performance - 尽管进行了 STOPKEY 优化，但前 N 个查询还是做了太多工作

Question

这会很长，所以这里有一个简短的总结来吸引你:我的
使用 COUNT STOPKEY 的前 N ​​个查询和 ORDER BY STOPKEY在它的计划中是
仍然无缘无故地缓慢。

现在，细节。它从一个缓慢的函数开始。在现实生活中它
涉及使用正则表达式的字符串操作。出于演示目的，
这是一个故意愚蠢的递归斐波那契算法。我找到它了
对于高达大约 25 的输入，速度非常快，大约 30，并且
35岁可笑。

-- I repeat: Please no advice on how to do Fibonacci correctly.
-- This is slow on purpose!
CREATE OR REPLACE FUNCTION tmp_fib (
  n INTEGER
)
  RETURN INTEGER
AS
BEGIN
  IF n = 0 OR n = 1 THEN
    RETURN 1;
  END IF;
  RETURN tmp_fib(n-2) + tmp_fib(n-1);
END;
/

现在一些输入:名称和数字的列表。

CREATE TABLE tmp_table (
  name VARCHAR2(20) UNIQUE NOT NULL,
  num NUMBER(2,0)
);
INSERT INTO tmp_table (name,num)
  SELECT 'Alpha',    10 FROM dual UNION ALL
  SELECT 'Bravo',    11 FROM dual UNION ALL
  SELECT 'Charlie',  33 FROM dual;

下面是一个慢查询的例子:使用慢 Fibonacci 函数来
选择 num 生成具有双位数的斐波那契数的行。

SELECT p.name, p.num
FROM tmp_table p
WHERE REGEXP_LIKE(tmp_fib(p.num), '(.)\1')
ORDER BY p.name;

这对于 11 和 33 是​​正确的，所以 Bravo和 Charlie在输出中。
运行大概需要5秒，几乎都是慢的 tmp_fib(33)的计算.所以我想做一个更快的版本
通过将其转换为 top-N 查询来降低查询速度。当 N=1 时，看起来像
这个:

SELECT * FROM (
  SELECT p.name, p.num
  FROM tmp_table p
  WHERE REGEXP_LIKE(tmp_fib(p.num), '(.)\1')
  ORDER BY p.name
)
WHERE ROWNUM <= 1;

现在它返回最高结果， Bravo .但是还是需要5秒
运行!唯一的解释是它仍在计算 tmp_fib(33) ，即使该计算的结果无关紧要
结果。应该可以决定 Bravo当时正在
要输出，因此无需测试 WHERE 条件
table 的其余部分。

我想也许优化器只需要被告知 tmp_fib价格昂贵。所以我试着告诉它，像这样:

ASSOCIATE STATISTICS WITH FUNCTIONS tmp_fib DEFAULT COST (999999999,0,0);

这改变了计划中的一些成本数字，但并没有
查询运行得更快。
SELECT * FROM v$version 的输出如果这是依赖于版本的:

Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for 64-bit Windows: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production

这是 top-1 查询的自动跟踪。它似乎声称
查询花了 1 秒，但事实并非如此。它运行了大约 5
秒。

NAME                        NUM
-------------------- ----------
Bravo                        11


Execution Plan
----------------------------------------------------------
Plan hash value: 548796432

-------------------------------------------------------------------------------------
| Id  | Operation               | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |           |     1 |    55 |     4  (25)| 00:00:01 |
|*  1 |  COUNT STOPKEY          |           |       |       |            |          |
|   2 |   VIEW                  |           |     1 |    55 |     4  (25)| 00:00:01 |
|*  3 |    SORT ORDER BY STOPKEY|           |     1 |    55 |     4  (25)| 00:00:01 |
|*  4 |     TABLE ACCESS FULL   | TMP_TABLE |     1 |    55 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<=1)
   3 - filter(ROWNUM<=1)
   4 - filter( REGEXP_LIKE (TO_CHAR("TMP_FIB"("P"."NUM")),'(.)\1'))

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
         27  recursive calls
          0  db block gets
         25  consistent gets
          0  physical reads
          0  redo size
        593  bytes sent via SQL*Net to client
        524  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          1  rows processed

更新 :正如我在评论中提到的，一个 INDEX提示对这个查询有很大帮助。被接受为正确答案就足够了，即使它不能很好地转化为我的真实世界场景。具有讽刺意味的是，甲骨文似乎从经验中吸取了教训，现在选择了 INDEX。默认计划；我必须告诉它 NO_INDEX重现原来的缓慢行为。

在实际场景中，我应用了更复杂的解决方案，将查询重写为 PL/SQL 函数。这是我的技术的外观，应用于 fib问题:

CREATE OR REPLACE PACKAGE tmp_package IS
  TYPE t_namenum IS TABLE OF tmp_table%ROWTYPE;
  FUNCTION get_interesting_names (howmany INTEGER) RETURN t_namenum PIPELINED;
END;
/

CREATE OR REPLACE PACKAGE BODY tmp_package IS
  FUNCTION get_interesting_names (howmany INTEGER) RETURN t_namenum PIPELINED IS
    CURSOR c IS SELECT name, num FROM tmp_table ORDER BY name;
    rec c%ROWTYPE;
    outcount INTEGER;
  BEGIN
    OPEN c;
    outcount := 0;
    WHILE outcount < howmany LOOP
      FETCH c INTO rec;
      EXIT WHEN c%NOTFOUND;
      IF REGEXP_LIKE(tmp_fib(rec.num), '(.)\1') THEN
        PIPE ROW(rec);
        outcount := outcount + 1;
      END IF;
    END LOOP;
  END;
END;
/

