01 Developing Successful Oracle Application

本章提要
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本章是概述性章节
1. 介绍了了解数据库内部结构对于开发的重要性
2. 介绍了如何才能开发好的数据库应用程序
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varchar2 类型定义时, 个人认为应该选择byte 类型, 即 varchar2(20), oracle 支持的最大的字符串是 varchar2(4000), 同时, 个人认为, 当你定义一个varchar2时, 首先预判这个地段大概能放多少内容, 比如20个字符,但是由于多字符集中, 比如utf8, 一个字符并不是对应一个字节, 比如有可能对应4个字节, 所以, 比如我们定义的字段有20个字, 那么, 我们需要定义80个字节以上, 比如 varchar2(100), 这样是比较保险的
1.
基本上 95% 的问题都可以通过 SQL解决, %5 PL/SQL 和 C 解决

自治事物的作用( 不推荐使用自治事物 )
    1) error-logging: 记录错误, 无论你的transaction是否提交, 都需要知道你曾经做过的内容
    2) demonstration concept: 用来演示, 比如你想演示两个独立的session 等

作为一个developer必须要基本上清楚database内部结构, 不能将database看成是一个"黑盒"
    举例: bitmap索引, 你需要了解bitmap索引是如何工作的, 否则就会出问题
    -- 01 test_autonomous_transaction&bitmap.sql

/*

 * Test autonomous_transaction, simulation two session

 * bitmap will lock the column, when uncommit transaction exist

 */



create table t

( test_flag varchar2(1));



create bitmap index t_idx on t(test_flag);



-- uncommit

insert into t values('N');



-- antonomous_transaction

declare

pragma autonomous_transaction;

begin

    insert into t values('Y');

end;

/

01 test_autonomous_transcation&bitmap

例子说明: 因为bitmap索引的特点是, 会对column增加锁, 所以正确的做法是对该列增加B*tree索引
    举例: 函数索引
    -- 02 function_index.sql
  

/*

 * This program will test index on function,

 * so when you want to use index on where condition, 

 * You need to use function. 

 */



create table t

(     id    number primary key,

    test_flag    varchar2(1),

    payload        varchar2(20)

);



-- if test_flag == 'N', return 'N'

create index t_idx on t(decode(test_flag, 'N', 'N'));    



insert into t

select r,

        case 

        when mod(r, 2) = 0 then 'N'

        else 'Y' 

        end,

        'payload ' || r

   from (select level r

           from dual

           connect by level <= 5);

/



select * from t;



create or replace function get_first_unlocked_row

return    t%rowtype

as

    resource_busy exception;

    pragma exception_init(resource_busy, -54);

    l_rec t%rowtype;

begin

    for x in (select rowid rid 

                from t

               where decode(test_flag, 'N', 'N') = 'N') -- index function

    loop

    begin

        select * into l_rec

          from t

         where rowid = x.rid and test_flag = 'N'

           for update nowait;

        return l_rec;

    exception

        when resource_busy then null;

        when no_data_found then null;

    end;

    end loop;

    return null;

end;

/



-- test function

declare

l_rec t%rowtype;

begin

    l_rec := get_first_unlocked_row;

             

    dbms_output.put_line('I got row ' || l_rec.id || ', ' || l_rec.payload);

    commit;

end;

/

-- result is 2.



declare

l_rec t%rowtype;

cursor c

is

select *

  from t

 where decode(test_flag, 'N', 'N') = 'N'    -- use decode function is for index

   for update

  skip locked;

begin

    open c;

    fetch c into l_rec;

    if (c%found)

    then

        dbms_output.put_line('I got row ' || l_rec.id || ', ' || l_rec.payload);

    end if;

    close c;

end;

/



-- result is 2



declare

l_rec t%rowtype;

pragma autonomous_transaction;

cursor c

is

select *

  from t

 where decode(test_flag, 'N', 'N') = 'N'    -- use decode function is for index

   for update

  skip locked;  -- if you don't use this statement, the process will hang and wait 

begin

    open c;

    fetch c into l_rec;

    if (c%found)

    then

        dbms_output.put_line('I got row ' || l_rec.id || ', ' || l_rec.payload);

    end if;

    close c;

    commit;

end;

/



-- result is 4

View Code

2.
怎样才能更好的创建application ?
1) understanding oracle architecture
2) Use a single connection in Oracle( 即多个查询使用一个connection, 这样当然节省资源)
3) Use Bind variables
    这里涉及到软解析和硬解析的概念, 另外, 如果不使用绑定变量, 有可能引起 SQL INJECTION(SQL注入),
    即用户输入了不合法数据, 使应用存在安全隐患.
    -- 03 & 04
    

