通過刪除'存在'和'不存在'來優化Oracle查詢

Question

我最近在oracle數據庫中移動了一段代碼，其中一位經驗豐富的開發人員提到我有太多的exists和not exists陳述和應該有一種方法可以將它們刪除，但是由於他不得不使用它並且不太記得它的工作原理，所以它已經太久了。目前，我回過頭來讓代碼更易於維護，因爲隨着業務邏輯/需求的變化，這部分代碼可能會在未來幾年內多次更改，並且我希望繼續優化它，同時使它更易於維護。

我嘗試過查找它，但是我可以找到的所有建議都是用not exists代替not in，並且不返回實際結果。

因此，我想知道什麼可以做，以優化出exists/not exists或是否有寫exists/not exists一種方式，以便（比我可能可以在一個更好的程度），甲骨文將在內部優化。

例如，如何優化以下內容？

UPDATE 
    SCOTT.TABLE_N N 
SET 
    N.VALUE_1 = 'Data!' 
WHERE 
    N.VALUE_2 = 'Y' 
    AND 
    EXISTS 
    (
     SELECT 
      1 
     FROM 
      SCOTT.TABLE_Q Q 
     WHERE 
      N.ID = Q.N_ID 
    ) 
    AND 
    NOT EXISTS 
    (
     SELECT 
      1 
     FROM 
      SCOTT.TABLE_W W 
     WHERE 
      N.ID = W.N_ID 
    ) 

Answer 1

你的聲明似乎完全沒有給我。

在任何優化任務中，都不要考慮模式。不要這樣想，「(not) exists不好，慢，(not) in是超級酷和快速」。

想想，數據庫在每個步驟上做了多少工作，以及如何測量它？

一個簡單的例子：

- NOT IN：

23:59:41 HR@sandbox> alter system flush buffer_cache; 

System altered. 

Elapsed: 00:00:00.03 
23:59:43 HR@sandbox> set autotrace traceonly explain statistics 
23:59:49 HR@sandbox> select country_id from countries where country_id not in (select country_id from locations); 

11 rows selected. 

Elapsed: 00:00:00.02 

Execution Plan 
---------------------------------------------------------- 
Plan hash value: 1748518851 

------------------------------------------------------------------------------------------ 
| Id | Operation    | Name   | Rows | Bytes | Cost (%CPU)| Time  | 
------------------------------------------------------------------------------------------ 
| 0 | SELECT STATEMENT  |     |  1 |  6 |  4 (0)| 00:00:01 | 
|* 1 | FILTER    |     |  |  |   |   | 
| 2 | NESTED LOOPS ANTI SNA|     | 11 | 66 |  4 (75)| 00:00:01 | 
| 3 | INDEX FULL SCAN  | COUNTRY_C_ID_PK | 25 | 75 |  1 (0)| 00:00:01 | 
|* 4 | INDEX RANGE SCAN | LOC_COUNTRY_IX | 13 | 39 |  0 (0)| 00:00:01 | 
|* 5 | TABLE ACCESS FULL | LOCATIONS  |  1 |  3 |  3 (0)| 00:00:01 | 
------------------------------------------------------------------------------------------ 

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

    1 - filter(NOT EXISTS (SELECT 0 FROM "LOCATIONS" "LOCATIONS" WHERE 
       "COUNTRY_ID" IS NULL)) 
    4 - access("COUNTRY_ID"="COUNTRY_ID") 
    5 - filter("COUNTRY_ID" IS NULL) 


Statistics 
---------------------------------------------------------- 
      0 recursive calls 
      0 db block gets 
     11 consistent gets 
      8 physical reads 
      0 redo size 
     446 bytes sent via SQL*Net to client 
     363 bytes received via SQL*Net from client 
      2 SQL*Net roundtrips to/from client 
      0 sorts (memory) 
      0 sorts (disk) 
     11 rows processed 

- NOT EXISTS

23:59:57 HR@sandbox> alter system flush buffer_cache; 

System altered. 

Elapsed: 00:00:00.17 
00:00:02 HR@sandbox> select country_id from countries c where not exists (select 1 from locations l where l.country_id = c.country_id); 

11 rows selected. 

Elapsed: 00:00:00.30 

Execution Plan 
---------------------------------------------------------- 
Plan hash value: 840074837 

------------------------------------------------------------------------------------- 
| Id | Operation   | Name   | Rows | Bytes | Cost (%CPU)| Time  | 
------------------------------------------------------------------------------------- 
| 0 | SELECT STATEMENT |     | 11 | 66 |  1 (0)| 00:00:01 | 
| 1 | NESTED LOOPS ANTI|     | 11 | 66 |  1 (0)| 00:00:01 | 
| 2 | INDEX FULL SCAN | COUNTRY_C_ID_PK | 25 | 75 |  1 (0)| 00:00:01 | 
|* 3 | INDEX RANGE SCAN| LOC_COUNTRY_IX | 13 | 39 |  0 (0)| 00:00:01 | 
------------------------------------------------------------------------------------- 

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

    3 - access("L"."COUNTRY_ID"="C"."COUNTRY_ID") 


Statistics 
---------------------------------------------------------- 
      0 recursive calls 
      0 db block gets 
      5 consistent gets 
      2 physical reads 
      0 redo size 
     446 bytes sent via SQL*Net to client 
     363 bytes received via SQL*Net from client 
      2 SQL*Net roundtrips to/from client 
      0 sorts (memory) 
      0 sorts (disk) 
     11 rows processed 

不限於在本示例讀取兩倍的數據庫塊並執行更復雜的過濾 - 問問自己，爲什麼你會選擇它而不是存在？

Answer 2

當你需要時，沒有理由避免使用EXISTS或NOT EXISTS。在你給出的例子中，這可能正是你想要使用的。

典型的困境是使用IN/NOT IN還是EXISTS/NOT EXISTS。它們的評估方式差異很大，根據具體情況可能會更快或更慢。

請參閱here瞭解更多詳情，而不是您可能想要的。

Answer 3

我不知道，如果它的速度要快得多，但這裏是寫出來的方式，而不EXISTS/NOT EXISTS：

MERGE INTO TABLE_N T 
USING (
    SELECT N.ID, 'Data!' AS NEW_VALUE_1 
    FROM SCOTT.TABLE_N N 
    INNER JOIN SCOTT.TABLE_Q Q 
     ON Q.N_ID = N.ID 
    LEFT JOIN SCOTT.TABLE_W W 
     ON W.N_ID = N.ID 
    WHERE N.VALUE_2 = 'Y' 
    AND W.ID IS NULL 
) X 
ON (T.ID = X.ID) 
WHEN MATCHED THEN UPDATE 
    SET T.VALUE_1 = X.NEW_VALUE_1;