使用Oracle树形查询应注意


Oracle提供了方便的树形查询功能,也就是connect by 语句。树形结构是很常见的,比如组织机构树,产品目录树等。本文不讲述connect by 如何使用,只是提出在使用树形查询时需要考虑的一个有关于性能方面的问题。

  这里提到的问题,主要是Oracle优化器在评估connect by 语句的cardinality时,存在的缺陷,下面将举例说明。

  在这个例子中所使用的表,是一个真实的生产系统中的表,BSS_ORG: 


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-->SQL> desc bss_org

  名称 是否为空? 类型

  ---------------------------- -------- --------------

  BSS_ORG_ID NOT NULL NUMBER(9)

  NAME NOT NULL VARCHAR2(64)

  BSS_PARENT_ORG_ID NUMBER(9)

  BSS_ORG_LEVEL_ID NOT NULL NUMBER(3)

  STATE NOT NULL VARCHAR2(3)

  STATE_DATE DATE

  BSS_ORG_CODE VARCHAR2(15)
  在这个BSS_ORG表中,BSS_ORG_ID是主键,BSS_PARENT_ORG_ID与BSS_ORG_ID形成上下层级关系。这个表的统计信息如下: 


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-->Table Number Empty Average Chain Average Global

  Name of Rows Blocks Blocks Space Count Row Len Stats

  --------------- -------------- -------- ------------ ------- -------- ------- ------

  BSS_ORG 5,739 52 0 0 0 53 YES

  Column Column Distinct Number

  Name Details Values Density Buckets

  ------------------------- ------------------------ ------------ --------- ------- --

  BSS_ORG_ID NUMBER(9,0) NOT NULL 5,739 .000174 1

  NAME VARCHAR2(64) NOT NULL 5,034 .000361 200

  BSS_PARENT_ORG_ID NUMBER(9,0) 905 .002189 200

  BSS_ORG_LEVEL_ID NUMBER(3,0) NOT NULL 6 .000087 6

  STATE VARCHAR2(3) NOT NULL 2 .000087 2

  STATE_DATE DATE 1,624 .001434 200

  BSS_ORG_CODE VARCHAR2(15) 5,639 .000179 200

  Table Number Empty Average Chain Average Global

  Name of Rows Blocks Blocks Space Count Row Len Stats

  --------------- -------------- -------- ------------ ------- -------- ------- ------

  BSS_ORG 5,739 52 0 0 0 53 YES

  Column Column Distinct Number

  Name Details Values Density Buckets

  ------------------------- ------------------------ ------------ --------- ------- --

  BSS_ORG_ID NUMBER(9,0) NOT NULL 5,739 .000174 1

  NAME VARCHAR2(64) NOT NULL 5,034 .000361 200

  BSS_PARENT_ORG_ID NUMBER(9,0) 905 .002189 200

  BSS_ORG_LEVEL_ID NUMBER(3,0) NOT NULL 6 .000087 6

  STATE VARCHAR2(3) NOT NULL 2 .000087 2

  STATE_DATE DATE 1,624 .001434 200

  BSS_ORG_CODE VARCHAR2(15) 5,639 .000179 200
  下面的测试,是在Oracle 11.1.0.6 for Windows版本下进行的测试。在Oracle 9i、Oracle 10g下测试的结果与Oracle 11g下测试的结果是相符的。当然这里谈到的问题是cardinality,因此在三个版本下,SQL的执行计划可能有所不同,但最终的结论是一致的。(BTW:从10g开始,connect by语句有一个新的执行步骤,称为CONNECT BY NO FILTERING,对应的Hint是no_connect_by_filtering)。


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-->Table Number Empty Average Chain Average Global

  Name of Rows Blocks Blocks Space Count Row Len Stats

  --------------- -------------- -------- ------------ ------- -------- ------- ------

  BSS_ORG 5,739 52 0 0 0 53 YES

  Column Column Distinct Number

  Name Details Values Density Buckets

  ------------------------- ------------------------ ------------ --------- ------- --

