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;
/

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

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);

/*
 * 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. ^^

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.

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

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));

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

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;

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.