Join Elimination

1.定义

The purpose of join elimination is to remove redundant joins—in other words, to completely avoid executing a join even if a SQL statement explicitly calls for it. The key information used by the query optimizer to decide whether it’s sensible to implement this query transformation is the availability of a foreign key that‘s either enforced or marked RELY. In addition, as of version 11.2, self-joins based on the primary key are also considered. This heuristic-based query transformation is especially useful when views containing joins are used. Note, however, that join elimination can also be applied to SQL statements without views.  

简单说就是消除不必要的连接,详细使用规则见分类。

2.参数

KSPPINM                                  KSPPSTVL   KSPPDESC
---------------------------------------- ---------- ------------------------------------------------------------
_optimizer_join_elimination_enabled         TRUE     optimizer join elimination enabled

hint：eliminate_join/no_eliminate_join

3.分类

1）主外键消除连接操作

• 关联条件为主外键字段
• 查询列只有子表

2）自连接消除连接操作

• 连接条件为主键或唯一索引字段

3）外连接消除连接操作

• 查询列所有字段都来自外连接驱动表
• 连接字段为被消除表上的主键或者唯一索引字段

4）连接位图索引消除连接操作

• 连接条件和连接位图索引的连接条件一致
• 查询列只有建立连接位图索引的表的字段
• 谓词条件有连接位图索引字段

4.测试

这里简单的测试第3种分类：外连接消除连接操作

sql文本：

 create unique index i_1 on lxy_1(object_id);

 select t.* from lxy t,lxy_1 t1 where t.object_id=t1.object_id(+);

执行计划：

Plan hash value: 1583004392
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      | 14243 |  1237K|    52   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| LXY  | 14243 |  1237K|    52   (0)| 00:00:01 |
--------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$FFB75F5A / T@SEL$1
Outline Data
-------------
  /*+
      BEGIN_OUTLINE_DATA
      FULL(@"SEL$FFB75F5A" "T"@"SEL$1")
      OUTLINE(@"SEL$1")
      ELIMINATE_JOIN(@"SEL$1" "T1"@"SEL$1")
      OUTLINE_LEAF(@"SEL$FFB75F5A")
      ALL_ROWS
      DB_VERSION('11.2.0.4')
      OPTIMIZER_FEATURES_ENABLE('11.2.0.4')
      IGNORE_OPTIM_EMBEDDED_HINTS
      END_OUTLINE_DATA
  */
可以看到查询块SEL$1的T1表被消除了，从执行计划也没有看到T1的相关对象出现，可以从10053中看到转换过程

10053：

JE:   Considering Join Elimination on query block SEL$1 (#0)
*************************
Join Elimination (JE)    
*************************
join elimination之前：
Query block (0x89b1dde0) before join elimination:
SQL:******* UNPARSED QUERY IS *******
SELECT "T"."OWNER" "OWNER","T"."OBJECT_NAME" "OBJECT_NAME","T"."SUBOBJECT_NAME" "SUBOBJECT_NAME","T"."OBJECT_ID" "OBJECT_ID","T"."DATA_OBJECT_ID" "DATA_OBJECT_ID","T"."OBJECT_TYPE" "OBJECT_TYPE","T"."CREATED" "CREATED","T"."LAST_DDL_TIME" "LAST_DDL_TIME","T"."TIMESTAMP" "TIMESTAMP","T"."STATUS" "STATUS","T"."TEMPORARY" "TEMPORARY","T"."GENERATED" "GENERATED","T"."SECONDARY" "SECONDARY","T"."NAMESPACE" "NAMESPACE","T"."EDITION_NAME" "EDITION_NAME" FROM "SYS"."LXY" "T","SYS"."LXY_1" "T1" WHERE "T"."OBJECT_ID"="T1"."OBJECT_ID"(+)

表移除：
JE:   eliminate table: LXY_1 (T1)
Registered qb: SEL$FFB75F5A 0x89b1dde0 (JOIN REMOVED FROM QUERY BLOCK SEL$1; SEL$1; "T1"@"SEL$1")

新查询块注册：

---------------------
QUERY BLOCK SIGNATURE
---------------------
  signature (): qb_name=SEL$FFB75F5A nbfros=1 flg=0
    fro(0): flg=0 objn=14727 hint_alias="T"@"SEL$1"

SQL:******* UNPARSED QUERY IS *******
SELECT "T"."OWNER" "OWNER","T"."OBJECT_NAME" "OBJECT_NAME","T"."SUBOBJECT_NAME" "SUBOBJECT_NAME","T"."OBJECT_ID" "OBJECT_ID","T"."DATA_OBJECT_ID" "DATA_OBJECT_ID","T"."OBJECT_TYPE" "OBJECT_TYPE","T"."CREATED" "CREATED","T"."LAST_DDL_TIME" "LAST_DDL_TIME","T"."TIMESTAMP" "TIMESTAMP","T"."STATUS" "STATUS","T"."TEMPORARY" "TEMPORARY","T"."GENERATED" "GENERATED","T"."SECONDARY" "SECONDARY","T"."NAMESPACE" "NAMESPACE","T"."EDITION_NAME" "EDITION_NAME" FROM "SYS"."LXY" "T" 
Query block SEL$FFB75F5A (#0) simplified
CVM: Considering view merge in query block SEL$FFB75F5A (#0)
OJE: Begin: find best directive for query block SEL$FFB75F5A (#0)
OJE: Considering outer-join elimination on query block SEL$FFB75F5A (#0)  --这里明确指出了是属于outer-join elimination
OJE:   outer-join not eliminated
OJE: End: finding best directive for query block SEL$FFB75F5A (#0)
query block SEL$1 transformed to SEL$FFB75F5A (#0)  --查询块从SEL$1转换成了 SEL$FFB75F5A

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