  BSS_ORG_ID NUMBER(9,0) NOT NULL 5,739 .000174 1

  NAME VARCHAR2(64) NOT NULL 5,034 .000361 200

  BSS_PARENT_ORG_ID NUMBER(9,0) 905 .002189 200

  BSS_ORG_LEVEL_ID NUMBER(3,0) NOT NULL 6 .000087 6

  STATE VARCHAR2(3) NOT NULL 2 .000087 2

  STATE_DATE DATE 1,624 .001434 200

  BSS_ORG_CODE VARCHAR2(15) 5,639 .000179 200

  Table Number Empty Average Chain Average Global

  Name of Rows Blocks Blocks Space Count Row Len Stats

  --------------- -------------- -------- ------------ ------- -------- ------- ------

  BSS_ORG 5,739 52 0 0 0 53 YES

  Column Column Distinct Number

  Name Details Values Density Buckets

  ------------------------- ------------------------ ------------ --------- ------- --

  BSS_ORG_ID NUMBER(9,0) NOT NULL 5,739 .000174 1

  NAME VARCHAR2(64) NOT NULL 5,034 .000361 200

  BSS_PARENT_ORG_ID NUMBER(9,0) 905 .002189 200

  BSS_ORG_LEVEL_ID NUMBER(3,0) NOT NULL 6 .000087 6

  STATE VARCHAR2(3) NOT NULL 2 .000087 2

  STATE_DATE DATE 1,624 .001434 200

  BSS_ORG_CODE VARCHAR2(15) 5,639 .000179 200
  上面的2条SQL,第1条实际应该返回的行数为5739,第2条SQL实际应该返回的行数为4,但是从执行计划上看,Oracle优化器评估的行数均为5739。


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-->Table Number Empty Average Chain Average Global

  Name of Rows Blocks Blocks Space Count Row Len Stats

  --------------- -------------- -------- ------------ ------- -------- ------- ------

  BSS_ORG 5,739 52 0 0 0 53 YES

  Column Column Distinct Number

  Name Details Values Density Buckets

  ------------------------- ------------------------ ------------ --------- ------- --

  BSS_ORG_ID NUMBER(9,0) NOT NULL 5,739 .000174 1

  NAME VARCHAR2(64) NOT NULL 5,034 .000361 200

  BSS_PARENT_ORG_ID NUMBER(9,0) 905 .002189 200

  BSS_ORG_LEVEL_ID NUMBER(3,0) NOT NULL 6 .000087 6

  STATE VARCHAR2(3) NOT NULL 2 .000087 2

  STATE_DATE DATE 1,624 .001434 200

  BSS_ORG_CODE VARCHAR2(15) 5,639 .000179 200

  Table Number Empty Average Chain Average Global

  Name of Rows Blocks Blocks Space Count Row Len Stats

  --------------- -------------- -------- ------------ ------- -------- ------- ------

  BSS_ORG 5,739 52 0 0 0 53 YES

  Column Column Distinct Number

  Name Details Values Density Buckets

  ------------------------- ------------------------ ------------ --------- ------- --

  BSS_ORG_ID NUMBER(9,0) NOT NULL 5,739 .000174 1

  NAME VARCHAR2(64) NOT NULL 5,034 .000361 200

  BSS_PARENT_ORG_ID NUMBER(9,0) 905 .002189 200

  BSS_ORG_LEVEL_ID NUMBER(3,0) NOT NULL 6 .000087 6

  STATE VARCHAR2(3) NOT NULL 2 .000087 2

  STATE_DATE DATE 1,624 .001434 200

  BSS_ORG_CODE VARCHAR2(15) 5,639 .000179 200
  这里在BSS_PARENT_ORG_ID列上建一个索引,是为了使执行计划与9i、10g下的一致。

