忽略Oracle DUP_VAL_ON_INDEX异常有多糟糕？

Question

如果用户至少查看过一次对象,我有一张表正在录制,因此:

 HasViewed
     ObjectID  number (FK to Object table)
     UserId    number (FK to Users table)

两个字段都不是NULL,并且一起形成主键.

我的问题是,因为我不关心有人看过一个对象多少次(在第一次之后),我有两个处理插入的选项.

• 执行SELECT计数(*)...如果未找到任何记录,请插入新记录.
• 总是只插入一条记录,如果它抛出DUP_VAL_ON_INDEX异常(表明已有这样的记录),就忽略它.

选择第二种选择的缺点是什么？

更新:

我想最好的方法是:"异常引起的开销是否比初始选择引起的开销更糟？"

Answer 1

我通常只是插入并捕获DUP_VAL_ON_INDEX异常,因为这是最简单的代码.这比插入前检查存在更有效.我不认为这是一个"难闻的气味"(可怕的短语!),因为我们处理的异常是由Oracle引发的 - 它不像将自己的异常作为流控制机制提升.

感谢Igor的评论我现在运行了两个不同的benchamrks:(1)除了第一个之外的所有插入尝试都是重复的,(2)其中所有插入都不是重复的.现实将介于两种情况之间.

注意:在Oracle 10.2.0.3.0上执行的测试.

案例1:大多数重复

似乎最有效的方法(通过一个重要因素)是检查存在何时插入:

prompt 1) Check DUP_VAL_ON_INDEX
begin
   for i in 1..1000 loop
      begin
         insert into hasviewed values(7782,20);
      exception
         when dup_val_on_index then
            null;
      end;
   end loop
   rollback;
end;
/

prompt 2) Test if row exists before inserting
declare
   dummy integer;
begin
   for i in 1..1000 loop
      select count(*) into dummy
      from hasviewed
      where objectid=7782 and userid=20;
      if dummy = 0 then
         insert into hasviewed values(7782,20);
      end if;
   end loop;
   rollback;
end;
/

prompt 3) Test if row exists while inserting
begin
   for i in 1..1000 loop
      insert into hasviewed
      select 7782,20 from dual
      where not exists (select null
                        from hasviewed
                        where objectid=7782 and userid=20);
   end loop;
   rollback;
end;
/

结果(运行一次以避免解析开销):

1) Check DUP_VAL_ON_INDEX

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.54
2) Test if row exists before inserting

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.59
3) Test if row exists while inserting

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.20

案例2:没有重复

prompt 1) Check DUP_VAL_ON_INDEX
begin
   for i in 1..1000 loop
      begin
         insert into hasviewed values(7782,i);
      exception
         when dup_val_on_index then
            null;
      end;
   end loop
   rollback;
end;
/

prompt 2) Test if row exists before inserting
declare
   dummy integer;
begin
   for i in 1..1000 loop
      select count(*) into dummy
      from hasviewed
      where objectid=7782 and userid=i;
      if dummy = 0 then
         insert into hasviewed values(7782,i);
      end if;
   end loop;
   rollback;
end;
/

prompt 3) Test if row exists while inserting
begin
   for i in 1..1000 loop
      insert into hasviewed
      select 7782,i from dual
      where not exists (select null
                        from hasviewed
                        where objectid=7782 and userid=i);
   end loop;
   rollback;
end;
/

结果:

1) Check DUP_VAL_ON_INDEX

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.15
2) Test if row exists before inserting

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.76
3) Test if row exists while inserting

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.71

在这种情况下,DUP_VAL_ON_INDEX会赢一英里.请注意,"插入前选择"在两种情况下都是最慢的.

因此,根据插入是否重复的相对可能性,您似乎应该选择选项1或3.