROW(g.f1, g.f2)
FROM graph g, search_graph sg
WHERE g.id = sg.link AND NOT cycle
WHERE g.id = sg.link AND NOT is_cycle
)
SELECT * FROM search_graph;
</programlisting>