深度嵌套子查询因子(CTE)的性能下降

Question

深度嵌套子查询因子(CTE)的性能下降

Ego*_*off 8 sql oracle benchmarking oracle11g

此查询包含16个相等的步骤.
每个步骤都在同一个数据集(单行)上进行相同的计算,
但最后的步骤需要花费太多时间.

with t0 as (select 0 as k from dual)
,t1 as (select k from t0 where k >= (select avg(k) from t0))
,t2 as (select k from t1 where k >= (select avg(k) from t1))
,t3 as (select k from t2 where k >= (select avg(k) from t2))
,t4 as (select k from t3 where k >= (select avg(k) from t3))
,t5 as (select k from t4 where k >= (select avg(k) from t4))
,t6 as (select k from t5 where k >= (select avg(k) from t5))
,t7 as (select k from t6 where k >= (select avg(k) from t6))
,t8 as (select k from t7 where k >= (select avg(k) from t7))
,t9 as (select k from t8 where k >= (select avg(k) from t8))
,t10 as (select k from t9 where k >= (select avg(k) from t9))
,t11 as (select k from t10 where k >= (select avg(k) from t10))
,t12 as (select k from t11 where k >= (select avg(k) from t11)) -- 0.5 sec
,t13 as (select k from t12 where k >= (select avg(k) from t12)) -- 1.3 sec
,t14 as (select k from t13 where k >= (select avg(k) from t13)) -- 4.5 sec
,t15 as (select k from t14 where k >= (select avg(k) from t14)) -- 30 sec
,t16 as (select k from t15 where k >= (select avg(k) from t15)) -- 4 min
select k from t16

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子查询t10立即完成,但整个查询(t16)需要4分钟才能完成.

Q1.
为什么相同数据的相同子查询的计算时间差别很大？

Q2.
它看起来像一个错误,因为它运行在Oracle 9速度非常快,在Oracle 11很慢
事实上,在长期和复杂的与子句的SELECT语句都将具有相同的行为方式.
这是一个已知的bug吗？(我无法访问metalink)
建议使用哪种解决方法？

Q3.
我必须为Oracle 11编写代码,我必须在单个select语句中完成所有计算.
我不能在两个单独的陈述中将我的长篇陈述分开以加速它.Oracle中
是否存在提示(或者可能是一些技巧)以使整个查询(t16)在合理的时间内完成(例如,在一秒内)？我试图找到这样一个但无济于事.
顺便说一句,执行计划非常好,而且成本表现为步数的线性函数(非指数).

Answer 1

Thi*_*Jet 7

Q1:似乎没有任何关于计算时间的信息,只是优化算法中的错误,它会在计算最佳执行计划时使其生气.

Q2:Oracle 11.X.0.X中存在许多已知和修复的错误,这些错误与嵌套查询和查询因子分解的优化有关.但是很难找到具体的问题.

Q3:有两个无证提示:materialize和inline,但没有他们中的一个为我工作,而我想你的例子.服务器配置或升级到11.2.0.3的某些更改可能会增加嵌套with子句的限制:对于我(在11.2.0.3 Win7/x86上),您的示例工作正常,但嵌套表的数量增加到30会挂起会话.

解决方法可能如下所示:

select k from (
select k, avg(k) over (partition by null) k_avg from ( --t16
  select k, avg(k) over (partition by null) k_avg from ( --t15
    select k, avg(k) over (partition by null) k_avg from ( --t14
      select k, avg(k) over (partition by null) k_avg from ( --t13
        select k, avg(k) over (partition by null) k_avg from ( --t12
          select k, avg(k) over (partition by null) k_avg from ( --t11
            select k, avg(k) over (partition by null) k_avg from ( --t10
              select k, avg(k) over (partition by null) k_avg from ( --t9
                select k, avg(k) over (partition by null) k_avg from ( --t8
                  select k, avg(k) over (partition by null) k_avg from ( --t7
                    select k, avg(k) over (partition by null) k_avg from ( --t6
                      select k, avg(k) over (partition by null) k_avg from ( --t5
                        select k, avg(k) over (partition by null) k_avg from ( --t4
                          select k, avg(k) over (partition by null) k_avg from ( --t3
                            select k, avg(k) over (partition by null) k_avg from ( --t2
                              select k, avg(k) over (partition by null) k_avg from ( -- t1
                                select k, avg(k) over (partition by null) k_avg from (select 0 as k from dual) t0
                              ) where k >= k_avg
                            ) where k >= k_avg
                          ) where k >= k_avg
                        ) where k >= k_avg
                      ) where k >= k_avg
                    ) where k >= k_avg
                  ) where k >= k_avg
                ) where k >= k_avg
              ) where k >= k_avg
            ) where k >= k_avg
          ) where k >= k_avg
        ) where k >= k_avg
      ) where k >= k_avg
    ) where k >= k_avg
  ) where k >= k_avg
) where k >= k_avg
)

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至少它为我的作品对30嵌套水平,然后用产生完全不同的执行计划WINDOW BUFFER和VIEW代替LOAD TABLE AS SELECT,SORT AGGREGATE和TABLE ACCESS FULL.