  这2条SQL返回的结果行数,与前面的2条SQL一样,分别是5739和4,但是从执行计划上看,Oracle优化器评估出来的行数都是6。

  从前面的两个测试来看,优化器评估出来的SQL返回的行数要么是5739(表BSS_ORG的总行数),要么是6(总行数/BSS_PARENT_ORG_ID的Disctint Values)。但无论如何,随着不同的start with条件,这个行数(cardinality)与实际返回的结果行数可能会存在非常大的差异。如果仅仅是测试中这样一个简单的SQL,实际上不会有什么问题,很容易出现问题的地方在于,一个复杂的SQL中,有类似于测试SQL这样的子查询,这样使得表连接的评估出现很大的偏差,这样容易引起非常大的性能问题。

  在9i下,如果BSS_PARENT_ORG_ID上如果没有索引,那么最后一个测试SQL的执行计划如下:


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--> SQL> explain plan for

  2 SELECT *

  3 FROM bss_org t

  4 START WITH bss_org_id = 832044754

  5 CONNECT BY bss_parent_org_id =

  6 PRIOR bss_org_id

  7 ;

  已解释。

  SQL> @showplan

  ----------------------------------------------------------------------------

  | Id | Operation | Name | Rows | Bytes | Cost |

  ----------------------------------------------------------------------------

  | 0 | SELECT STATEMENT | | 5739 | 297K| 9 |

  |* 1 | CONNECT BY WITH FILTERING | | | | |

  | 2 | TABLE ACCESS BY INDEX ROWID| BSS_ORG | | | |

  |* 3 | INDEX UNIQUE SCAN | PK_BSS_ORG | 1 | 7 | 1 |

  | 4 | HASH JOIN | | | | |

  | 5 | CONNECT BY PUMP | | | | |

  | 6 | TABLE ACCESS FULL | BSS_ORG | 5739 | 297K| 9 |

  | 7 | TABLE ACCESS FULL | BSS_ORG | 5739 | 297K| 9 |

  ----------------------------------------------------------------------------

  Predicate Information (identified by operation id):

  ---------------------------------------------------

  1 - filter("T"."BSS_ORG_ID"=832044754)

  3 - access("T"."BSS_ORG_ID"=832044754)

  SQL> explain plan for

  2 SELECT *

  3 FROM bss_org t

  4 START WITH bss_org_id = 832044754

  5 CONNECT BY bss_parent_org_id =

  6 PRIOR bss_org_id

  7 ;

  已解释。

  SQL> @showplan

  ----------------------------------------------------------------------------

  | Id | Operation | Name | Rows | Bytes | Cost |

  ----------------------------------------------------------------------------

  | 0 | SELECT STATEMENT | | 5739 | 297K| 9 |

  |* 1 | CONNECT BY WITH FILTERING | | | | |

  | 2 | TABLE ACCESS BY INDEX ROWID| BSS_ORG | | | |

  |* 3 | INDEX UNIQUE SCAN | PK_BSS_ORG | 1 | 7 | 1 |

  | 4 | HASH JOIN | | | | |

  | 5 | CONNECT BY PUMP | | | | |

  | 6 | TABLE ACCESS FULL | BSS_ORG | 5739 | 297K| 9 |

  | 7 | TABLE ACCESS FULL | BSS_ORG | 5739 | 297K| 9 |

  ----------------------------------------------------------------------------

  Predicate Information (identified by operation id):

  ---------------------------------------------------

  1 - filter("T"."BSS_ORG_ID"=832044754)

  3 - access("T"."BSS_ORG_ID"=832044754)
  这里9i的优化器评估出来的cardinality为5739,而11g与此同样的执行计划,评估的cardinality是6。

  前段时间就遇上由于connect by语句引起的性能问题。数据库为Oracle 9208,开始由于bss_org表的bss_parent_org_id列上没有索引,导致connect by部分得到的cardinality为5739,结果SQL性能非常差,在bss_parent_org_id上建索引后,执行计划改变,connect by 部分得到的cardinality为6,SQL性能大幅提升。

  对于Oracle优化器不能准确评估connect by 语句的cardinality,目前没有比较好的解决办法。必要的时候只有考虑使用Hint了


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