让Oracle将OR连接的谓词转换为UNION ALL操作

Question

UNION在某些情况下,UNION ALL查询可以胜过使用OR-connected谓词的等效查询.据我所知,这部分是因为子UNION选择可以并行执行,因此它们可以有自己的特定于OR连接谓词的每个部分的"子计划" ,由于更简单的适用查询转换,这可能更加优化.

但是OR,即使将子查询因子应用于UNION ALL解决方案,写连接谓词通常也更具可读性和简洁性.我的问题是:有没有办法向Oracle表明,应该将单个昂贵的OR连接谓词转换为UNION ALL操作？如果有这样的提示/方法,在什么情况下可以应用它(例如,是否需要在谓词中涉及的列上出现任何约束等)？一个例子:

CREATE TABLE a AS
  SELECT 1 x, 2 y FROM DUAL UNION ALL
  SELECT 2 x, 1 y FROM DUAL;

-- This query...
SELECT * FROM a
WHERE x = 1 OR y = 1

-- Is sometimes outperformed by this one, for more complex table sources...
-- Note: in my case, I can safely apply UNION ALL. I know the two predicates to
-- be mutually exclusive.
SELECT * FROM a
WHERE x = 1
UNION ALL
SELECT * FROM a
WHERE y = 1

注意,我知道/*+ USE_CONCAT */提示:

SELECT /*+ USE_CONCAT */ * FROM a
WHERE x = 1 OR y = 1

但它似乎没有产生我需要的东西(UNION ALL执行计划中没有强制操作):

-------------------------------------------
| Id  | Operation         | Name | E-Rows |
-------------------------------------------
|   0 | SELECT STATEMENT  |      |        |
|*  1 |  TABLE ACCESS FULL| A    |      2 |
-------------------------------------------

也许,这个提示有一些限制？我有Oracle 11g2可用于此.

Answer 1

我相信这可能与您在谓词中使用的列上存在的索引有关OR。

我在 11gR2 中使用以下内容进行了测试。

create table scott.test as 
select level l, 
       decode(mod(level,2), 1, 1, 2) x, 
       decode(mod(level,2), 1, 2, 1) y, 
       dbms_random.value(1, 3) z from dual 
connect by level < 1000;
/

begin
   dbms_stats.gather_table_stats('scott', 'test');
end;
/

然后我在 TOAD 中解释了以下查询，( EXPLAIN PLAN FOR)

select x, y, z from scott.test
    where (floor(z) = 1 and x = 1) or (floor(z) = 2 and y = 1)
    ;

SELECT STATEMENT Optimizer Mode=ALL_ROWS        10          4                                
  TABLE ACCESS FULL COS_DM.TEST 10      280     4   

select /*+ USE_CONCAT */ x, y, z from scott.test
where (floor(z) = 1 and x = 1) or (floor(z) = 2 and y = 1)
;

SELECT STATEMENT Optimizer Mode=ALL_ROWS        10          4                                
  TABLE ACCESS FULL COS_DM.TEST 10      280     4                                


select x, y, z from test where (floor(z) = 1 and x = 1)
union all
select x, y, z from test where (floor(z) = 2 and y = 1)
;

SELECT STATEMENT Optimizer Mode=ALL_ROWS        10          8                                
  UNION-ALL                                              
    TABLE ACCESS FULL   COS_DM.TEST 5   140     4                                
    TABLE ACCESS FULL   COS_DM.TEST 5   140     4                                

所以看来提示不起作用。然后我向 x 和 y 列添加了一个索引：

create index test_x on test (x, y);

begin
   dbms_stats.gather_table_stats('scott', 'test');
end;
/

现在重新运行查询：

select x, y, z from scott.test
    where (floor(z) = 1 and x = 1) or (floor(z) = 2 and y = 1)
    ;

SELECT STATEMENT Optimizer Mode=ALL_ROWS        10          4                                
  TABLE ACCESS FULL COS_DM.TEST 10      280     4   

select /*+ USE_CONCAT */ x, y, z from scott.test
where (floor(z) = 1 and x = 1) or (floor(z) = 2 and y = 1)
;

SELECT STATEMENT Optimizer Mode=ALL_ROWS        10          8                                
  CONCATENATION                                              
    TABLE ACCESS FULL   COS_DM.TEST 5   140     4                                
    TABLE ACCESS FULL   COS_DM.TEST 5   140     4                                

select x, y, z from test where (floor(z) = 1 and x = 1)
union all
select x, y, z from test where (floor(z) = 2 and y = 1)
;

SELECT STATEMENT Optimizer Mode=ALL_ROWS        10          8                                
  UNION-ALL                                              
    TABLE ACCESS FULL   COS_DM.TEST 5   140     4                                
    TABLE ACCESS FULL   COS_DM.TEST 5   140     4                                

看起来在添加索引后（即使它没有被使用）优化器最终决定使用提示！

也许你可以试试这个？