更新

刚刚安装了11.2.0.4(Win7/32bit)并根据初始查询进行测试.优化器行为没有任何改变.
即使使用inline(未记录的)或RULE(不推荐的)提示,也不可能直接影响CBO行为.可能是一些Guru知道一些变体,但它对我来说是一个绝密(也是谷歌:-).
如果主select语句分成一个部分并放入返回一组行的函数(函数返回sys_refcursor或强类型游标),则可以在合理的时间内在一个select语句中执行操作,但如果查询不是一个选择在运行时构造.
使用XML的解决方法是可行的,~~但这种变体看起来像是通过屁眼洞移除扁桃体~~(对不起):

.

select
  extractvalue(column_value,'/t/somevalue') abc
from 
  table(xmlsequence((
    select t2 from (
      select
        t0,
        t1,
        (   
          select xmlagg(
                   xmlelement("t", 
                     xmlelement("k1",extractvalue(t1t.column_value,'/t/k1')), 
                     xmlelement("somevalue", systimestamp))
                  )
          from 
            table(xmlsequence(t0)) t0t, 
            table(xmlsequence(t1)) t1t  
          where 
            extractvalue(t1t.column_value,'/t/k1') >= (
              select avg(extractvalue(t1t.column_value, '/t/k1')) from table(xmlsequence(t1))
            )                                              
            and 
            extractvalue(t0t.column_value,'/t/k2') > 6
        ) t2
      from (
        select
          t0,
          (
            select xmlagg(
                     xmlelement("t", 
                       xmlelement("k1",extractvalue(column_value,'/t/k1')), 
                       xmlelement("somevalue", sysdate))
                    )
            from table(xmlsequence(t0))   
            where 
              extractvalue(column_value,'/t/k1') >= (
                select avg(extractvalue(column_value, '/t/k1')) from table(xmlsequence(t0))
              )
          ) t1
        from (
          select
            xmlagg(xmlelement("t", xmlelement("k1", level), xmlelement("k2", level + 3))) t0
          from dual connect by level < 5
        )
      )
    )
  )))

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关于上面的奇怪代码的另一个问题是,此变体仅适用于with数据集没有大量行的情况.

Answer 2

Jon*_*ler 5

（这不是完整的答案。希望此处的信息可以帮助其他人提供更好的答案。）

Q1：优化器通过内联所有内容来重写查询。每个新的公共表表达式的内部语句的大小都会加倍，并且语句会迅速变得巨大。例如，T15 生成 3,162,172 个字符的查询。

跟踪语句的代码：

sqlplus user/pass@orcl

alter session set events '10053 trace name context forever, level 1';

with t0 as (select 0 as k from dual)
,t1 as (select k from t0 where k >= (select avg(k) from t0))
,t2 as (select k from t1 where k >= (select avg(k) from t1))
select k from t2;

exit;


sqlplus user/pass@orcl

alter session set events '10053 trace name context forever, level 1';

with t0 as (select 0 as k from dual)
,t1 as (select k from t0 where k >= (select avg(k) from t0))
,t2 as (select k from t1 where k >= (select avg(k) from t1))
,t3 as (select k from t2 where k >= (select avg(k) from t2))
select k from t3;
exit;

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如果比较两个跟踪文件，就会发现有很多差异，但大多数差异看起来很小。真正的区别仅在于字符串后面的一行：Stmt: ******* UNPARSED QUERY IS *******。如果跟踪较大的查询，请小心打开跟踪文件。并非所有编辑器都能处理如此长的行。T20 文件有 250MB！

格式化后来自第一条跟踪的 SQL：

SELECT "T1"."K" "K"
  FROM (SELECT 0 "K"
          FROM "SYS"."DUAL" "DUAL"
         WHERE 0 >= (SELECT AVG(0) "AVG(K)" FROM "SYS"."DUAL" "DUAL")) "T1"
 WHERE "T1"."K" >=
       (SELECT AVG("T1"."K") "AVG(K)"
          FROM (SELECT 0 "K"
                  FROM "SYS"."DUAL" "DUAL"
                 WHERE 0 >= (SELECT AVG(0) "AVG(K)" FROM "SYS"."DUAL" "DUAL")) "T1")

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格式化后来自第二个跟踪的 SQL：

SELECT "T2"."K" "K"
  FROM (SELECT "T1"."K" "K"
          FROM (SELECT 0 "K"
                  FROM "SYS"."DUAL" "DUAL"
                 WHERE 0 >= (SELECT AVG(0) "AVG(K)" FROM "SYS"."DUAL" "DUAL")) "T1"
         WHERE "T1"."K" >=
               (SELECT AVG("T1"."K") "AVG(K)"
                  FROM (SELECT 0 "K"
                          FROM "SYS"."DUAL" "DUAL"
                         WHERE 0 >=
                               (SELECT AVG(0) "AVG(K)" FROM "SYS"."DUAL" "DUAL")) "T1")) "T2"
 WHERE "T2"."K" >=
       (SELECT AVG("T2"."K") "AVG(K)"
          FROM (SELECT "T1"."K" "K"
                  FROM (SELECT 0 "K"
                          FROM "SYS"."DUAL" "DUAL"
                         WHERE 0 >=
                               (SELECT AVG(0) "AVG(K)" FROM "SYS"."DUAL" "DUAL")) "T1"
                 WHERE "T1"."K" >=
                       (SELECT AVG("T1"."K") "AVG(K)"
                          FROM (SELECT 0 "K"
                                  FROM "SYS"."DUAL" "DUAL"
                                 WHERE 0 >= (SELECT AVG(0) "AVG(K)"
                                               FROM "SYS"."DUAL" "DUAL")) "T1")) "T2")

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Q2：我不会说每个“复杂”公用表表达式都会以相同的方式运行。我见过更大的 CTE。似乎只有极端的嵌套才是问题所在。我在 Oracle 支持上找不到任何明显的错误。

ThinkJet 的代码看起来是一个很好的解决方法。嵌套内联视图比嵌套公用表表达式更常见。

Q3：可能有一个提示可以防止这种行为，但我不确定它是什么。希望通过显示查询的转换版本，其他人可以猜测如何修复它。

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