案例：建立合适的索引

--建表
CREATE TABLE staffs (staff_id NUMBER(6) NOT NULL, first_name VARCHAR2(20), last_name VARCHAR2(25), employment_id VARCHAR2(10), section_id NUMBER(4), state_name VARCHAR2(10), city VARCHAR2(10));
CREATE TABLE sections(section_id NUMBER(4), place_id NUMBER(4), section_name VARCHAR2(20));
CREATE TABLE states(state_id NUMBER(4));
CREATE TABLE places(place_id NUMBER(4), state_id  NUMBER(4));
--优化前查询
EXPLAIN SELECT staff_id,first_name,last_name,employment_id,state_name,city 
FROM staffs,sections,states,places 
WHERE sections.section_name='Sales' 
AND staffs.section_id = sections.section_id 
AND sections.place_id = places.place_id 
AND places.state_id = states.state_id 
ORDER BY staff_id;
--优化后查询
CREATE INDEX loc_id_pk ON places(place_id);
CREATE INDEX state_c_id_pk ON states(state_id);

EXPLAIN SELECT staff_id,first_name,last_name,employment_id,state_name,city 
FROM staffs,sections,states,places 
WHERE sections.section_name='Sales' 
AND staffs.section_id = sections.section_id 
AND sections.place_id = places.place_id 
AND places.state_id = states.state_id 
ORDER BY staff_id;