РИТ++ 2017, Backend Conf Зал Сан-Паулу, 6 июня, 10:00 Тезисы: http://backendconf.ru/2017/abstracts/2784.html Опубликовав в своём блоге знаменитую заметку о переезде с PostgreSQL на MySQL, Uber наделал много шума в постгресовом сообществе. Для многих из разработчиков PostgreSQL это стало толчком к осознанию несовершенства постгресового табличного движка (который пока всё ещё один). В результате было разработано множество патчей, нацеленных на преодоление проблем, указанных Uber'ом, которые в чём-то пересекаются, а в чём-то даже противоречат друг другу. Среди этих патчей можно отметить indirect indexes (индексы, которые ссылаются на значение primary key), WARM (write-amplification reduction method – уменьшение избыточности записи), RDS (recently dead storage – хранилище для недавно устаревших версий). Также ведётся обсуждение подключаемых табличных движков и undo log'а. ...

1. Наш ответ Uber’у
Александр Коротков
Postgres Professional

2. Russian developers of PostgreSQL:
Alexander Korotkov, Teodor Sigaev, Oleg Bartunov
▶ Speakers at PGCon, PGConf: 20+ talks
▶ GSoC mentors
▶ PostgreSQL commi ers (1+1 in progress)
▶ Conference organizers
▶ 50+ years of expertship: development, audit, consul ng
▶ Postgres Professional co-founders
PostgreSQL CORE
▶ Locale support
▶ PostgreSQL extendability:
GiST(KNN), GIN, SP-GiST
▶ Full Text Search (FTS)
▶ NoSQL (hstore, jsonb)
▶ Indexed regexp search
▶ Create AM & Generic WAL
▶ Table engines (WIP)
Extensions
▶ intarray
▶ pg_trgm
▶ ltree
▶ hstore
▶ plantuner
▶ jsquery
▶ RUM
▶ imgsmlr
Alexander Korotkov Наш ответ Uber’у 2 / 31

3. Disclaimer
▶ I’m NOT a MySQL expert. I didn’t even touch MySQL since 2011...
▶ This talk express my own opinion, not PostgreSQL community
posi on, not even Postgres Professional official posi on.
▶ Uber’s guys knows be er which database they should use.
Alexander Korotkov Наш ответ Uber’у 3 / 31

4. What happened?
▶ Uber migrated from MySQL to PostgreSQL in 2012.
▶ Uber migrated from PostgreSQL to MySQL in 2016.
▶ PostgreSQL to MySQL migra on made a log of buzz in PostgreSQL
community.
Alexander Korotkov Наш ответ Uber’у 4 / 31

5. Why did it happen?
▶ Uber migrated from MySQL to PostgreSQL for “a bunch of reasons,
but one of the most important was availability of PostGIS” ¹
▶ Uber migrated from PostgreSQL to MySQL “some of the drawbacks
they found with Postgres” ²
¹https://www.yumpu.com/en/document/view/53683323/migrating-uber-from-mysql-to-postgresql
²https://eng.uber.com/mysql-migration/
Alexander Korotkov Наш ответ Uber’у 5 / 31

6. Uber’s complaints to PostgreSQL
Uber claims following “PostgreSQL limita ons”:
▶ Inefficient architecture for writes
▶ Inefficient data replica on
▶ Issues with table corrup on
▶ Poor replica MVCC support
▶ Difficulty upgrading to newer releases
Alexander Korotkov Наш ответ Uber’у 6 / 31

7. PostgreSQL’s vs. InnoDB’s storage formats
Alexander Korotkov Наш ответ Uber’у 7 / 31

8. PostgreSQL storage format
▶ Both primary and secondary indexes point to loca on (blkno, offset) of tuple (row
version) in the heap.
▶ When tuple is moved to another loca on, all corresponding index tuples should be
inserted to the indexes.
▶ Heap contains both live and dead tuples.
▶ VACUUM cleans up dead tuples and corresponding index tuples in a bulk manner.
Alexander Korotkov Наш ответ Uber’у 8 / 31

