为了使遍历算法不返回到已经访问过的边缘，实际上可以将访问过的边缘保持在某处。正如您已经发现的那样，使用字符串连接不会取得很大的成功。但是，还有其他可用的“值级联”技术可用…
您必须拥有一个方便的架构级标量集合供您使用：
create or replace type arr_strings is table of varchar2(64);
然后，您可以在每次迭代中将访问的边收集到该收集中：
with nondirected$ as (
select from_id, to_id, from_id||'-'||to_id as edge_desc
from edge
where from_id != to_id
union all
select to_id, from_id, from_id||'-'||to_id as edge_desc
from edge
where (to_id, from_id) not in (
select from_id, to_id
from edge
)
),
graph$(lvl, from_id, to_id, edge_desc, visited_edges) as (
select 1, from_id, to_id, edge_desc,
arr_strings(edge_desc)
from nondirected$ R
where from_id in (&nodes)
--
union all
--
select
lvl+1,
Y.from_id, Y.to_id, Y.edge_desc,
X.visited_edges multiset union arr_strings(Y.edge_desc)
from graph$ X
join nondirected$ Y
on Y.from_id = X.to_id
where not exists (
select 1
from table(X.visited_edges) Z
where Y.edge_desc = Z.column_value
)
)
search breadth first by edge_desc set order_id
cycle edge_desc set is_cycle to 1 default 0,
ranked_graph$ as (
select C.*,
row_number() over (partition by edge_desc order by lvl, order_id) as rank$
from graph$ C
-- where is_cycle = 0
)
select *
from ranked_graph$
--where rank$ <= 1
order by lvl, order_id
;
笔记
在sql中，您不能这样做。您提到的Postgresql解决方案可以做到这一点。但是，在Oracle中，您不能这样做。对于每个遍历联接，您都必须测试所有其他遍历联接的行。这将意味着某种聚合或分析……Oracle禁止并抛出ORA异常。
不过，您可以在PL / sql中执行此操作。它应该具有多少性能，取决于要在数据库中预取边缘时要花费多少内存，以及愿意从“当前”节点遍历图时要花费多少sql往返次数，或者是否愿意使用与您宁愿不加入常规arr_output集合的情况相比，它还有更多的内存可将访问的节点保持在按边索引的精美索引中l_visited_nodes。您有多种选择，明智地选择。
无论如何，对于最大量使用sql引擎的最简单情况，这可能是您正在寻找的代码…
create or replace
package pkg_so_recursive_traversal
is
type rec_output is record (
from_id edge.from_id%type,
to_id edge.to_id%type,
lvl integer
);
type arr_output is table of rec_output;
function traverse_a_graph
( i_from in arr_strings
, i_is_directed in varchar2 default 'NO' )
return arr_output
pipelined;
end pkg_so_recursive_traversal;
/
create or replace
package body pkg_so_recursive_traversal
is
function traverse_a_graph
( i_from in arr_strings
, i_is_directed in varchar2 )
return arr_output
pipelined
is
l_next_edges arr_output;
l_current_edges arr_output;
l_visited_edges arr_output := arr_output();
l_out rec_output;
i pls_integer;
l_is_directed varchar2(32) := case when i_is_directed = 'YES' then 'YES' else 'NO' end;
begin
select E.from_id, E.to_id, 0
bulk collect into l_next_edges
from table(i_from) F
join edge E
on F.column_value in (E.from_id, case when l_is_directed = 'YES' then null else E.to_id end)
where E.from_id != E.to_id;
l_out.lvl := 0;
loop
dbms_output.put_line(l_next_edges.count());
exit when l_next_edges.count() <= 0;
l_out.lvl := l_out.lvl + 1;
-- spool the edges to output
i := l_next_edges.first();
while i is not null loop
l_out.from_id := l_next_edges(i).from_id;
l_out.to_id := l_next_edges(i).to_id;
pipe row(l_out);
i := l_next_edges.next(i);
end loop;
l_current_edges := l_next_edges;
l_visited_edges := l_visited_edges multiset union l_current_edges;
-- find next edges
select unique E.from_id, E.to_id, 0
bulk collect into l_next_edges
from table(l_current_edges) CE
join edge E
on CE.to_id in (E.from_id, case when l_is_directed = 'YES' then null else E.to_id end)
or l_is_directed = 'NO' and CE.from_id in (E.from_id, E.to_id)
where E.from_id != E.to_id
and not exists (
select 1
from table(l_visited_edges) VE
where VE.from_id = E.from_id
and VE.to_id = E.to_id
);
end loop;
return;
end;
end pkg_so_recursive_traversal;
/
当调用的起始节点A并考虑图形是无向的时…
select *
from table(pkg_so_recursive_traversal.traverse_a_graph(
i_from => arr_strings('A'),
i_is_directed => 'NO'
));
…它产生…
FROM_ID TO_ID LVL
---------- ---------- ----------
A B 1
C A 1
C E 2
B D 2
C F 2
E B 2
G D 3
H F 3
G I 4
笔记
602392714
清零编程群