/*

 * This program will show use bind variable is very effect

 */



create table t(x int);



create or replace procedure proc1

as

begin

        for i in 1..10000

        loop

                execute immediate

                'insert into t values(:x)'

                using i;

        end loop;

end;

/



create or replace procedure proc2

as

begin

        for i in 1..10000

        loop

                execute immediate

                'insert into t values('|| i ||')';

        end loop;

end;

/



-- use runstats_pkg test to procedure

exec runstats_pkg.rs_start;

exec proc1;

exec runstats_pkg.rs_middle;

exec proc2;

exec runstats_pkg.rs_stop(10000);

03

/*

 * This program will test SQL injection when you don't use bind variable.

 */



create or replace procedure inj(p_date in date)

as

    l_rec    all_users%rowtype;

    c        sys_refcursor;

    l_query    long;

begin

    l_query := '

        select *

          from all_users

         where created = ''' || p_date || '''';

        dbms_output.put_line(l_query);

        open c for l_query;



        for i in 1..5

        loop

                fetch c into l_rec;

                exit when c%notfound;

                dbms_output.put_line(l_rec.username || '.....');

        end loop;

        close c;

end;

/



-- you want to show

exec inj(sysdate);



-- show the dangerous part about this inj procedure

create table user_pw

( uname varchar2(30) primary key,

  pw varchar2(30)

);



insert into user_pw(uname, pw)

values('TKYTE', 'TO SECRET');

COMMIT;

-- now, some user don't know user_pw table exist, simulation this table

-- +is very important.

grant execute on inj to scott;



-- so now, scott connect the database, and do as below

alter session set nls_date_format = '"''union select tname, 0, null from tab--"';

exec leon.inj(sysdate);

/*

 * The result is:

 * -----------------------------------------

   select *

     from all_users

    where created = ''union select tname, 0, null from tab--'

 * -----------------------------------------

 * we know table information, some important table.

 */

-- in this way, scott can see the table user_pw(very important table)

-- now they want to try to select this important table.

select * from leon.user_pw;  -- but they can not, because they don't have privilege.



alter session set nls_date_format = '"''union select tname || cname, 0, null from col--"';

exec leon.inj(sysdate);

/*

 * The result is:

 * -----------------------------------------

   select *

     from all_users

    where created = ''union select tname || cname, 0, null from col--'

 * -----------------------------------------

 * we know the column information in some important table.

 */



-- use bind variable to pertect you.

create or replace procedure NOT_inj(p_date in date)

as

    l_rec    all_users%rowtype;

    s        sys_refcursor;

    l_query    long;

begin

        l_query := '

        select *

          from all_users

         where created = :x';

        dbms_output.put_line(l_query);

        open c for l_query using P_DATE;

        for i in 1..5

        loop

                fetch c into l_rec;

                exit when c%notfound;

                dbms_output.put_line(l_rec.username || '....');

        end loop;

        close c;

end;

/



-- test not_inj

exec not_inj(sysdate);

/*

 * The result is:

 * -----------------------------------------

   select *

     from all_users

    where created = :x

 * -----------------------------------------

 */



-- so from now on, you must use bind variable. ^^

04

4) understanding concurrency control
    并发是很难控制的, 而且很容易导致我们的应用程序出现问题, 锁可以用来处理并发, 但是如果锁利用不当,
    就会给程序的可扩展和多用户操作造成阻碍, 所以, in your database is vital if you are to develop a scalable,
    correct application.
    举例: 控制并发
    -- 05 control_concurrency.sql
   

create table resources

( resource_name    varchar2(25) primary key,

  other_data varchar2(25)

);



create table schedules

( resource_name varchar2(25) references resources,

  start_time date,

  end_time date

);



-- the purpose about schedules is when someone want to modify table resources

-- he needs to check the schedules status, but if someone

-- modify the table schedules, and you select schedules, you can not find 

-- the result modified.

select count(*) 

  from schedules

 where resource_name = :resource_name

   and (start_time < :new_start_time)

   and (end_time > :new_end_time);

-- so the correct selectment is as below:

select * from resources where resource_name := resource_name for update;

-- for update is very important, it is lock on the row. so if someone want to

-- modify the table, you will know it.