9. Update in PostgreSQL
▶ New tuple is inserted to the heap, previous tuple is marked as
deleted.
▶ Index tuples poin ng to new tuple are inserted to all indexes.
Alexander Korotkov Наш ответ Uber’у 9 / 31

10. Heap-Only-Tuple (HOT) PostgreSQL
▶ When no indexed columns are updated and new version of row can
fit the same page, then HOT is used and only heap is updated.
▶ Microvacuum can be used to free required space in the page for HOT.
Alexander Korotkov Наш ответ Uber’у 10 / 31

11. Update in MySQL
▶ Table rows are placed in the primary index itself. Updates are performed in-place.
Old version of rows are placed to special segment (undo log).
▶ When secondary indexed column is updated, then new index tuple is inserted while
previous index tuple is marked as deleted.
Alexander Korotkov Наш ответ Uber’у 11 / 31

12. Updates: InnoDB in comparison with PostgreSQL
Pro:
▶ Update of few indexed columns is cheaper.
▶ Update, which don’t touch indexed columns, doesn’t depend on page
free space in the page
Cons:
▶ Update of majority of indexed columns is more expensive.
▶ Secondary index scan is slower.
▶ Primary key update is disaster.
Alexander Korotkov Наш ответ Uber’у 12 / 31

13. Uber example for write-amplifica on in PostgreSQL
CREATE TABLE users (id SERIAL PRIMARY KEY,
first TEXT,
last TEXT,
birth_year INTEGER);
CREATE INDEX ix_users_first_last ON users (first, last);
CREATE INDEX ix_users_birth_year ON users (birth_year);
UPDATE users SET birth_year = 1986 WHERE id = 1;
1. Write the new row tuple to the tablespace
2. Update the primary key index to add a record for the new tuple
3. Update the (first, last) index to add a record for the new tuple
4. Update the birth_year index to add a record for the new tuple
5. Previous ac ons are protected by WAL log.
Alexander Korotkov Наш ответ Uber’у 13 / 31

14. Uber example for write-amplifica on: MySQL vs. PostgreSQL
Alexander Korotkov Наш ответ Uber’у 14 / 31

15. Uber example for write-amplifica on: MySQL vs. PostgreSQL
PostgreSQL
1. Write the new row tuple to the
tablespace
2. Insert new tuple to primary key index
3. Insert new tuple to (first, last) index
4. Insert new tuple to birth_year index
5. Previous ac ons are protected by WAL
log.
MySQL
1. Update row in-place
2. Write old version of row to the rollback
segment
3. Insert new tuple to birth_year index
4. Mark old tuple of birth_year index as
obsolete
5. Previous ac ons are protected by
innodb log
6. Write update record to binary log
Assuming we have replica on turned on
Alexander Korotkov Наш ответ Uber’у 15 / 31

16. Pending patches: WARM (write-amplifica on reduc on
method)
▶ Behaves like HOT, but works also when some of index columns are
updated.
▶ New index tuples are inserted only for updated index columns.
https://www.postgresql.org/message-id/flat/20170110192442.ocws4pu5wjxcf45b%40alvherre.pgsql
Alexander Korotkov Наш ответ Uber’у 16 / 31

17. Pending patches: indirect indexes
▶ Indirect indexes are indexes which points to primary key value instead of pointer to
heap.
▶ Indirect index is not updates un l corresponding column is updated.
https://www.postgresql.org/message-id/20161018182843.xczrxsa2yd47pnru@alvherre.pgsql
Alexander Korotkov Наш ответ Uber’у 17 / 31

18. Ideas: RDS (recently dead store)
▶ Recently dead tuples (deleted but visible for some transac ons) are
displaced into special storage: RDS.
▶ Heap tuple headers are le in the heap.
Alexander Korotkov Наш ответ Uber’у 18 / 31

19. Idea: undo log
▶ Displace old version of rows to undo log.
▶ New index tuples are inserted only for updated index columns. Old index tuples are
marked as expired.
▶ Move row to another page if new version doesn’t fit the page.
https://www.postgresql.org/message-id/flat/CA%2BTgmoZS4_CvkaseW8dUcXwJuZmPhdcGBoE_
GNZXWWn6xgKh9A%40mail.gmail.com
Alexander Korotkov Наш ответ Uber’у 19 / 31

