Question

表上的级联删除是否比单个删除语句更有效（在单个plsql块中执行）？

Answer 1

cascade delete的作用是发出个别删除声明。

检查以下测试用例：

create table parent 
 (parent_id number, 
  parent_name varchar2(30), 
  constraint parent_pk primary key (parent_id) using index);

create table child 
 (child_id   number,
  parent_id  number,
  child_name varchar2(30),
  constraint child_pk primary key (parent_id, child_id) using index,
  constraint child_fk01 foreign key (parent_id) 
    references parent (parent_id) 
      on delete cascade;
 );


insert into parent
 (parent_id, parent_name)
select object_id, object_name from dba_objects where rownum <= 10000;

begin
  for i in 1..10
  loop
    insert into child
      (child_id, parent_id, child_name)
    select i, parent_id, parent_name
      from parent;
  end loop;
end;
/

exec dbms_stats.gather_table_stats (tabname => 'PARENT', cascade => true);
exec dbms_stats.gather_table_stats (tabname => 'CHILD', cascade => true);

exec dbms_monitor.session_trace_enable;
alter table child drop constraint child_fk01;
alter table child add constraint child_fk01 foreign key (parent_id)
  references parent (parent_id) on delete cascade enable novalidate ;
delete from parent;
rollback;

在跟踪文件中，您将找到如下所示的行：

  delete from "<MY_SCHEMA_NAME>"."CHILD" where "PARENT_ID" = :1
END OF STMT
PARSE #6:c=0,e=182,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=4,tim=1293353992514766
EXEC #6:c=0,e=545,p=0,cr=2,cu=32,mis=1,r=10,dep=1,og=4,tim=1293353992515354
EXEC #6:c=0,e=233,p=0,cr=2,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992515644
EXEC #6:c=0,e=238,p=0,cr=2,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992515931
EXEC #6:c=0,e=252,p=0,cr=2,cu=32,mis=0,r=10,dep=1,og=4,tim=1293353992516229
EXEC #6:c=0,e=231,p=0,cr=2,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992516507
EXEC #6:c=0,e=227,p=0,cr=2,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992516782
EXEC #6:c=0,e=244,p=0,cr=2,cu=32,mis=0,r=10,dep=1,og=4,tim=1293353992517072
EXEC #6:c=0,e=219,p=0,cr=2,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992517337
EXEC #6:c=0,e=236,p=0,cr=3,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992517622
EXEC #6:c=0,e=235,p=0,cr=2,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992517921
EXEC #6:c=0,e=229,p=0,cr=2,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992518196
EXEC #6:c=0,e=246,p=0,cr=2,cu=32,mis=0,r=10,dep=1,og=4,tim=1293353992518487
EXEC #6:c=0,e=234,p=0,cr=2,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992518767
EXEC #6:c=6999,e=570,p=0,cr=2,cu=30,mis=0,r=10,dep=1,og=4,tim=1293353992519383

对于CHILD中删除的每条记录，Oracle就PARENT发出删除声明。

另一个问题是两者中哪一个更有效：

DELETE FROM CHILD WHERE PARENT_ID = 1;
DELETE FROM PARENT WHERE PARENT_ID = 1;

VS

DELETE FROM PARENT WHERE PARENT_ID = 1;

都启用了on delete cascade。令人沮丧的是，在上面的第一种情况中，Oracle将探测子表上的外键索引，以查看是否存在需要级联的任何行。如果不存在任何行，则Oracle不会执行级联删除。

Answer 2

您无法比较这两个选项。它不是性能问题，而是更多的设计和结构。

如果使用主键/外键设计数据库，使用级联删除比使用手动搜索更容易删除哪些列和表上有外键并生成加工SQL语句。

级联删除功能的主要优点是它允许您减少执行删除操作所需的SQL语句数量

Answer 3

如果要级联删除并且没有定义外键，可以使用以下内容：

DELETE FROM my_table
 WHERE ROWID IN
     ( SELECT ROWID
         FROM my_table
        START WITH (condition_on_the_row_that_you_want_to_delete)
      CONNECT BY PRIOR (primary_key) = (self_foreign_key)
     )

Oracle级联删除

3 个答案: