MySQL random paging to get non duplicate data

demand

From the list page of post bar

Click into the video details, and you can slide up and down to brush the video

It is required that each time you enter the same video details, the video display that slides up and down should not be repeated

analysis

The difficulty of this requirement is that the video list given by the back end is different from the details and the repetition is minimized
programme ：

• If according to id The default sorting is random id then >id Number paging return , There will be a large number of continuous duplicate data
• according to order by rand() return , The probability of recurrence meets the demand, but the amount of data in post bar is relatively large , use rand() Return efficiency is worrying

When used for a large table , This may slow down use ORDER BY RAND() The execution of the function . This is because MySQL You need to sort the entire table to find random values , It's a time-consuming process

Today we are going to talk about the third of the above two schemes , namely MD5( Primary key id) Generating uniqueness 32 Bit character field , When requesting a list from the details page ,, adopt mysql Of substr Function randomly from 32 A part of bit data is intercepted and sorted , On the next page, the data of the next page is still returned according to the intercepted parameters . When entering this detail again, randomly generate the intercepted digits

Realization

SELECT
	`posts`.`post_id`,
	MD5( `posts_id` ) AS `uid`,
	`posts`.`user_id`,
	`posts`.`post_content`,
	`posts`.`created_at` 
FROM
	(
	SELECT
		`post_id`,
		`user_id`,
		`post_content`,
		`created_at` 
	FROM
		`posts` 
	WHERE
		( `posts`.`deleted_at` = 0 ) 
	) `posts` 
ORDER BY
	SUBSTR(uid, 23, 6 ) 
	offset 0
	LIMIT 30

As above sql Shown , But the first page of the request list can be randomly generated 0-32 The number of as SUBSTR(uid, { KaTeX parse error: Expected 'EOF', got '}' at position 6: start}̲, { len} ) Of $startParameters\; ,\; If\; you\; want\; to\; start\; the\; second\; step,\; you\; need\; the\; front\; end\; to\; pass\; the\; previous\; interception\; parameters\; ,\; The\; back\; end\; obtains\; the\; data\; of\; the\; next\; page\; according\; to\; the\; interception\; parameters\; .$len The parameters of can be written directly 4-6 position

 		// If the intercepted parameters are not transmitted, they will be regenerated 
		$subKey = $params['sub_key'] ?? 0;
        if (empty($subKey)) {
    
            $subKey = rand(0, 32);
        }

Next through page Replace offset Parameters can be paged perfectly

When exiting the list and requesting again, re intercepting the character sorting will obtain different data and achieve perfect paging

Our test data is 3 Ten thousand
use rand() Pagination llimit 10 offset 10000 need 0.122~148s
use substr Paged in the form of llimit 10 offset 10000 need 0.082-0.084