1. Efficient Pagination Using MySQL Surat Singh Bhati (surat@yahoo-inc.com) Rick James (rjames@yahoo-inc.com) Yahoo Inc Percona Performance Conference 2009

2. Outline 1. Overview – Common pagination UI pattern – Sample table and typical solution using OFFSET – Techniques to avoid large OFFSET – Performance comparison – Concerns -2-

3. Common Patterns -3-

4. Basics First step toward having efficient pagination over large data set – Use index to filter rows (resolve WHERE) – Use same index to return rows in sorted order (resolve ORDER) Step zero – http://dev.mysql.com/doc/refman/5.1/en/mysql-indexes.html – http://dev.mysql.com/doc/refman/5.1/en/order-by-optimization.html – http://dev.mysql.com/doc/refman/5.1/en/limit-optimization.html -4-

5. Using Index KEY a_b_c (a, b, c) ORDER may get resolved using Index – ORDER BY a – ORDER BY a,b – ORDER BY a, b, c – ORDER BY a DESC, b DESC, c DESC WHERE and ORDER both resolved using index: – WHERE a = const ORDER BY b, c – WHERE a = const AND b = const ORDER BY c – WHERE a = const – WHERE a = const AND b > const ORDER BY b, c ORDER BY b, c ORDER will not get resolved uisng index (file sort) – ORDER BY a ASC, b DESC, c DESC /* mixed sort direction */ – WHERE g = const ORDER BY b, c /* a prefix is missing */ – WHERE a = const ORDER BY c /* b is missing */ – WHERE a = const ORDER BY a, d /* d is not part of index */ -5-

6. Sample Schema CREATE TABLE `message` ( `id` int(11) NOT NULL AUTO_INCREMENT, `title` varchar(255) COLLATE utf8_unicode_ci NOT NULL, `user_id` int(11) NOT NULL, `content` text COLLATE utf8_unicode_ci NOT NULL, `create_time` int(11) NOT NULL, `thumbs_up` int(11) NOT NULL DEFAULT '0', /* Vote Count */ PRIMARY KEY (`id`), KEY `thumbs_up_key` (`thumbs_up`,`id`) ) ENGINE=InnoDB mysql> show table status like 'message' \G Engine: InnoDB Version: 10 Row_format: Compact Rows: 50000040 /* 50 Million */ Avg_row_length: 565 Data_length: 28273803264 /* 26 GB */ Index_length: 789577728 /* 753 MB */ Data_free: 6291456 Create_time: 2009-04-20 13:30:45 Two use case: • Paginate by time, recent message one page one • Paginate by thumps_up, largest value on page one -6-

7. Typical Query 1. Get the total records SELECT count(*) FROM message 2. Get current page SELECT * FROM message ORDER BY id DESC LIMIT 0, 20 • http://domain.com/message?page=1 • ORDER BY id DESC LIMIT 0, • 20 http://domain.com/message?page=2 • ORDER BY id DESC LIMIT 20, • 20 http://domain.com/message?page=3 • ORDER BY id DESC LIMIT 40, 20 Note: id is auto_increment, same as create_time order, no need to create index on create_time, save space – -7-

8. Explain mysql> explain SELECT * FROM message ORDER BY id DESC LIMIT 10000, 20\G ***************** 1. row ************** id: 1 select_type: SIMPLE table: message type: index possible_keys: NULL key: PRIMARY key_len: 4 ref: NULL rows: 10020 Extra: 1 row in set (0.00 sec) – it can read rows using index scan and execution will stop as soon as it finds required rows. – LIMIT 10000, 20 means it has to read 10020 and throw away 10000 rows, then return next 20 rows. -8-

9. Performance Implications – Larger OFFSET is going to increase active data set, MySQL has to bring data in memory that is never returned to caller. – Performance issue is more visible when your have database that can't fit in main memory. – Small percentage of request with large OFFSET would be able to hit disk I/O Disk I/O bottleneck – In order to display “21 to 40 of 1000,000” , some one has to count 1000,000 rows. -9-

10. Simple Solution – Do not display total records, does user really care? – Do not let user go to deep pages, redirect him http://en.wikipedia.org/wiki/Internet_addiction_disorder after certain number of pages - 10 -

11. Avoid Count(*) 1. Never display total messages, let user see more message by clicking 'next' 2. Do not count on every request, cache it, display stale count, user do not care about 324533 v/s 324633 3. Display 41 to 80 of Thousands 4. Use pre calculated count, increment/decrement value as insert/delete happens. - 11 -

12. Solution to avoid offset 1. Change User Interface – No direct jumps to Nth page 2. LIMIT N is fine, Do not use LIMIT M,N – Provide extra clue about from where to start given page – Find the desired records using more restricted WHERE using given clue and ORDER BY and LIMIT N without OFFSET) - 12 -

