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法律研究如何在 Oracle 中快取查詢結果？
問題陳述
您希望提高常用查詢的效能。
解決方案
我們必須使用 Oracle 的結果快取來儲存常用 SQL 的查詢結果，以便在將來執行相同查詢時可以快速檢索它們。
結果快取是 Oracle 11g 的新增功能，它允許我們將常用查詢的結果儲存在記憶體中，以便快速輕鬆地檢索。
示例
SELECT /*+ result_cache */ e.class_id, min_fees, max_fees FROM students e ,jobs j WHERE e.class_id = j.class_id GROUP BY e.class_id, min_fees, max_fees;
為了演示如何使用它，我們將檢查上述 sql 的 explain_plan。
示例
EXPLAIN PLAN FOR SELECT /*+ result_cache */ e.class_id, min_fees, max_fees FROM students e ,jobs j WHERE e.class_id = j.class_id GROUP BY e.class_id, min_fees, max_fees; SELECT * FROM table(DBMS_XPLAN.DISPLAY (FORMAT=>'ALL'));
輸出
--------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 19 | 475 | 4 (25)| 00:00:01 | | 1 | RESULT CACHE | a1pfq5gpbag4wgj0zth2xasdj1 | 19 | 475 | 4 (25)| 00:00:01 | | 2 | HASH GROUP BY | | 19 | 475 | 4 (25)| 00:00:01 | | 3 | NESTED LOOPS SEMI | | 19 | 475 | 3 (0)| 00:00:01 | | 4 | TABLE ACCESS FULL| JOBS | 19 | 304 | 3 (0)| 00:00:01 | |* 5 | INDEX RANGE SCAN | stu_JOB_IX | 107 | 963 | 0 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------------
然後，我們將查詢 V$RESULT_CACHE_OBJECTS 檢視，透過檢視 explain 計劃中的快取 ID 值來驗證查詢的結果是否儲存在結果快取中。
示例
SELECT ID, type, status, creation_timestamp, column_count AS cols, row_count AS "rows" FROM V$RESULT_CACHE_OBJECTS WHERE CACHE_ID = 'a1pfq5gpbag4wgj0zth2xasdj1';
示例
1947 Result Published 11/NOV/20 3 19
結果快取提示在新增到 SQL 時，將覆蓋任何資料庫、表或會話級別的結果快取設定。在將提示新增到 SQL 之前，您需要驗證資料庫上結果快取的配置。
使用結果快取可以顯著提高查詢時間，因為首先會在結果快取中檢查查詢結果，如果結果存在，則只需從記憶體中提取它們，並且不會執行查詢。結果快取最適合用於產生相同結果的頻繁執行的查詢。
為了顯示效能改進，上述 SQL 運行了兩次，第一次沒有提示，第二次有提示。
示例
SELECT /*+ no_result_cache */ e.class_id, min_fees, max_fees FROM students e ,jobs j WHERE e.class_id = j.class_id GROUP BY e.class_id, min_fees, max_fees;
輸出
19 rows selected. Elapsed: 00:00:17.20
示例
SELECT /*+ result_cache */ e.class_id, min_fees, max_fees FROM students e ,jobs j WHERE e.class_id = j.class_id GROUP BY e.class_id, min_fees, max_fees;
輸出
19 rows selected. Elapsed: 00:00:01.03
用於上述問題的資料庫
示例
DROP TABLE students; COMMIT; CREATE TABLE students ( student_id NUMBER(6) , first_name VARCHAR2(20) , last_name VARCHAR2(25) , email VARCHAR2(40) , phone_number VARCHAR2(20) , join_date DATE , class_id VARCHAR2(20) , fees NUMBER(8,2) , professor_id NUMBER(6) , department_id NUMBER(4) ) ;
示例
CREATE UNIQUE INDEX stu_id_pk ON students (student_id) ;
INSERT INTO students VALUES (100,'SMITH','JAMES','SMITH.JAMES@notreal.com','111.111.1245',TO_DATE('17-06-2003','DD-MM-YYYY'),'INS_CHAIRMAN',24000,NULL,NULL);
INSERT INTO students VALUES (101,'JOHNSON','JOHN','JOHNSON.JOHN@notreal.com','111.111.1246',TO_DATE('21-09-2005','DD-MM-YYYY'),'INS_VP',17000,100,90);
INSERT INTO students VALUES (102,'WILLIAMS','ROBERT','WILLIAMS.ROBERT@notreal.com','111.111.1247',TO_DATE('13-01-2001','DD-MM-YYYY'),'INS_VP',17000,100,90);
INSERT INTO students VALUES (103,'BROWN','MICHAEL','BROWN.MICHAEL@notreal.com','111.111.1248',TO_DATE('03-01-2006','DD-MM-YYYY'),'INS_STAFF',9000,102,60);
INSERT INTO students VALUES (104,'JONES','WILLIAM','JONES.WILLIAM@notreal.com','111.111.1249',TO_DATE('21-05-2007','DD-MM-YYYY'),'INS_STAFF',6000,103,60);
INSERT INTO students VALUES (105,'MILLER','DAVID','MILLER.DAVID@notreal.com','111.111.1250',TO_DATE('25-06-2005','DD-MM-YYYY'),'INS_STAFF',4800,103,60);
INSERT INTO students VALUES (106,'DAVIS','RICHARD','DAVIS.RICHARD@notreal.com','111.111.1251',TO_DATE('05-02-2006','DD-MM-YYYY'),'INS_STAFF',4800,103,60);
INSERT INTO students VALUES (107,'GARCIA','CHARLES','GARCIA.CHARLES@notreal.com','111.111.1252',TO_DATE('07-02-2007','DD-MM-YYYY'),'INS_STAFF',4200,103,60);
INSERT INTO students VALUES (108,'RODRIGUEZ','JOSEPH','RODRIGUEZ.JOSEPH@notreal.com','111.111.1253',TO_DATE('17-08-2002','DD-MM-YYYY'),'CL_PHY',12008,101,100);
INSERT INTO students VALUES (109,'WILSON','THOMAS','WILSON.THOMAS@notreal.com','111.111.1254',TO_DATE('16-08-2002','DD-MM-YYYY'),'CL_MATH',9000,108,100);
INSERT INTO students VALUES (110,'MARTINEZ','CHRISTOPHER','MARTINEZ.CHRISTOPHER@notreal.com','111.111.1255',TO_DATE('28-09-2005','DD-MM-YYYY'),'CL_MATH',8200,108,100);
INSERT INTO students VALUES (111,'ANDERSON','DANIEL','ANDERSON.DANIEL@notreal.com','111.111.1256',TO_DATE('30-09-2005','DD-MM-YYYY'),'CL_MATH',7700,108,100);
INSERT INTO students VALUES (112,'TAYLOR','PAUL','TAYLOR.PAUL@notreal.com','111.111.1257',TO_DATE('07-03-2006','DD-MM-YYYY'),'CL_MATH',7800,108,100);
INSERT INTO students VALUES (113,'THOMAS','MARK','THOMAS.MARK@notreal.com','111.111.1258',TO_DATE('07-12-2007','DD-MM-YYYY'),'CL_MATH',6900,108,100);
COMMIT;
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