performance - SQLite 索引性能

Question

我有以下布局的表格
email, item_id, json在哪里

email是一个字符串

item_id是以毫秒为单位的 unix 时间戳

json是与 JSON1 扩展一起使用的项目数据

我在该表上也有一个多列索引 email, id
我以 WHERE email = 'asd' AND item_id > ... AND item_id < ... 的样式执行了很多查询

我已经和 MongoDB 打交道太多年了，所以我习惯了不处理数据库规范化，而只是使用最简单的 SQL 表布局。

在手机上，对于 35000 个项目的查询，对上述样式的查询最多可能需要一秒钟。该索引确实被使用了。

当我通过使用 email, email_id 创建一个新表来规范化数据库时，我会获得显着的性能提升吗？并将原来的更改为email_id, item_id, json并开始通过 JOINS 查询？在那种情况下 email, email_id将包含大约 2-5 个项目和 email_id, item_id, json数以千计。

Answer 1

使用 3 测试原始，使用 JOIN 和附加选项的查询，使用子查询而不是连接来根据电子邮件地址获取电子邮件 ID，并将其与 email_id 进行比较。子查询名列前茅，原来的表现最差。

结果是:-

SELECT * FROM original WHERE email = 'email3@ouremail.com' AND item_id > 7800 AND item_id < 2404327029516376406

OK Time: 0.199s

SELECT * FROM item WHERE email_id = (SELECT email.email_id FROM email WHERE email.email = 'email3@ouremail.com') AND item_id > 7800 AND item_id < 2404327029516376406

OK Time: 0.082s

SELECT * FROM item JOIN email ON item.email_id = email.email_id WHERE email.email = 'email3@ouremail.com' AND item_id > 7800 AND item_id < 2404327029516376406

OK Time: 0.109s

以下用于创建和测试:-

DROP TABLE IF EXISTS original;
CREATE TABLE IF NOT EXISTS original (email TEXT, item_id INTEGER, json BLOB, PRIMARY KEY(email,item_id));
WITH RECURSIVE cnt(x,y,z) 
AS (
    SELECT 'email'||(1 + ABS(random() / (9223372036854775807 / 5)))||'@ouremail.com',
                ABS(random()),
          randomblob(ABS(random() / (9223372036854775807 / 40) ))
            UNION ALL SELECT 
              'email'||(1 + ABS(random() / (9223372036854775807 / 5)))||'@ouremail.com',
                ABS(random()),
                randomblob(ABS(random() / (9223372036854775807 / 40)))  
            FROM cnt LIMIT 350000
)
INSERT INTO original SELECT * FROM cnt;


DROP TABLE IF EXISTS email;
CREATE TABLE IF NOT EXISTS email (email_id INTEGER PRIMARY KEY, email TEXT);
INSERT INTO email SELECT DISTINCT null,email FROM original;


DROP TABLE IF EXISTS item;
CREATE TABLE IF NOT EXISTS item (email_id, item_id, json);
INSERT INTO item SELECT 
    (SELECT email_id FROM email WHERE original.email = email.email),
        item_id,
        json FROM original;


SELECT * FROM original WHERE email = 'email3@ouremail.com' AND item_id > 7800 AND item_id < 2404327029516376406;
SELECT * FROM item WHERE email_id = (SELECT email.email_id FROM email WHERE email.email = 'email3@ouremail.com') AND item_id > 7800 AND item_id < 2404327029516376406; 
SELECT * FROM item JOIN email ON item.email_id = email.email_id WHERE email.email = 'email3@ouremail.com' AND item_id > 7800 AND item_id < 2404327029516376406;

你可能会更糟糕的是运行以下命令并查看输出。

EXPLAIN QUERY PLAN SELECT * FROM original WHERE email = 'email3@ouremail.com' AND item_id > 7800 AND item_id < 2404327029516376406;
EXPLAIN QUERY PLAN SELECT * FROM item WHERE email_id = (SELECT email.email_id FROM email WHERE email.email = 'email3@ouremail.com') AND item_id > 7800 AND item_id < 2404327029516376406;
EXPLAIN QUERY PLAN SELECT * FROM item JOIN email ON item.email_id = email.email_id WHERE email.email = 'email3@ouremail.com' AND item_id > 7800 AND item_id < 2404327029516376406;