Question

我想在一个范围内找到前10个未使用的手动序列号。

请在下面找到我的问题：

select X1.* From
    (Select Rownum seq_number From Dual Connect By Rownum <= 
         (Select LPAD(9,(UTC.DATA_PRECISION - UTC.DATA_SCALE),9) 
          From User_Tab_Columns UTC 
          where UTC.Table_Name = 'Table_Name' And UTC.Column_Name = 'seq_number')) X1,
Table_Name X2
Where X1.seq_number = X2.seq_number (+)
  And X2.Rowid is Null
  And Rownum <= 10

虽然它提供了所需的输出，但我担心[如果有]造成的负载，因为我们将每天多次使用此查询。

请告知是否有办法优化此查询。

其他信息：在Table_Name T2上，在（seq_number）

上定义了唯一索引

工作示例：

create table TEMP_TABLE_NAME ( seq_number number(6))

insert into TEMP_TABLE_NAME 
select distinct trunc(dbms_random.VALUE(1,5000)) seq_number 
from dual
connect by rownum <= 1000


create unique index TEMP_TABLE_NAME_IDX on TEMP_TABLE_NAME(seq_Number)


SELECT T1.*
  FROM (    SELECT ROWNUM seq_number
              FROM DUAL
        CONNECT BY ROWNUM <=
                      (SELECT LPAD (9,(UTC.DATA_PRECISION - UTC.DATA_SCALE),9)
                         FROM User_Tab_Columns UTC
                        WHERE     UTC.Table_Name = 'TEMP_TABLE_NAME'
                              AND UTC.Column_Name = 'SEQ_NUMBER')) T1,
       TEMP_TABLE_NAME T2
 WHERE     T1.seq_number = T2.seq_number(+)
       AND T2.ROWID IS NULL
       AND ROWNUM <= 10

对我来说，我的查询给出了以下输出。表中随机创建的数字包括7＆amp; 8所以他们被忽略了。重点是获得前10个未使用的数字。

Answer 1

首先我会替换这个复杂的子查询：

Select Rownum seq_number From Dual Connect By Rownum <= 
         (Select LPAD(9,(UTC.DATA_PRECISION - UTC.DATA_SCALE),9) 
          From User_Tab_Columns UTC 
          where UTC.Table_Name = 'Table_Name' And UTC.Column_Name = 'seq_number')

这一个：

Select Rownum As seq_number From Dual 
Connect By Rownum <= (Select max( seq_number ) + 10 From TEMP_TABLE_NAME )

甚至是一个简单的常数：

Select Rownum As seq_number From Dual Connect By Rownum <= 1000000

你的子查询坦率地说不适用于一个非常基本的案例：

create table TEMP_TABLE_NAME(
  seq_number NUMBER
);

SELECT LPAD (9,(UTC.DATA_PRECISION - UTC.DATA_SCALE),9) as x , 
       UTC.DATA_PRECISION, UTC.DATA_SCALE, UTC.COLUMN_NAME
FROM User_Tab_Columns UTC
WHERE     UTC.Table_Name = 'TEMP_TABLE_NAME'
  AND UTC.Column_Name = 'SEQ_NUMBER'
;

X        DATA_PRECISION DATA_SCALE COLUMN_NAME
-------- -------------- ---------- -----------
  (null)         (null)     (null) SEQ_NUMBER

第二个案例：

create table TEMP_TABLE_NAME(
  seq_number NUMBER(15,0)
);

在这种情况下，子查询尝试生成999999999999999行，这很快就会导致内存不足错误

SELECT count(*) FROM (
 SELECT ROWNUM seq_number
              FROM DUAL
        CONNECT BY ROWNUM <=
                      (SELECT LPAD (9,(UTC.DATA_PRECISION - UTC.DATA_SCALE),9)
                         FROM User_Tab_Columns UTC
                        WHERE     UTC.Table_Name = 'TEMP_TABLE_NAME'
                              AND UTC.Column_Name = 'SEQ_NUMBER')
);

ORA-30009: Not enough memory for CONNECT BY operation
30009. 0000 -  "Not enough memory for %s operation"
*Cause:    The memory size was not sufficient to process all the levels of the
           hierarchy specified by the query.
*Action:   In WORKAREA_SIZE_POLICY=AUTO mode, set PGA_AGGREGATE_TARGET to
           a reasonably larger value.
           Or, in WORKAREA_SIZE_POLICY=MANUAL mode, set SORT_AREA_SIZE to a
           reasonably larger value.

其次，您的查询不具有确定性!!!
它强烈依赖于物理表结构，并且不使用ORDER BY子句强加正确的顺序请记住 - ＆gt; Wikipedia - ORDER BY

ORDER BY 是对结果集中的行进行排序的唯一方法。的无在这个子句中，关系数据库系统可以返回任何行订单。如果需要订购，则必须提供ORDER BY 应用程序发送的SELECT语句。

考虑这个测试用例：

create table TEMP_TABLE_NAME 
as SELECT * FROM (
    select rownum as seq_number , t.*
    from ALL_OBJECTS t
    cross join ( select * from dual connect by level <= 10)
    where rownum <= 100000
)
ORDER BY DBMS_RANDOM.Value;
create unique index TEMP_TABLE_NAME_IDX on TEMP_TABLE_NAME(seq_Number);

select count(*) from TEMP_TABLE_NAME;
  COUNT(*)
----------
    100000

DELETE FROM TEMP_TABLE_NAME
WHERE seq_number between 10000 and 10002
  OR seq_number between 20000 and 20002
  OR seq_number between 30000 and 30002
  OR seq_number between 40000 and 40002
  OR seq_number between 50000 and 50002
  OR seq_number between 60000 and 60002
  ;

