如何更改 Oracle 中的 JOIN 顺序?
问题:
您的查询在多个表上有 JOIN 条件，但存在性能问题，Oracle 优化器未选择您想要的连接顺序。
解决方案:
Oracle 有两个提示:ORDERED 提示和 LEADING 提示，它们可用于影响查询中使用的连接顺序。
ORDERED 提示
您正在运行 SQL 来连接两个表(学生和部门)，因为您想要获取每个学生的部门名称。通过将 ORDERED 提示放入查询中，您可以看到提示如何改变执行访问路径。
示例
EXPLAIN PLAN FOR SELECT first_name, department_name FROM students e, departments d WHERE e.department_id = d.department_id; SELECT * FROM table(DBMS_XPLAN.DISPLAY (FORMAT=>'BASIC'));
输出
---------------------------------------------------------- | Id | Operation | Name | ---------------------------------------------------------- | 0 | SELECT STATEMENT | | | 1 | MERGE JOIN | | | 2 | TABLE ACCESS BY INDEX ROWID| DEPARTMENTS | | 3 | INDEX FULL SCAN | DEPT_ID_PK | | 4 | SORT JOIN | | | 5 | VIEW | index$_join$_001 | | 6 | HASH JOIN | | | 7 | INDEX FAST FULL SCAN | stu_DEPARTMENT_IX | | 8 | INDEX FAST FULL SCAN | stu_NAME_IX | ----------------------------------------------------------
示例
EXPLAIN PLAN FOR SELECT /*+ ordered */ first_name, department_name FROM students e, departments d WHERE e.department_id = d.department_id;
输出
----------------------------------------------------- | Id | Operation | Name | ----------------------------------------------------- | 0 | SELECT STATEMENT | | | 1 | HASH JOIN | | | 2 | VIEW | index$_join$_001 | | 3 | HASH JOIN | | | 4 | INDEX FAST FULL SCAN| stu_DEPARTMENT_IX | | 5 | INDEX FAST FULL SCAN| stu_NAME_IX | | 6 | TABLE ACCESS FULL | DEPARTMENTS | -----------------------------------------------------
LEADING 提示
与使用 ORDERED 提示的示例一样，您可以同样控制指定查询的连接顺序。LEADING 提示的不同之处在于，您可以在提示本身内指定连接顺序，而 ORDERED 提示则是在查询的 FROM 子句中指定连接顺序
示例
EXPLAIN PLAN FOR SELECT /*+ leading(departments, students) */ first_name, department_name FROM students e, departments d WHERE e.department_id = d.department_id; SELECT * FROM table(DBMS_XPLAN.DISPLAY (FORMAT=>'BASIC'));
输出
---------------------------------------------------------- | Id | Operation | Name | ---------------------------------------------------------- | 0 | SELECT STATEMENT | | | 1 | MERGE JOIN | | | 2 | TABLE ACCESS BY INDEX ROWID| DEPARTMENTS | | 3 | INDEX FULL SCAN | DEPT_ID_PK | | 4 | SORT JOIN | | | 5 | VIEW | index$_join$_001 | | 6 | HASH JOIN | | | 7 | INDEX FAST FULL SCAN | stu_DEPARTMENT_IX |
提示将节省优化器处理所有可能的连接顺序以确定最佳连接顺序的时间。提示可以提高查询性能，尤其是当查询中要连接的表数量增加时。
使用这两种提示中的任何一种时，您都会指示优化器有关表的连接顺序。因此，了解提示将提高查询的性能至关重要。Oracle 建议尽可能使用 LEADING 提示而不是 ORDERED 提示，因为 LEADING 提示具有更多的内置多功能性。指定 ORDERED 提示时，您可以从 FROM 子句中的表列表中指定连接顺序，而使用 LEADING 提示时，您可以在提示本身中指定连接顺序。
数据准备:问题所用的数据如下所示。数据完全是出于演示目的而编造的。
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;