13. Find the clue 150 111 102 101 100 98 97 96 95 94 93 92 91 90 89 Page One <a href=”/page=2;last_seen=100;dir=next>Next</a> <a href=”/page=1;last_seen=98;dir=prev>Prev</a> Page Two <a href=”/page=3;last_seen=94;dir=next>Next</a> <a href=”/page=3;last_seen=93;dir=prev>Prev</a> Page Three <a href=”/page=4;last_seen=89;dir=prev>Next</a> - 13 -

14. Solution using clue Next Page: http://domain.com/forum?page=2&last_seen=100&dir=next WHERE id < 100 /* last_seen * ORDER BY id DESC LIMIT $page_size /* No OFFSET*/ Prev Page: http://domain.com/forum?page=1&last_seen=98&dir=prev WHERE id > 98 /* last_seen * ORDER BY id ASC LIMIT $page_size /* No OFFSET*/ Reverse given 10 rows before sending to user - 14 -

15. Explain mysql> explain SELECT * FROM message WHERE id < '49999961' ORDER BY id DESC LIMIT 20 \G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: message type: range possible_keys: PRIMARY key: PRIMARY key_len: 4 ref: NULL Rows: 25000020 /* ignore this */ Extra: Using where 1 row in set (0.00 sec) - 15 -

16. What about order by non unique values? 99 Page One 99 98 98 98 98 Page Two 98 97 97 10 We can't do: WHERE thumbs_up < 98 ORDER BY thumbs_up DESC /* It will return few seen rows */ Can we say this: WHERE thumbs_up <= 98 AND <extra_con> ORDER BY thumbs_up DESC - 16 -

17. Add more condition • Consider thumbs_up as major number – if we have additional minor number, we can use combination of major & minor as extra condition • Find additional column (minor number) – we can use id primary key as minor number - 17 -

18. Solution First Page SELECT thumbs_up, id FROM message ORDER BY thumbs_up DESC, id DESC LIMIT $page_size +-----------+----+ | thumbs_up | id | +-----------+----+ | 99 | 14 | | 99 | 2 | | 98 | 18 | | 98 | 15 | | 98 | 13 | +-----------+----+ Next Page SELECT thumbs_up, id FROM message WHERE thumbs_up <= 98 AND (id < 13 OR thumbs_up < 98) ORDER BY thumbs_up DESC, id DESC LIMIT $page_size +-----------+----+ | thumbs_up | id | +-----------+----+ | 98 | 10 | | 98 | 6 | | 97 | 17 | - 18 -

19. Make it better.. Query: SELECT * FROM message WHERE thumbs_up <= 98 AND (id < 13 OR thumbs_up < 98) ORDER BY thumbs_up DESC, id DESC LIMIT 20 Can be written as: SELECT m2.* FROM message m1, message m2 WHERE m1.id = m2.id AND m1.thumbs_up <= 98 AND (m1.id < 13 OR m1.thumbs_up < 98) ORDER BY m1.thumbs_up DESC, m1.id DESC LIMIT 20; - 19 -

20. Explain *************************** 1. row *************************** id: 1 select_type: SIMPLE table: m1 type: range possible_keys: PRIMARY,thumbs_up_key key: thumbs_up_key /* (thumbs_up,id) */ key_len: 4 ref: NULL Rows: 25000020 /*ignore this, we will read just 20 rows*/ Extra: Using where; Using index /* Cover */ *************************** 2. row *************************** id: 1 select_type: SIMPLE table: m2 type: eq_ref possible_keys: PRIMARY key: PRIMARY key_len: 4 ref: forum.m1.id rows: 1 Extra: - 20 -

21. Performance Gain (Primary Key Order) - 21 -

22. Performance Gain (Secondary Key Order) - 22 -

23. Throughput Gain • Throughput Gain while hitting first 30 pages: – Using LIMIT OFFSET, N • 600 query/sec – Using LIMIT N (no OFFSET) • 3.7k query/sec - 23 -

24. Bonus Point Product issue with LIMIT M, N User is reading a page, in the mean time some records may be added to previous page. Due to insert/delete pages records are going to move forward/backward as rolling window: – User is reading messages on 4th page – While he was reading, one new message posted (it would be there on page one), all pages are going to move one message to next page. – User Clicks on Page 5 – One message from page got pushed forward on page 5, user has to read it again No such issue with news approach - 24 -

25. Drawback Search Engine Optimization Expert says: Let bot reach all you pages with fewer number of deep dive Two Solutions: • Read extra rows – Read extra rows in advance and construct links for few previous & next pages • Use small offset – Do not read extra rows in advance, just add links for few past & next pages with required offset & last_seen_id on current page – Do query using new approach with small offset to display desired page – file:///Users/surat/Desktop/Picture%2043.png Additional concern: Dynamic urls, last_seen is not constant over time. - 25 -

26. Thanks - 26 -