Oracle 递归

  当对象存在父节点、子节点时，通过特定的方式获取父节点、子节点数据构建树状结构或其它形式结构时，通常都会使用递归，如:一个公司有多个部门、每个部门下可能有多个小部门，小部门下面又有组….为了数据容易管理和维护,通过构建合适的表结构存储这些数据,以下示例以省市县为例学习了解递归:
1.创建存储省市县数据表:

create table tb_distree
(
id number,
name varchar2(300),
pid number
)
/
 remark 添加主外键
alter table tb_distree add (
constraints pk_id primary key(id),
constraints fk_pid foreign key(pid) references tb_distree(id)
)
/

2.初始化数据:

insert into tb_distree(id,name) values(1,'云南省');
insert into tb_distree(id,name,pid) values(2,'昆明市',1);
insert into tb_distree(id,name,pid) values(3,'临沧市',1);
insert into tb_distree(id,name,pid) values(4,'丽江市',1);
insert into tb_distree(id,name,pid) values(5,'云县',3);
insert into tb_distree(id,name,pid) values(6,'凤庆',3);
insert into tb_distree(id,name,pid) values(7,'幸福',3);
insert into tb_distree(id,name,pid) values(8,'盘龙区',2);
insert into tb_distree(id,name,pid) values(9,'五华区',2);
insert into tb_distree(id,name,pid) values(10,'西山区',2);

3.递归查询语法:

select column... from table_name
where ....  过滤条件
start with ...  递归开始点
connect by prior .... 优先级

4.递归查询数据:
4.1 从父节点开始查询出所有父节点和子节点:

SQL> select id,name,pid from tb_distree start with pid is null connect by prior id=pid;

        ID NAME                PID
---------- ------------ ----------
         1 云南省
         2 昆明市                1
         8 盘龙区                2
         9 五华区                2
        10 西山区                2
         3 临沧市                1
         5 云县                  3
         6 凤庆                  3
         7 幸福                  3
         4 丽江市                1

10 rows selected.

4.2 查询某个节点的父节点:

 SQL> select id,name,pid from tb_distree start with name='云县' connect by prior pid=id;

        ID NAME                PID
---------- ------------ ----------
         5 云县                  3
         3 临沧市                1
         1 云南省

在上例中"云县"属于"临沧市","临沧市"属于"云南省";对于从父节点递归到子节点,优先级条件为子节点id等于父节点id;对于从子节点到父节点递归,方向刚好相反;
4.3 通过层次查询出父节点和某个子节点:

SQL> select id,name,pid,level from tb_distree where level in(1,2) start with pid is null connect by prior id=pid;

        ID NAME                PID      LEVEL
---------- ------------ ---------- ----------
         1 云南省                           1
         2 昆明市                1          2
         3 临沧市                1          2
         4 丽江市                1          2

层次也很重要,某些时候要修改某个节点父节点或子节点时会很有用;
5. 递归查询效率:    

SQL>select/*+ selectDG1 */ id,name,pid from tb_distree start with pid is null connect by prior id=pid;

SQL> select sql_id,sql_text from v$sql where sql_text like '%selectDG1%' ;
SQL_ID          SQL_TEXT
--------------- --------------------------------------------------
2wnu324ga4n0y   select sql_id,sql_text from v$sql where sql_text l
                ike '%selectDG1%'

d4g89bucsbvzd   select/*+ selectDG1 */ id,name,pid from tb_distree
                 start with pid is null connect by prior id=pid


SQL>select * from table(dbms_xplan.display_cursor('d4g89bucsbvzd',null,'advanced allstats last peeked_binds'));
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------
SQL_ID  d4g89bucsbvzd, child number 0
-------------------------------------
select/*+ selectDG1 */ id,name,pid from tb_distree start with pid is
null connect by prior id=pid

Plan hash value: 1466399788

------------------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                               | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
------------------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                        |            |      1 |        |       |    18 (100)|          |     10 |00:00:00.01 |      22 |

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------
|*  1 |  CONNECT BY NO FILTERING WITH START-WITH|            |      1 |        |       |            |          |     10 |00:00:00.01 |      22 |
|   2 |   TABLE ACCESS FULL                     | TB_DISTREE |      1 |     10 |  1780 |    17   (0)| 00:00:01 |     10 |00:00:00.01 |      22 |
------------------------------------------------------------------------------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

   1 - SEL$1
   2 - SEL$2 / TB_DISTREE@SEL$2

Outline Data

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------
  /*+
      BEGIN_OUTLINE_DATA
      IGNORE_OPTIM_EMBEDDED_HINTS
      OPTIMIZER_FEATURES_ENABLE('11.2.0.3')
      DB_VERSION('11.2.0.3')
      ALL_ROWS
      OUTLINE_LEAF(@"SEL$2")
      OUTLINE_LEAF(@"SEL$3")
      OUTLINE_LEAF(@"SEL$4")

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------------------------------------
      OUTLINE_LEAF(@"SET$1")
      OUTLINE_LEAF(@"SEL$1")
      NO_ACCESS(@"SEL$1" "connect$_by$_work$_set$_006"@"SEL$1")
      NO_CONNECT_BY_FILTERING(@"SEL$1")
      CONNECT_BY_COMBINE_SW(@"SEL$1")
      FULL(@"SEL$4" "TB_DISTREE"@"SEL$4")
      FULL(@"SEL$3" "connect$_by$_pump$_002"@"SEL$3")
      FULL(@"SEL$3" "TB_DISTREE"@"SEL$3")
      LEADING(@"SEL$3" "connect$_by$_pump$_002"@"SEL$3" "TB_DISTREE"@"SEL$3")
      USE_HASH(@"SEL$3" "TB_DISTREE"@"SEL$3")
      FULL(@"SEL$2" "TB_DISTREE"@"SEL$2")

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------
      END_OUTLINE_DATA
  */

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("PID"=PRIOR NULL)
       filter("PID" IS NULL)

 10046 trace:
Rows (1st) Rows (avg) Rows (max)  Row Source Operation
 675 ---------- ---------- ----------  ---------------------------------------------------
 676         10         10         10  CONNECT BY NO FILTERING WITH START-WITH (cr=22 pr=0 pw=0 time=269 us)
 677         10         10         10   TABLE ACCESS FULL TB_DISTREE (cr=22 pr=0 pw=0 time=118 us cost=17 size=1780 card=10)
 678
 679
 680 Elapsed times include waiting on following events:
 681   Event waited on                             Times   Max. Wait  Total Waited
 682   ----------------------------------------   Waited  ----------  ------------
 683   row cache lock                                  3        0.00          0.00
 684   Disk file operations I/O                        1        0.00          0.00
 685   db file sequential read                         3        0.03          0.04
 686   SQL*Net message to client                       2        0.00          0.00
 687   SQL*Net message from client                     2        0.00          0.00
 688 ********************************************************************************

在递归表上创建索引是没有用的,都必须进行全表扫描,当然通常这样的表也不会很大,如果数据量的确很大,建议将表中的节点拆分成多个表提高效率;