SELECT * FROM TABLE(tmp_package.get_interesting_names(1));

感谢阅读问题并运行测试并帮助我理解执行计划的响应者，我将按照他们的建议处理这个问题。

Answer 1

后续评论，因为这太大了。在 11.2.0.3 (OEL) 下运行，您的查询:

SELECT * FROM (
  SELECT p.name, p.num
  FROM tmp_table p
  WHERE REGEXP_LIKE(tmp_fib(p.num), '(.)\1')
  ORDER BY p.name
)
WHERE ROWNUM <= 1;

NAME                        NUM
-------------------- ----------
Bravo                        11 

Elapsed: 00:00:00.094
Plan hash value: 1058933870

----------------------------------------------------------------------------------
| Id  | Operation            | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |           |     1 |    25 |     4  (25)| 00:00:01 |
|*  1 |  COUNT STOPKEY       |           |       |       |            |          |
|*  2 |   VIEW               |           |     3 |    75 |     4  (25)| 00:00:01 |
|   3 |    SORT ORDER BY     |           |     3 |    75 |     4  (25)| 00:00:01 |
|   4 |     TABLE ACCESS FULL| TMP_TABLE |     3 |    75 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<=1)
   2 - filter( REGEXP_LIKE (TO_CHAR("TMP_FIB"("NUM")),'(.)\1'))

Note
-----
   - dynamic sampling used for this statement (level=2)

请注意 SORT ORDER BY 中的变化从你看到的，以及相应的 rows值。将 order-by 移动到子选择中看起来更像你的:

SELECT * FROM (
  SELECT * FROM (
    SELECT p.name, p.num
    FROM tmp_table p
    ORDER BY p.name
  )
  WHERE REGEXP_LIKE(tmp_fib(num), '(.)\1')
)
WHERE ROWNUM <= 1;

NAME                        NUM
-------------------- ----------
Bravo                        11 

Elapsed: 00:00:07.894
Plan hash value: 548796432

-------------------------------------------------------------------------------------
| Id  | Operation               | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |           |     1 |    25 |   171  (99)| 00:00:03 |
|*  1 |  COUNT STOPKEY          |           |       |       |            |          |
|   2 |   VIEW                  |           |     1 |    25 |   171  (99)| 00:00:03 |
|*  3 |    SORT ORDER BY STOPKEY|           |     1 |    25 |   171  (99)| 00:00:03 |
|*  4 |     TABLE ACCESS FULL   | TMP_TABLE |     1 |    25 |   170  (99)| 00:00:03 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<=1)
   3 - filter(ROWNUM<=1)
   4 - filter( REGEXP_LIKE (TO_CHAR("TMP_FIB"("P"."NUM")),'(.)\1'))

Note
-----
   - dynamic sampling used for this statement (level=2)

不知道这在您的实际场景中会有多大帮助或实用性，但在这种情况下(无论如何，在我的环境中)，在所有获取的列中添加索引 - 以获得完整索引扫描而不是完整表扫描 - 似乎改变行为:

CREATE INDEX tmp_index ON tmp_table(name, num);

index TMP_INDEX created.

SELECT * FROM (
  SELECT p.name, p.num
  FROM tmp_table p
  WHERE REGEXP_LIKE(tmp_fib(p.num), '(.)\1')
  ORDER BY p.name
)
WHERE ROWNUM <= 1;

NAME                        NUM
-------------------- ----------
Bravo                        11 

Elapsed: 00:00:00.093
Plan hash value: 1841475998

-------------------------------------------------------------------------------
| Id  | Operation         | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |           |     1 |    25 |     1   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY    |           |       |       |            |          |
|*  2 |   VIEW            |           |     3 |    75 |     1   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN| TMP_INDEX |     3 |    75 |     1   (0)| 00:00:01 |
-------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<=1)
   2 - filter( REGEXP_LIKE (TO_CHAR("TMP_FIB"("NUM")),'(.)\1'))

Note
-----
   - dynamic sampling used for this statement (level=2)

SELECT * FROM (
  SELECT * FROM (
    SELECT p.name, p.num
    FROM tmp_table p
    ORDER BY p.name
  )
  WHERE REGEXP_LIKE(tmp_fib(num), '(.)\1')
)
WHERE ROWNUM <= 1;

NAME                        NUM
-------------------- ----------
Bravo                        11 

Elapsed: 00:00:00.093
Plan hash value: 1841475998

-------------------------------------------------------------------------------
| Id  | Operation         | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |           |     1 |    25 |     1   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY    |           |       |       |            |          |
|   2 |   VIEW            |           |     1 |    25 |     1   (0)| 00:00:01 |
|*  3 |    INDEX FULL SCAN| TMP_INDEX |     1 |    25 |     1   (0)| 00:00:01 |
-------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<=1)
   3 - filter( REGEXP_LIKE (TO_CHAR("TMP_FIB"("P"."NUM")),'(.)\1'))

Note
-----
   - dynamic sampling used for this statement (level=2)

顺便说一句，在我用任何一个 rownum 跑了几次之后我最终开始得到的变体 ORA-01000: maximum open cursors exceeded错误。我在每次运行结束时放下对象但保持连接。我认为这表明某处存在另一个错误，尽管可能与您看到的内容无关，因为即使使用索引扫描也会发生。