GaussDB 实操——数据库性能调优实验2

Id 字段

• explain 执行效果：

EXPLAIN SELECT *
FROM subject
WHERE id = 1;

• id 相同时，执行顺序从上到下

EXPLAIN SELECT subject.*
FROM subject, student_score, teacher
WHERE subject.id = student_id
AND subject.teacher_id = teacher.id;

读取顺序：subject -> teacher -> student_score。

• Id 不同。

EXPLAIN SELECT score.*
FROM student_score score
WHERE subject_id = (
SELECT id
FROM subject
WHERE teacher_id = (
SELECT id
FROM teacher
WHERE id = 2
)
);

如果是子查询，id 的序号会递增，id 的值越大优先级越高，越先被执行。

读取顺序：teacher -> subject -> student_score

• id 相同与不同都存在时。

EXPLAIN SELECT subject.*
FROM subject
LEFT JOIN teacher ON subject.teacher_id = teacher.id
UNION
SELECT subject.*
FROM subject
RIGHT JOIN teacher ON subject.teacher_id = teacher.id;

id 如果相同，可以认为是一组，从上往下顺序执行。
在所有组中，id 值越大，优先级越高，越先执行。

读取顺序：2.teacher -> 2.subject -> 1.subject -> 1.teacher。