【YashanDB知识库】filter or改写问题

问题现象

当filter中出现or的时候，会导致filter无法走索引或者走hash join，就需要进行改写，例如：

create table test_tab1(col1 int, col2 int, col3 int);
create table test_tab2(col4 int, col5 int, col6 int);

begin
for i in 1..10000 loop
  insert into test_tab1 values (i,i+1,i+2);
  insert into test_tab2 values (i,i+1,i+2);
end
 loop;
commit;
end;
/

exec DBMS_STATS.GATHER_TABLE_STATS('SYS', 'test_tab1', null, 1, FALSE, 'FOR ALL COLUMNS SIZE AUTO', 4, 'AUTO', TRUE);
exec DBMS_STATS.GATHER_TABLE_STATS('SYS', 'test_tab2', null, 1, FALSE, 'FOR ALL COLUMNS SIZE AUTO', 4, 'AUTO', TRUE);

select * from test_tab1, test_tab2 where col1=col4 or col2=col5;

select * from test_tab1, test_tab2 where col1=col4
union all
select * from test_tab1, test_tab2 where col1<>col4 and col2=col5;

create unique index idx1 on test_tab1(col1);
create unique index idx2 on test_tab1(col2);

exec DBMS_STATS.GATHER_TABLE_STATS('SYS', 'test_tab1', null, 1, FALSE, 'FOR ALL COLUMNS SIZE AUTO', 4, 'AUTO', TRUE);
exec DBMS_STATS.GATHER_TABLE_STATS('SYS', 'test_tab2', null, 1, FALSE, 'FOR ALL COLUMNS SIZE AUTO', 4, 'AUTO', TRUE);

select * from test_tab1 where col1=5 or col2=5;

select * from test_tab1 where col1=5
union all
select * from test_tab1 where col1<>5 and col2=5;

执行计划：

SQL> explain select * from test_tab1, test_tab2 where col1=col4 or col2=col5;

PLAN_DESCRIPTION
----------------------------------------------------------------
SQL hash value: 1783492608
Optimizer: ADOPT_C

+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| Id | Operation type | Name | Owner | Rows | Cost(%CPU) | Partition info |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| 0 | SELECT STATEMENT | | | | | |
|* 1 | NESTED LOOPS INNER | | | 15003| 16082( 0)| |
| 2 | TABLE ACCESS FULL | TEST_TAB1 | SYS | 10000| 24( 0)| |
| 3 | TABLE ACCESS FULL | TEST_TAB2 | SYS | 10000| 24( 0)| |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+

Operation Information (identified by operation id):
---------------------------------------------------

   1 - Predicate : filter("TEST_TAB1"."COL2" = "TEST_TAB2"."COL5" OR "TEST_TAB1"."COL1" = "TEST_TAB2"."COL4")

16 rows fetched.

SQL> explain select * from test_tab1, test_tab2 where col1=col4
union all
   3 select * from test_tab1, test_tab2 where col1<>col4 and col2=col5;

PLAN_DESCRIPTION
----------------------------------------------------------------
SQL hash value: 2495885085
Optimizer: ADOPT_C

+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| Id | Operation type | Name | Owner | Rows | Cost(%CPU) | Partition info |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| 0 | SELECT STATEMENT | | | | | |
| 1 | VIEW | | | 20000| 111( 0)| |
| 2 | UNION ALL | | | 20000| 111( 0)| |
|* 3 | HASH JOIN INNER | | | 10000| 54( 0)| |
| 4 | TABLE ACCESS FULL | TEST_TAB1 | SYS | 10000| 24( 0)| |
| 5 | TABLE ACCESS FULL | TEST_TAB2 | SYS | 10000| 24( 0)| |
|* 6 | HASH JOIN INNER | | | 10000| 54( 0)| |
| 7 | TABLE ACCESS FULL | TEST_TAB1 | SYS | 10000| 24( 0)| |
| 8 | TABLE ACCESS FULL | TEST_TAB2 | SYS | 10000| 24( 0)| |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+

Operation Information (identified by operation id):
---------------------------------------------------

   3 - Predicate : access("TEST_TAB1"."COL1" = "TEST_TAB2"."COL4")
   6 - Predicate : access("TEST_TAB1"."COL2" = "TEST_TAB2"."COL5")
                   filter("TEST_TAB1"."COL1" <> "TEST_TAB2"."COL4")

23 rows fetched.