如果索引存在，则结果为OK：

SELECT T1.*
  FROM (    SELECT ROWNUM seq_number
              FROM DUAL
        CONNECT BY ROWNUM <= 1000000
) T1,
       TEMP_TABLE_NAME T2
 WHERE     T1.seq_number = T2.seq_number(+)
       AND T2.ROWID IS NULL
       AND ROWNUM <= 10
;

SEQ_NUMBER
----------
     10000
     10001
     10002
     20000
     20001
     20002
     30000
     30001
     30002
     40000

但是当有人删除索引时会发生什么，或者由于某些原因决定不使用该索引的优化器呢？根据定义：如果没有ORDER BY，关系数据库系统可以按任何顺序返回行。 我使用提示模拟这些案例：

SELECT /*+ NO_INDEX(T2) */ T1.*
  FROM (    SELECT ROWNUM seq_number
              FROM DUAL
        CONNECT BY ROWNUM <= 1000000
) T1,
       TEMP_TABLE_NAME T2
 WHERE     T1.seq_number = T2.seq_number(+)
       AND T2.ROWID IS NULL
       AND ROWNUM <= 10
;

SEQ_NUMBER
----------
    213856
    910281
    668862
    412743
    295487
    214762
    788486
    346216
    777734
    806457

以下查询使用ORDER BY子句强制执行正确的顺序，并提供重现结果，无论是否存在正确的索引。
我使用推荐的ANSI SQL LEFT JOIN子句而不是过时的WHERE .... (+)语法。

SELECT  * FROM (
    SELECT /*+ NO_INDEX(T2) */ T1.*
      FROM (    SELECT ROWNUM seq_number
                  FROM DUAL
            CONNECT BY ROWNUM <= 1000000
    ) T1 
    LEFT JOIN TEMP_TABLE_NAME T2
    ON T1.seq_number = T2.seq_number
    WHERE T2.ROWID IS NULL
    ORDER BY T1.seq_number
)
WHERE ROWNUM <= 10

<强>性能
检查性能的最简单方法是进行测试 - 运行查询10到100次并测量时间：

SET TIMING ON;
DECLARE
   x NUMBER;
BEGIN
   FOR i IN 1..10 LOOP
      SELECT sum( seq_number ) INTO x
      FROM (
           SELECT  * FROM (
            SELECT T1.*
              FROM (    SELECT ROWNUM seq_number
                          FROM DUAL
                    CONNECT BY ROWNUM <= 1000000
            ) T1 
            LEFT JOIN TEMP_TABLE_NAME T2
            ON T1.seq_number = T2.seq_number
            WHERE T2.ROWID IS NULL
            ORDER BY T1.seq_number
            )
            WHERE ROWNUM <= 10
        );
    END LOOP;
END;
/

PL/SQL procedure successfully completed.

Elapsed: 00:00:11.750

10次 - 11.75秒，因此一次查询需要1,2秒。

下一个版本CONNECT BY中的限制使用子查询：

SET TIMING ON;
DECLARE
   x NUMBER;
BEGIN
   FOR i IN 1..10 LOOP
      SELECT sum( seq_number ) INTO x
      FROM (
           SELECT  * FROM (
            SELECT T1.*
              FROM (    SELECT ROWNUM seq_number
                          FROM DUAL
                    CONNECT BY ROWNUM <= (Select max( seq_number ) + 10 From TEMP_TABLE_NAME ) 
            ) T1 
            LEFT JOIN TEMP_TABLE_NAME T2
            ON T1.seq_number = T2.seq_number
            WHERE T2.ROWID IS NULL
            ORDER BY T1.seq_number
            )
            WHERE ROWNUM <= 10
        );
    END LOOP;
END;
/
PL/SQL procedure successfully completed.

Elapsed: 00:00:00.986

好多了 - 只有100毫秒由此得出结论，CONNECT BY部分成本最高。

使用具有预生成的数字序列的表的另一种尝试，最多1毫升（物化视图的种类），而不是每次在内存中动态生成数字的CONNECT BY子查询：

create table seq(
   seq_number int primary key
)
ORGANIZATION INDEX ;

INSERT INTO seq 
SELECT level FROM dual
CONNECT BY LEVEL <= 1000000;

SET TIMING ON;
DECLARE
   x NUMBER;
BEGIN
   FOR i IN 1..10 LOOP
      SELECT sum( seq_number ) INTO x
      FROM (
           SELECT  * FROM (
            SELECT T1.*
            FROM seq T1 
            LEFT JOIN TEMP_TABLE_NAME T2
            ON T1.seq_number = T2.seq_number
            WHERE T2.ROWID IS NULL
            ORDER BY T1.seq_number
            )
            WHERE ROWNUM <= 10
        );
    END LOOP;
END;
/

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.398

这是最快的 - 只有40毫秒

第一个1200毫秒，最后一个40毫秒 - 快30倍（3000％）。

获取前10个未使用的manual_sequence数字

1 个答案: