一、簡要說明：
Prediceate(謂詞)：一個查詢中的WHERE限制條件。
在查看執行計划的信息中，經常會看到兩個謂詞filter和access，它們的區別是什么，理解了這個兩個詞對我們解讀Oracle的執行計划信息會有所幫助。
簡單地說，執行計划如果顯示是access，就表示這個謂詞條件的值將會影響數據的訪問路徑(表還是索引)，而filter表示謂詞條件的值並不會影響數據訪問路徑，只起到過濾的作用。
二、舉例說明：
----創建一張表echo----
1 SQL> create table echo as select * from dba_objects; 2 3 Table created. 4 5 SQL> set autotrace trace exp; 6 SQL> set linesize 150; 7 SQL> select * from echo where object_id=1000; 8 9 Execution Plan 10 ---------------------------------------------------------- 11 Plan hash value: 642657756 12 13 -------------------------------------------------------------------------- 14 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 15 -------------------------------------------------------------------------- 16 | 0 | SELECT STATEMENT | | 12 | 2484 | 289 (1)| 00:00:04 | 17 |* 1 | TABLE ACCESS FULL| ECHO | 12 | 2484 | 289 (1)| 00:00:04 | 18 -------------------------------------------------------------------------- 19 20 Predicate Information (identified by operation id): 21 --------------------------------------------------- 22 23 1 - filter("OBJECT_ID"=1000) ----因為表echo沒有創建索引，執行計划沒有選擇數據訪問路徑的余地，謂詞條件在這里只是起到數據過濾的作用，所以使用了filter。 24 25 Note 26 ----- 27 - dynamic sampling used for this statement (level=2) 28 ----創建索引的情況----
29 SQL> create index echo_ind on echo(object_id); 30 31 Index created. 32 33 SQL> select * from echo where object_id=1000; 34 35 Execution Plan 36 ---------------------------------------------------------- 37 Plan hash value: 1345159126 38 39 ---------------------------------------------------------------------------------------- 40 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 41 ---------------------------------------------------------------------------------------- 42 | 0 | SELECT STATEMENT | | 1 | 207 | 2 (0)| 00:00:01 | 43 | 1 | TABLE ACCESS BY INDEX ROWID| ECHO | 1 | 207 | 2 (0)| 00:00:01 | 44 |* 2 | INDEX RANGE SCAN | ECHO_IND | 1 | | 1 (0)| 00:00:01 | 45 ---------------------------------------------------------------------------------------- 46 47 Predicate Information (identified by operation id): 48 --------------------------------------------------- 49 50 2 - access("OBJECT_ID"=1000) ----謂詞條件影響到數據訪問的路徑，選擇了索引，所以用access。 51 52 Note 53 ----- 54 - dynamic sampling used for this statement (level=2)