SQL> explain select * from test_tab1 where col1=5 or col2=5;

PLAN_DESCRIPTION
----------------------------------------------------------------
SQL hash value: 3573759094
Optimizer: ADOPT_C

+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| Id | Operation type | Name | Owner | Rows | Cost(%CPU) | Partition info |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| 0 | SELECT STATEMENT | | | | | |
|* 1 | TABLE ACCESS FULL | TEST_TAB1 | SYS | 2| 25( 0)| |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+

Operation Information (identified by operation id):
---------------------------------------------------

   1 - Predicate : filter("TEST_TAB1"."COL1" = 5 OR "TEST_TAB1"."COL2" = 5)

14 rows fetched.

SQL> explain select * from test_tab1 where col1=5
union all
   3 select * from test_tab1 where col1<>5 and col2=5;

PLAN_DESCRIPTION
----------------------------------------------------------------
SQL hash value: 2073754267
Optimizer: ADOPT_C

+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| Id | Operation type | Name | Owner | Rows | Cost(%CPU) | Partition info |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| 0 | SELECT STATEMENT | | | | | |
| 1 | VIEW | | | 2| 1( 0)| |
| 2 | UNION ALL | | | 2| 1( 0)| |
| 3 | TABLE ACCESS BY INDEX ROWID | TEST_TAB1 | SYS | 1| 1( 0)| |
|* 4 | INDEX UNIQUE SCAN | IDX1 | SYS | 1| 1( 0)| |
|* 5 | TABLE ACCESS BY INDEX ROWID | TEST_TAB1 | SYS | 1| 1( 0)| |
|* 6 | INDEX UNIQUE SCAN | IDX2 | SYS | 1| 1( 0)| |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+

Operation Information (identified by operation id):
---------------------------------------------------

   4 - Predicate : access("TEST_TAB1"."COL1" = 5)
   5 - Predicate : filter("TEST_TAB1"."COL1" <> 5)
   6 - Predicate : access("TEST_TAB1"."COL2" = 5)

21 rows fetched.

执行时间：

SQL> select * from test_tab1, test_tab2 where col1=col4 or col2=col5;

...

10000 rows fetched.

Elapsed: 00:00:45.263

SQL> select * from test_tab1, test_tab2 where col1=col4
union all
   3 select * from test_tab1, test_tab2 where col1<>col4 and col2=col5;

...

10000 rows fetched.

Elapsed: 00:00:00.447

SQL> select * from test_tab1 where col1=5 or col2=5;

        COL1 COL2 COL3
------------ ------------ ------------
           4 5 6
           5 6 7

2 rows fetched.

Elapsed: 00:00:00.004

SQL> select * from test_tab1 where col1=5
union all
   2 3 select * from test_tab1 where col1<>5 and col2=5;

        COL1 COL2 COL3
------------ ------------ ------------
           5 6 7
           4 5 6

2 rows fetched.

Elapsed: 00:00:00.000

问题的风险及影响

性能会大幅下降。

问题影响的版本

截止2024年4月份，最新版本依然有这个问题。

问题发生原因

问题单：优化器支持or改为了集合操作（CONCATENATION）需求没有实现。

解决方法以及规避方法

理论上有两种解决方案，一种是使用or索引，另一种是改写为集合操作，改写为集合操作的覆盖面会更大一些。

问题分析以及处理过程

1、可以通过观察oracle的执行计划，是否出现CONCATENATION这个算子，如果出现说明计划被改写为集合操作；

2、观察filter中是否有or导致了无法走hash join或者阻碍了走索引计划。

经验总结

执行计划并不是建了索引就可以选上索引的，需要将条件改为可以走索引才行。