Oracle 优化器无关的过滤谓词？
Oracle Optimizer Extraneous Filter Predicate?
为什么即使同一索引的访问谓词保证过滤谓词始终为真，Oracle 仍然在索引上应用过滤谓词？
drop table index_filter_child
;
drop table index_filter_parent
;
create table index_filter_parent
as
select level id, chr(mod(level - 1, 26) + ascii('A')) code from dual connect by level <= 26
;
create table index_filter_child
as
with
"C" as (select chr(mod(level - 1, 26) + ascii('A')) code from dual connect by level <= 26)
select rownum id, C1.code from C C1, C C2
;
exec dbms_stats.gather_table_stats('USER','INDEX_FILTER_PARENT')
;
exec dbms_stats.gather_table_stats('USER','INDEX_FILTER_CHILD')
;
create index ix_index_filter_parent on index_filter_parent(code)
;
create index ix_index_filter_child on index_filter_child(code)
;
select P.*
from index_filter_parent "P"
join index_filter_child "C"
on C.code = P.code
where P.code in('A','Z') --same result if we predicate instead on C.code in('A','Z')
;
--------------------------------------------------------------------------------------------------------------
| id | Operation | name | rows | Bytes | cost (%CPU)| time |
--------------------------------------------------------------------------------------------------------------
| 0 | select statement | | 5 | 35 | 4 (0)| 00:00:01 |
| 1 | nested LOOPS | | 5 | 35 | 4 (0)| 00:00:01 |
| 2 | INLIST ITERATOR | | | | | |
| 3 | table access by index ROWID| INDEX_FILTER_PARENT | 2 | 10 | 2 (0)| 00:00:01 |
|* 4 | index range scan | IX_INDEX_FILTER_PARENT | 2 | | 1 (0)| 00:00:01 |
|* 5 | index range scan | IX_INDEX_FILTER_CHILD | 2 | 4 | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("P"."CODE"='A' or "P"."CODE"='Z')
5 - access("C"."CODE"="P"."CODE")
filter("C"."CODE"='A' or "C"."CODE"='Z') <========== why is this needed?
鉴于 access("C"."CODE"="P"."CODE") 保证 C.code 是 'A' 或 'Z'，为什么需要 5 中的过滤谓词？
提前致谢。
Oracle 12.1企业版。
这是“传递闭包”转换的结果：您可以在此处阅读更多信息：
- Transitivity and Transitive Closure (Doc ID 68979.1) Doc id 68979.1
- Jonathan Lewis - Cartesian Merge Join
- Jonathan Lewis - Transitive Closure (or, even better, in his book "Cost Based Oracle Fundamentals")
如果您获得 CBO 跟踪（alter session set events '10053 trace name context forever, level 1' 或 alter session set events 'trace[SQL_Optimizer.*]），您将看到转换发生在选择连接方法和访问路径之前。它允许 CBO 分析更多不同的访问路径并选择最佳可用计划。此外，在自适应计划的情况下，它允许 oracle 动态更改连接方法。
例如，您可以获得这样的计划：
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 52 | 364 | 4 (0)| 00:00:01 |
|* 1 | HASH JOIN | | 52 | 364 | 4 (0)| 00:00:01 |
| 2 | INLIST ITERATOR | | | | | |
| 3 | TABLE ACCESS BY INDEX ROWID BATCHED| INDEX_FILTER_PARENT | 2 | 10 | 2 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IX_INDEX_FILTER_PARENT | 2 | | 1 (0)| 00:00:01 |
| 5 | INLIST ITERATOR | | | | | |
|* 6 | INDEX RANGE SCAN | IX_INDEX_FILTER_CHILD | 52 | 104 | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("C"."CODE"="P"."CODE")
4 - access("P"."CODE"='A' OR "P"."CODE"='Z')
6 - access("C"."CODE"='A' OR "C"."CODE"='Z')
事实上，您可以使用事件 10155: CBO disable generation of transitive OR-chains.
禁用它
你的例子：
alter session set events '10155';
explain plan for
select P.*
from index_filter_parent "P"
join index_filter_child "C"
on C.code = P.code
where P.code in('A','Z');
结果：
SQL> alter session set events '10155';
Session altered.
SQL> explain plan for
2 select P.*
3 from index_filter_parent "P"
4 join index_filter_child "C"
5 on C.code = P.code
6 where P.code in('A','Z') ;
Explained.
SQL> @xplan typical
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------
Plan hash value: 2543178509
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 52 | 364 | 4 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 52 | 364 | 4 (0)| 00:00:01 |
| 2 | INLIST ITERATOR | | | | | |
| 3 | TABLE ACCESS BY INDEX ROWID BATCHED| INDEX_FILTER_PARENT | 2 | 10 | 2 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IX_INDEX_FILTER_PARENT | 2 | | 1 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | IX_INDEX_FILTER_CHILD | 26 | 52 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("P"."CODE"='A' OR "P"."CODE"='Z')
5 - access("C"."CODE"="P"."CODE")
Note
-----
- this is an adaptive plan
22 rows selected.
