Oracle优先级队列的实现之道（enque oracle）
Oracle：优先级队列的实现之道
优先级队列是一种特殊的队列，其中每个元素都有一个优先级。元素以降序排列，即优先级最高的元素排在队列最前面，而优先级最低的元素排在队列最后面。在实际应用中，优先级队列的实现非常重要，这就需要掌握一些优先级队列的实现之道。
Oracle 数据库提供了一些实现优先级队列的方式，以下是其中几种：
1.使用堆数据结构
堆是一种基于树结构的数据结构，在堆中，父节点的优先级大于其所有子节点。在 Oracle 数据库中，可以使用堆来实现优先级队列。下面是一个使用堆实现优先级队列的 SQL 代码：
“`sql
create or replace type heap_element as object(
priority number,
data varchar2(200)
)
create or replace type heap_list as table of heap_element
create or replace type heap_queue as object(
heap heap_list,
static function cmp(h1 heap_element, h2 heap_element) return number
)
not final
create or replace type body heap_queue as
static function cmp(h1 heap_element, h2 heap_element) return number is
begin
return h1.priority – h2.priority;
end cmp;
member function insert(new_element heap_element) return self as result is
begin
self.heap.extend(1);
self.heap(self.heap.count) := new_element;
for i in reverse self.heap.first+1..self.heap.last loop
if cmp(self.heap(i),self.heap(i-1)) > 0 then
self.heap(i-1).data,self.heap(i).data:=self.heap(i).data,self.heap(i-1).data;
self.heap(i-1).priority,self.heap(i).priority:=self.heap(i).priority,self.heap(i-1).priority;
else
exit;
end if;
end loop;
return self;
end insert;
member function remove() return heap_element is
tmp heap_element;
begin
if self.heap.count = 0 then
rse_application_error(-20001,’Heap is empty!’);
else
tmp := self.heap(1);
self.heap(1) := self.heap(self.heap.count);
self.heap.trim(self.heap.count-1);
for i in reverse self.heap.first..self.heap.last-1 loop
if cmp(self.heap(i),self.heap(i-1)) > 0 then
self.heap(i-1).data,self.heap(i).data:=self.heap(i).data,self.heap(i-1).data;
self.heap(i-1).priority,self.heap(i).priority:=self.heap(i).priority,self.heap(i-1).priority;
else
exit;
end if;
end loop;
end if;
return tmp;
end remove;
end;
在上述代码中，heap_element 表示堆中的一个元素，其中包含一个优先级 priority 和数据 data；heap_list 表示堆，其中包含一组元素 heap_element；heap_queue 表示优先级队列，其中包含一个堆 heap 和一个 static function cmp，cmp 函数用于比较堆中两个元素的优先级，insert 函数用于向堆中插入元素，remove 函数用于从堆中删除元素，并返回删除的元素。
2.使用 B 树数据结构
B 树是一种常用的数据结构，它对于大型数据集合非常有效。在 Oracle 数据库中，也可以使用 B 树来实现优先级队列。以下是一个使用 B 树实现优先级队列的 SQL 代码：
```sqlcreate or replace type btree_element as object(
priority number, data varchar2(200)
)
create or replace type btree_list as table of btree_element
create or replace type btree_node as object( children btree_list,
parent ref btree_node, sup_prior number
)
create or replace type btree_queue as object( root ref btree_node,
static function cmp(b1 btree_element, b2 btree_element) return number)
not final
create or replace type body btree_queue as static function cmp(b1 btree_element, b2 btree_element) return number is
begin return b1.priority - b2.priority;
end cmp;
static function find_node(parent btree_node,b btree_element,p number) return btree_node is n btree_node;
c btree_element; i number := 1;
begin if parent sup_prior >= p then
return parent; end if;
for i in parent.children.first..parent.children.last loop if cmp(b, parent.children(i))
c := parent.children(i); n := find_node(c,c, b.priority);
if n is null then return parent;
else return n;
end if; end if;
end loop;
return parent; end find_node;
member function insert(b btree_element) return self as result is cur_node btree_node;
new_node btree_node; parent_node btree_node;
is_full boolean := false; pivoted boolean := false;
begin if self.root is null then
self.root := btree_node(null,null,null); self.root.children := btree_list(b);
else cur_node := find_node(self.root,self.root,b.priority);
new_node := btree_node(null,ref cur_node,null); new_node.children := btree_list(b);
if cur_node.children.last+1 > 3 then is_full := true;
end if;
while is_full loop parent_node := cur_node.parent;
new_node.sup_prior := new_node.children(1).priority;
if parent_node is null then new_node.parent := btree_node(null,null,null);
self.root := new_node.parent; new_node.parent.children := btree_list();
new_node.parent.children.extend(2); new_node.parent.children(1) := cur_node;
new_node.parent.children(2) := new_node; new_node.parent.sup_prior := new_node.sup_prior;
cur_node.parent := new_node.parent; new_node.parent.parent := null;
is_full := false; self.root.children(1).parent := self.root;
elsif parent_node.children.last+1 parent_node.children.extend(1);
for i in reverse parent_node.children.first+1..parent_node.children.last loop if cmp(new_node.parent.sup_prior, parent_node.children(i-1).sup_prior) > 0 then
parent_node.children(i).data,parent_node.children(i-1).data:=parent_node.children(i-1).data,parent_node.children(i).data; parent_node.children(i).priority,parent_node.children(i-1).priority:=parent_node.children(i-1).priority,parent_node.children(i).priority;
else parent_node.children(i) := new_node.parent;
new_node.parent.parent := parent_node; is_full := false;
new_node.parent.sup_prior := new_node.children(1).priority; cur_node.parent := new_node.parent;
new_node.parent.children(1) := cur_node; new_node.parent.children.extend(2);
new_node.parent.children(2) := new_node; new_node.parent.parent := parent_node;
new_node.parent.parent.sup_prior := new_node.parent.sup_prior; self.root := self.root.parent;
self.root.children(1).parent := self.root; exit;
end if; end loop;
else new_node.parent := btree_node(null,null,null);
cur_node := parent_node; parent_node := parent_node.parent;
new_node.parent.children := btree_list(cur_node.children(cur_node.children.last)); cur_node.children.trim(cur_node.children.last);
new_node.parent.children.extend(1); new_node.parent.children(1) := new_node;
new_node.parent.sup_prior := new_node.children(1).priority; end if;
end loop; end if;
return self; end insert;
member function remove() return btree_element is cur_node btree_node;
left btree_element; begin
if self.root is null then rse_application_error(-20001,'Heap is empty!');
elsif self.root.children.first is self.root.children.last then left := self.root.children(1);
self.root := null; else
cur_node := self.root.children.first; while true loop
if cur_node.children.last is not null then cur_node := cur_node.children.last;
else left := cur_node.children(last-1);
cur_node.children.trim(last-1); break;
end if; end loop;
while cur_node is not self.root do left_data := left.data;
left_prior := left.priority