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5) implementing locking ( 使用锁, 上边已经有例子了)
6) flashback
    SCN: This SCN is Oracle’s internal clock: every time a commit occurs, this clock ticks upward (increments).
    flashback举例:
    -- 06 flashback_example.sql
   

variable scn number;

exec :scn := dbms_flashback.get_system_change_number;

print scn;



select count(*) from emp;    -- 14 rows

delete from emp;

select count(*) from emp;     -- 0 rows



-- use flashback, as of scn, as of timestamp

select count(*)

        :scn then_scn,

        dbms_flashback.get_system_change_number now_scn

  from emp as of scn :scn;    -- get the time point at :scn

-- the result is

/*

COUNT(*) THEN_SCN NOW_SCN 

---------- ---------- ---------- 

14        6294536 6294537

*/

-- you can see the result is 14.

commit;        -- commit the transaction

select cnt_now, cnt_then, :scn then_scn,

        dbms_flashback.get_system_change_number now scn

    from (select count(*) cnt_now from emp),

         (select count(*) cnt_then, from emp as of scn :scn)

/

/*

CNT_NOW CNT_THEN THEN_SCN NOW_SCN 

---------- ---------- ---------- ---------- 

14               14 6294536 6294552

 */



flashback table emp to scn :scn;    -- the data return

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7) Read Consistency and Non-Blocking Reads
    读一致性, 并且没有读锁.
8) Database Independence
    当需要数据库迁移时, 你要知道, 即便是相同的数据库, 比如都是 oracle, 那么它们的内部运作机制也可能不一样, 这也有可能
    给你带来一些问题, 更别提那些不同数据库之间的迁移问题, 比如: 将标准SQL转换成plsql:
    07 convert_SQL_to_PLSQL.sql
   

declare

    l_some_varibale    varchar2(25);

begin

    if (some_condition)

    then

        l_some_variable := f(...);

    end if;

    

    for x in (select * from t where x = l_some_variable)

    loop

        ...

    end loop;

-- as this statement

-- in oracle, this query return no data when l_some_variable was not set to a specific value

-- in sybase or sql server, the query would find rows where x was set to a null value.



-- in oracle null can not use = to set condition, like as below:

select * from dual where null = null;  -- return 0 rows selected

select * from dual where null <> null; -- return 0 rows selected

select * from dual where null is null; -- return X.



-- to solve this problem 

select * from t

 where nvl(x, -1) = nvl(l_some_variable, -1);



-- and you need to create a function index

create index t_idx on t(nvl(x, -1));

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9) The impact of standards
    SQL99 is an ANSI/ISO standard for databases, 各个数据库在实现这个标准时有不同, 另外这个标准有些时候也是有问题的, 所以
    你只要专注你目前使用的数据库的标准.
10) Layered Programming (分层Programming)
    Let’s say you are programming using JDBC, 不同的数据库之间, 要使用不同的方法实现, 并且利用存储过程实现.
11) Knowing What's Out There
    不是完全了解SQL的特性
    example: inline views
   

-- inline views

select p.id, c1_sum1, c2_sum2

  from p,

        (select id, sum(q1) c1_sum1

           from c1

          group by id) c1,

        (select id, sum(q2) c2_sum2

           from c2

          group by id) c2

    where p.id = c1.id

      and p.id = c2.id



-- sub query that run another query per row

select p.id,

        (select sum(q1) from c1 where c1.id = p.id) c1_sum1,

        (select sum(q2) from c2 where c2.id = p.id) c2_sum2

  from p

 where p.name = '1234'



-- sub factory with clause

with c1_vw as

(select id, sum(q1) c1_sum1

   from c1

  group by id),

c2_vw as

(select id, sum(q2) c2_sum2

   from c2

  group by id),

c1_c2 as

(select c1.id, c1.c1_sum1, c2.c2_sum2

   from c1_vw c1, c2_vw c2

  where c1.id = c2.id)

select p.id, c1_sum1, c2_sum2

  from p, c1_c2

 where p.id = c1_c2.id

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12) sloving problems simply
    选择简单的办法来解决事情, 比如想控制某个用户只能一个session连接上来, 简单的办法是:
    09 do_job_easy_way.sql
   

create profile one_session limit sessions_per_user 1;



alter user scott profile one_session;



alter system set resource_limit = true;

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13) How Do i make it run faster ?
    80%以上的性能问题都出现在设计实现级, 而不是数据库级. 在对数据库上运行的应用进行优化之前, 不要对数据库进行优化.