这将会很长，所以这里有一个快速摘要来吸引你：我的
其计划中COUNT STOPKEY和ORDER BY STOPKEY的前N个查询是
没有充分理由仍然很慢。
现在，细节。它以慢速功能开始。在现实生活中 涉及使用regexp进行字符串操作。出于演示目的， 这是一个故意愚蠢的递归Fibonacci算法。我找到了 对于大约25的输入来说非常快，大约30的速度慢，并且 35岁时很荒谬。
-- 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;
/
现在有些输入：名单和数字列表。
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;
以下是慢查询的示例：使用慢速Fibonacci函数 选择其num生成带有加倍数字的Fibonacci数的行。
SELECT p.name, p.num
FROM tmp_table p
WHERE REGEXP_LIKE(tmp_fib(p.num), '(.)\1')
ORDER BY p.name;
对于11和33都是如此，因此Bravo和Charlie在输出中。
运行大约需要5秒钟，几乎所有这些都很慢
计算tmp_fib(33)。所以我想做一个更快的版本
通过将其转换为前N个查询来缓慢查询。 N = 1，看起来像
这样：
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。但它仍需要5秒钟
跑步！唯一的解释是它仍在计算中
tmp_fib(33)，即使计算结果无关紧要
结果。应该能够确定Bravo正在进行
要输出，所以不需要测试WHERE条件
其余的表。
我认为可能只需要告诉优化器
tmp_fib很贵。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，但它没有 查询运行得更快。
SELECT * FROM v$version的输出，如果这是版本相关的：
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。它似乎在宣称 查询耗时1秒，但事实并非如此。它跑了大约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，INDEX提示有助于此查询。尽管它不能很好地转化为我的现实场景，但接受它作为正确的答案已经足够好了。而且具有讽刺意味的是，Oracle似乎从经验中吸取了教训，现在默认选择INDEX计划;我必须告诉它NO_INDEX重现原始的缓慢行为。
在现实场景中，我应用了一个更复杂的解决方案，将查询重写为PL / SQL函数。以下是我的技术外观，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));
感谢响应者阅读问题并运行测试并帮助我理解执行计划，并且我会处理这个问题但是他们建议。
答案 0 :(得分：2)
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与您所看到的内容的变化以及相应的rows值。将订单移动到子选项看起来更像您的：
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)
不知道这在您的真实场景中会有多大帮助或实用，但在这种情况下（在我的环境中，无论如何），在所有提取的列中添加索引 - 获取完整的索引扫描而不是完整的索引扫描 - 似乎改变了行为：
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，aftr我运行了几次rownum变种后，我最终开始出现ORA-01000: maximum open cursors exceeded错误。我在每次运行结束时都丢弃了这些对象，但保持连接状态。我认为这表明某个地方存在另一个错误，但可能与你所看到的无关，因为即使使用索引扫描它也会发生。
答案 1 :(得分：1)
兴趣显然已经消失，所以我只是在自我回答中总结可能的解决方案。