如您所见，谓词已消失。
PS。传递谓词的其他事件：
- ORA-10155: CBO 禁止生成可传递的 OR 链
- ORA-10171: CBO 禁用传递连接谓词
- ORA-10179: CBO 关闭传递谓词替换
- ORA-10195: CBO 不对传递谓词使用检查约束
为什么即使同一索引的访问谓词保证过滤谓词始终为真，Oracle 仍然在索引上应用过滤谓词？
drop table index_filter_child
;
drop table index_filter_parent
;
create table index_filter_parent
as
select level id, chr(mod(level - 1, 26) + ascii('A')) code from dual connect by level <= 26
;
create table index_filter_child
as
with
"C" as (select chr(mod(level - 1, 26) + ascii('A')) code from dual connect by level <= 26)
select rownum id, C1.code from C C1, C C2
;
exec dbms_stats.gather_table_stats('USER','INDEX_FILTER_PARENT')
;
exec dbms_stats.gather_table_stats('USER','INDEX_FILTER_CHILD')
;
create index ix_index_filter_parent on index_filter_parent(code)
;
create index ix_index_filter_child on index_filter_child(code)
;
select P.*
from index_filter_parent "P"
join index_filter_child "C"
on C.code = P.code
where P.code in('A','Z') --same result if we predicate instead on C.code in('A','Z')
;
--------------------------------------------------------------------------------------------------------------
| id | Operation | name | rows | Bytes | cost (%CPU)| time |
--------------------------------------------------------------------------------------------------------------
| 0 | select statement | | 5 | 35 | 4 (0)| 00:00:01 |
| 1 | nested LOOPS | | 5 | 35 | 4 (0)| 00:00:01 |
| 2 | INLIST ITERATOR | | | | | |
| 3 | table access by index ROWID| INDEX_FILTER_PARENT | 2 | 10 | 2 (0)| 00:00:01 |
|* 4 | index range scan | IX_INDEX_FILTER_PARENT | 2 | | 1 (0)| 00:00:01 |
|* 5 | index range scan | IX_INDEX_FILTER_CHILD | 2 | 4 | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("P"."CODE"='A' or "P"."CODE"='Z')
5 - access("C"."CODE"="P"."CODE")
filter("C"."CODE"='A' or "C"."CODE"='Z') <========== why is this needed?
鉴于 access("C"."CODE"="P"."CODE") 保证 C.code 是 'A' 或 'Z'，为什么需要 5 中的过滤谓词？
提前致谢。
Oracle 12.1企业版。
这是“传递闭包”转换的结果：您可以在此处阅读更多信息：
- Transitivity and Transitive Closure (Doc ID 68979.1) Doc id 68979.1
- Jonathan Lewis - Cartesian Merge Join
- Jonathan Lewis - Transitive Closure (or, even better, in his book "Cost Based Oracle Fundamentals")
如果您获得 CBO 跟踪（alter session set events '10053 trace name context forever, level 1' 或 alter session set events 'trace[SQL_Optimizer.*]），您将看到转换发生在选择连接方法和访问路径之前。它允许 CBO 分析更多不同的访问路径并选择最佳可用计划。此外，在自适应计划的情况下，它允许 oracle 动态更改连接方法。
例如，您可以获得这样的计划：
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 52 | 364 | 4 (0)| 00:00:01 |
|* 1 | HASH JOIN | | 52 | 364 | 4 (0)| 00:00:01 |
| 2 | INLIST ITERATOR | | | | | |
| 3 | TABLE ACCESS BY INDEX ROWID BATCHED| INDEX_FILTER_PARENT | 2 | 10 | 2 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IX_INDEX_FILTER_PARENT | 2 | | 1 (0)| 00:00:01 |
| 5 | INLIST ITERATOR | | | | | |
|* 6 | INDEX RANGE SCAN | IX_INDEX_FILTER_CHILD | 52 | 104 | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("C"."CODE"="P"."CODE")
4 - access("P"."CODE"='A' OR "P"."CODE"='Z')
6 - access("C"."CODE"='A' OR "C"."CODE"='Z')
事实上，您可以使用事件 10155: CBO disable generation of transitive OR-chains.
你的例子：
alter session set events '10155';
explain plan for
select P.*
from index_filter_parent "P"
join index_filter_child "C"
on C.code = P.code
where P.code in('A','Z');
结果：
SQL> alter session set events '10155';
Session altered.
SQL> explain plan for
2 select P.*
3 from index_filter_parent "P"
4 join index_filter_child "C"
5 on C.code = P.code
6 where P.code in('A','Z') ;
Explained.
SQL> @xplan typical
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------
Plan hash value: 2543178509
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 52 | 364 | 4 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 52 | 364 | 4 (0)| 00:00:01 |
| 2 | INLIST ITERATOR | | | | | |
| 3 | TABLE ACCESS BY INDEX ROWID BATCHED| INDEX_FILTER_PARENT | 2 | 10 | 2 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IX_INDEX_FILTER_PARENT | 2 | | 1 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | IX_INDEX_FILTER_CHILD | 26 | 52 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("P"."CODE"='A' OR "P"."CODE"='Z')
5 - access("C"."CODE"="P"."CODE")
Note
-----
- this is an adaptive plan
22 rows selected.
如您所见，谓词已消失。
PS。传递谓词的其他事件：
- ORA-10155: CBO 禁止生成可传递的 OR 链
- ORA-10171: CBO 禁用传递连接谓词
- ORA-10179: CBO 关闭传递谓词替换
- ORA-10195: CBO 不对传递谓词使用检查约束