[20170816]Join Elimination Bug.txt
https://jonathanlewis.wordpress.com/2017/08/14/join-elimination-bug/
--//自己重复测试1次.
1.环境:
SCOTT@book> @ &r/ver1
PORT_STRING VERSION BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
rem Script: join_eliminate_bug_2.sql
rem Author: Jonathan Lewis
rem Dated: Dec 2012
drop table child purge;
drop table parent purge;
create table parent (
id number(4),
name varchar2(10),
constraint par_pk primary key (id)
deferrable initially immediate
)
;
create table child(
id_p number(4)
constraint chi_fk_par
references parent,
id number(4),
name varchar2(10),
constraint chi_pk primary key (id_p, id)
)
;
insert into parent values (1,'Smith');
insert into parent values (2,'Jones');
insert into child values(1,1,'Simon');
insert into child values(1,2,'Sally');
insert into child values(2,1,'Jack');
insert into child values(2,2,'Jill');
commit;
begin
dbms_stats.gather_table_stats(user,'child');
dbms_stats.gather_table_stats(user,'parent');
end;
/
set serveroutput off
select
chi.*
from
child chi,
parent par
where
par.id = chi.id_p
;
ID_P ID NAME
---------- ---------- --------------------
1 1 Simon
1 2 Sally
2 1 Jack
2 2 Jill
select * from table(dbms_xplan.display_cursor);
SCOTT@book> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID bf70xtmkt5wxv, child number 0
-------------------------------------
select chi.* from child chi, parent par
where par.id = chi.id_p
Plan hash value: 2406669797
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 (100)| |
| 1 | TABLE ACCESS FULL| CHILD | 4 | 48 | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------
14 rows selected.
--//可以发现取消了连接查询,仅仅查询child就知道结果,但是如果执行如下,延迟约束,问题就再现了.
On a single column join, with referential integrity in place, and no columns other than the primary key involved, the
optimizer eliminates table parent from the query. But if I now defer the primary key constraint on parent and duplicate
every row (which ought to duplicate the query result), watch what happens with the query:
set constraint par_pk deferred;
insert into parent (id,name) values (1,'Smith');
insert into parent (id,name) values (2,'Jones');
alter system flush shared_pool;
select
chi.*
from
child chi,
parent par
where
par.id = chi.id_p
;
ID_P ID NAME
---------- ---------- --------------------
1 1 Simon
1 2 Sally
2 1 Jack
2 2 Jill
SCOTT@book> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID bf70xtmkt5wxv, child number 0
-------------------------------------
select chi.* from child chi, parent par
where par.id = chi.id_p
Plan hash value: 2406669797
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 (100)| |
| 1 | TABLE ACCESS FULL| CHILD | 4 | 48 | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------
14 rows selected.
--//在这样的情况下存在错误,加入提示no_eliminate_join就能避免这个问题.
select /*+ no_eliminate_join(@sel$1 par@sel$1) */
chi.*
from
child chi,
parent par
where
par.id = chi.id_p
;
ID_P ID NAME
---------- ---------- --------------------
1 1 Simon
1 1 Simon
1 2 Sally
1 2 Sally
2 1 Jack
2 1 Jack
2 2 Jill
2 2 Jill
8 rows selected.
SCOTT@book> @ &r/dpc '' outline
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID g7udbfpxa8wxw, child number 0
-------------------------------------
select /*+ no_eliminate_join(@sel$1 par@sel$1) */ chi.* from
child chi, parent par where par.id = chi.id_p
Plan hash value: 1687613841
------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 (100)| |
| 1 | NESTED LOOPS | | 4 | 60 | 3 (0)| 00:00:01 |
| 2 | TABLE ACCESS FULL| CHILD | 4 | 48 | 3 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | PAR_PK | 1 | 3 | 0 (0)| |
------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$1 / CHI@SEL$1
3 - SEL$1 / PAR@SEL$1
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('11.2.0.4')
DB_VERSION('11.2.0.4')
ALL_ROWS
OUTLINE_LEAF(@"SEL$1")
FULL(@"SEL$1" "CHI"@"SEL$1")
INDEX(@"SEL$1" "PAR"@"SEL$1" ("PARENT"."ID"))
LEADING(@"SEL$1" "CHI"@"SEL$1" "PAR"@"SEL$1")
USE_NL(@"SEL$1" "PAR"@"SEL$1")
END_OUTLINE_DATA
*/
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("PAR"."ID"="CHI"."ID_P")
--//当然这样提交会报错的,破坏了唯一性约束.
SCOTT@book> commit ;
commit
*
ERROR at line 1:
ORA-02091: transaction rolled back
ORA-00001: unique constraint (SCOTT.PAR_PK) violated
