转载自老熊的三分地:http://www.laoxiong.net/category/oracle/oraperf
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：
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：
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）。
SQL> explain plan for
2 SELECT bss_org_id
3 FROM bss_org t
4 START WITH bss_org_id = 1
5 CONNECT BY bss_parent_org_id = PRIOR bss_org_id;
已解释。
SQL> @showplan
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5739 | 80346 | 16 (0)| 00:00:01 |
|* 1 | CONNECT BY NO FILTERING WITH START-WITH| | | | | |
| 2 | TABLE ACCESS FULL | BSS_ORG | 5739 | 80346 | 16 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("BSS_PARENT_ORG_ID"=PRIOR "BSS_ORG_ID")
filter("BSS_ORG_ID"=1)
SQL> explain plan for
2 SELECT bss_org_id
3 FROM bss_org t
4 START WITH bss_org_id = 832044754
5 CONNECT BY bss_parent_org_id = PRIOR bss_org_id;
已解释。
SQL> @showplan
---------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5739 | 80346 | 16 (0)| 00:00:01 |
|* 1 | CONNECT BY NO FILTERING WITH START-WITH| | | | | |
| 2 | TABLE ACCESS FULL | BSS_ORG | 5739 | 80346 | 16 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("BSS_PARENT_ORG_ID"=PRIOR "BSS_ORG_ID")
filter("BSS_ORG_ID"=832044754)
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。
SQL> create index bss_org_idx1 on bss_org(bss_parent_org_id);
索引已创建。
SQL> explain plan for
2 SELECT *
3 FROM bss_org t
4 START WITH bss_org_id = 1
5 CONNECT BY bss_parent_org_id = PRIOR bss_org_id;
已解释。
SQL> @showplan
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 6 | 318 | 4 (0)| 00:00:01 |
|* 1 | CONNECT BY WITH FILTERING | | | | | |
| 2 | TABLE ACCESS BY INDEX ROWID | BSS_ORG | 1 | 53 | 2 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | PK_BSS_ORG | 1 | | 1 (0)| 00:00:01 |
| 4 | NESTED LOOPS | | | | | |
| 5 | CONNECT BY PUMP | | | | | |
| 6 | TABLE ACCESS BY INDEX ROWID| BSS_ORG | 6 | 318 | 4 (0)| 00:00:01 |
|* 7 | INDEX RANGE SCAN | BSS_ORG_IDX1 | 6 | | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("BSS_PARENT_ORG_ID"=PRIOR "BSS_ORG_ID")
3 - access("BSS_ORG_ID"=1)
7 - access("BSS_PARENT_ORG_ID"=PRIOR "BSS_ORG_ID")
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 = PRIOR bss_org_id;
已解释。
SQL> @showplan
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 6 | 318 | 4 (0)| 00:00:01 |
|* 1 | CONNECT BY WITH FILTERING | | | | | |
| 2 | TABLE ACCESS BY INDEX ROWID | BSS_ORG | 1 | 53 | 2 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | PK_BSS_ORG | 1 | | 1 (0)| 00:00:01 |
| 4 | NESTED LOOPS | | | | | |
| 5 | CONNECT BY PUMP | | | | | |
| 6 | TABLE ACCESS BY INDEX ROWID| BSS_ORG | 6 | 318 | 4 (0)| 00:00:01 |
|* 7 | INDEX RANGE SCAN | BSS_ORG_IDX1 | 6 | | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("BSS_PARENT_ORG_ID"=PRIOR "BSS_ORG_ID")
3 - access("BSS_ORG_ID"=832044754)
7 - access("BSS_PARENT_ORG_ID"=PRIOR "BSS_ORG_ID")
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：
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。