讓Oracle將OR連接謂詞轉換爲UNION ALL操作

Question

UNION和UNION ALL在某些情況下，使用OR連接的謂詞可以使查詢性能優於等效查詢。據我所知，這部分是因爲UNION子查詢可以並行執行，因此他們可以擁有與連接謂詞的每個部分相關的自己的「子計劃」，由於更簡單的適用查詢轉換，該子查詢可能更加優化。

但是，編寫OR - 即使將子查詢因子應用於UNION ALL解決方案，連接謂詞通常也更具可讀性和簡潔性。我的問題是：是否有辦法向Oracle表明，單個昂貴的OR謂詞應該轉換爲UNION ALL操作？如果有這樣的提示/方法，在什麼情況下可以應用（例如，謂詞所涉及的列中是否存在任何約束）？舉個例子：

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，我知道了/*+ USE_CONCAT */暗示：

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

但它似乎並沒有產生我需要什麼（在執行計劃中沒有強制UNION ALL操作）：

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

也許，這個提示有一些限制嗎？我有Oracle 11g2可用於此。

Answer 1

我認爲這可能與您在OR謂詞中使用的列上存在的索引有關。

我在11gR2中使用以下測試。

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; 
/

我再解釋蟾蜍以下查詢，（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         

所以它似乎暗示不工作。然後我添加一個索引到X & Y列：

create index test_x on test (x, y); 

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

現在重新運行查詢：

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         

看來，添加索引（即使它不使用）優化後畢竟決定使用提示！

也許你可以試試這個？