Oracle联接消除在联接两个以上的表时未按预期工作
联接两个表时，联接消除工作正常：Oracle联接消除在联接两个以上的表时未按预期工作,oracle,sqlperformance,Oracle,Sqlperformance,联接两个表时，联接消除工作正常： SQL> set lines 200; SQL> SQL> select * from v$version; BANNER CON_ID
SQL> set lines 200;
SQL>
SQL> select * from v$version;
BANNER CON_ID
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Oracle Database 12c Release 12.1.0.1.0 - 64bit Production 0
PL/SQL Release 12.1.0.1.0 - Production 0
CORE 12.1.0.1.0 Production 0
TNS for Linux: Version 12.1.0.1.0 - Production 0
NLSRTL Version 12.1.0.1.0 - Production 0
SQL>
SQL> create table t01 (
2 id integer,
3 apk varchar2(255 char),
4 constraint pk_01 primary key (id)
5 );
Tabelle wurde erstellt.
SQL> create table t02 (
2 id integer,
3 apk varchar2(255 char),
4 id_t01 integer,
5 constraint pk_02 primary key (id),
6 constraint fk_02 foreign key (id_t01) references t01(id)
7 );
Tabelle wurde erstellt.
SQL> create table t03 (
2 id integer,
3 apk varchar2(255 char),
4 id_t02 integer,
5 constraint pk_03 primary key (id),
6 constraint fk_03 foreign key (id_t02) references t02(id)
7 );
Tabelle wurde erstellt.
SQL> create index ix_t03 on t03(id_t02);
Index wurde erstellt.
SQL> create index ix_t02 on t02(id_t01);
Index wurde erstellt.
SQL> insert into t01 (id, apk)
2 select level, to_char(level)
3 from dual
4 connect by level <= 1000;
1000 Zeilen erstellt.
SQL> insert into t02(id, apk, id_t01)
2 select id, apk, id from t01;
1000 Zeilen erstellt.
SQL> insert into t03(id, apk, id_t02)
2 select id, apk, id from t01;
1000 Zeilen erstellt.
SQL> commit;
Transaktion mit COMMIT abgeschlossen.
SQL>
SQL> exec dbms_stats.gather_table_stats(null, 'T01', method_opt=>'for all columns size skewonly', cascade=>true);
PL/SQL-Prozedur erfolgreich abgeschlossen.
SQL> exec dbms_stats.gather_table_stats(null, 'T02', method_opt=>'for all columns size skewonly', cascade=>true);
PL/SQL-Prozedur erfolgreich abgeschlossen.
SQL> exec dbms_stats.gather_table_stats(null, 'T03', method_opt=>'for all columns size skewonly', cascade=>true);
PL/SQL-Prozedur erfolgreich abgeschlossen.
SQL> commit;
Transaktion mit COMMIT abgeschlossen.
SQL>
SQL> set autotrace traceonly explain;
SQL>
SQL> select t02.id
2 from t02
3 left join t01 on t01.id = t02.id_t01;
Ausführungsplan
----------------------------------------------------------
--------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
--------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 8000 | 2 |
| 1 | INDEX FAST FULL SCAN| PK_02 | 1000 | 8000 | 2 |
--------------------------------------------------------------
SQL> select t03.id
2 from t03
3 left join t02 on t02.id = t03.id_t02
4 left join t01 on t01.id = t02.id_t01;
Ausführungsplan
----------------------------------------------------------
------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 16000 | 18 |
| 1 | NESTED LOOPS OUTER| | 1000 | 16000 | 18 |
| 2 | TABLE ACCESS FULL| T03 | 1000 | 8000 | 18 |
| 3 | INDEX UNIQUE SCAN| PK_02 | 1 | 8 | 0 |
------------------------------------------------------------
select t03.id
from t03
left join t02 on t02.id = t03.id_t02
left join t01 on t01.id = t02.id_t01;
id_t02 t02_rowid
873 AAEiyFAAVAAA49bAAA
874 AAEiyFAAVAAA49bAAB
...
create index ix_t0302 on t03(id_t02, id);
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 160 | 2 (0)| 00:00:01 |
| 1 | NESTED LOOPS OUTER| | 10 | 160 | 2 (0)| 00:00:01 |
| 2 | INDEX FULL SCAN | IX_T0302 | 10 | 80 | 1 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN| PK_02 | 1 | 8 | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------
*编辑以纠正我自己优化器必须认为查询计划比备选方案执行得更有效。你确定这是个问题吗？如果是，您是否尝试过使用查询提示，例如没有嵌套循环等？pk_03上的快速完全扫描成本为2（请参阅t02和t03之间的连接），t01、t02和t03之间的连接的执行计划成本为18。这正是我不明白的，为什么不选择成本较低=2的计划？我的生产数据库中的实际查询被表示为一个视图，该视图被许多其他查询使用；虽然我可以尝试调整这个特定的示例，但我不可能对所有可能的查询组合使用查询提示。您的代码在版本12.1.0.2中工作正常。我建议您检查support.oracle.com。@Matthew McPeak看起来这个问题实际上与oracle版本有关：它在oracle 12.2.0.1.0和11.2.0.4.0中正常工作-尚未检查12.1.0.2。非常感谢。
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 160 | 2 (0)| 00:00:01 |
| 1 | NESTED LOOPS OUTER| | 10 | 160 | 2 (0)| 00:00:01 |
| 2 | INDEX FULL SCAN | IX_T0302 | 10 | 80 | 1 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN| PK_02 | 1 | 8 | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------