20. Idea: pluggable table engines
Owns
▶ Ways to scan and modify tables.
▶ Access methods implementa ons.
Shares
▶ Transac ons, snapshots.
▶ WAL.
https://www.pgcon.org/2016/schedule/events/920.en.html
Alexander Korotkov Наш ответ Uber’у 20 / 31

21. Types of replica on
▶ Statement-level – stream wri ng queries to the slave.
▶ Row-level – stream updated rows to the slave.
▶ Block-level – stream blocks and/or block deltas to the slave.
Alexander Korotkov Наш ответ Uber’у 21 / 31

22. Replica on types in PostgreSQL vs. MySQL
Replica on Type MySQL PostgreSQL
Statement-level buil n pgPool-II
Row-level buil n pgLogical
Londiste
Slony ...
Block-level N/A buil n
Alexander Korotkov Наш ответ Uber’у 22 / 31

23. Uber’s replica on comparison
▶ Uber compares MySQL replica on versus PostgreSQL replica on.
▶ Actually, Uber compares MySQL row-level replica on versus
PostgreSQL block-level replica on.
▶ That happened because that me PostgreSQL had buil n block-level
replica on, but didn’t have buil n row-level replica on.
Simultaneously, MySQL had buil n row-level replica on, but didn’t
have buil n block-level replica on.
Alexander Korotkov Наш ответ Uber’у 23 / 31

24. Uber’s complaints to PostgreSQL block-level replica on
▶ Replica on stream transfers all the changes at block-level including
“write-amplifica on”. Thus, it requires very high-bandwidth channel.
In turn, that makes geo-distributed replica on harder.
▶ There are MVCC limita ons for read-only requires on replica. Apply of
VACUUM changes conflicts with read-only queries which could see
the data VACUUM is going to delete.
Alexander Korotkov Наш ответ Uber’у 24 / 31

25. Is row-level replica on superior over block-level replica on?
Alibaba works on adding block-level replica on to InnoDB. Zhai Weixiang,
database developer from Alibaba considers following advantages of
block-level replica on: ³
▶ Be er performance: higher throughput and lower response me
▶ Write less data (turn off binary log and g d), and only one fsync to make
transac on durable
▶ Less recovery me
▶ Replica on
▶ Less replica on latency
▶ Ensure data consistency (most important for some sensi ve clients)
³https://www.percona.com/live/data-performance-conference-2016/sessions/
physical-replication-based-innodb
Alexander Korotkov Наш ответ Uber’у 25 / 31

26. Replica read-only query MVCC conflict with VACUUM
Possible op ons:
▶ Delay the replica on,
▶ Cancel read-only query on replica,
▶ Provide a feedback to master about row versions which could be demanded.
Undo log would do be er, we wouldn’t have to choose...
Alexander Korotkov Наш ответ Uber’у 26 / 31

27. More about replica on and write-amplifica on
MySQL row-level replica on
PostgreSQL row-level replica on (pgLogical)
Alexander Korotkov Наш ответ Uber’у 27 / 31

28. Major version upgrade with pg_upgrade
Alexander Korotkov Наш ответ Uber’у 28 / 31

29. Major version upgrade with pgLogical
https://www.depesz.com/2016/11/08/major-version-upgrading-with-minimal-downtime/
Alexander Korotkov Наш ответ Uber’у 29 / 31

30. Other Uber notes
▶ PostgreSQL 9.2 had data corrup on bug. It was fixed long me ago. Since that me
PostgreSQL automated tests system was significantly improved to evade such bugs in future.
▶ pread is faster than seek + read. Thats really gives 1.5% accelera on on read-only
benchmark. ⁴
▶ PostgreSQL advises to setup rela vely small shared_buffers and rely on OS cache, while
“InnoDB storage engine implements its own LRU in something it calls the InnoDB buffer
pool”. PostgreSQL also implements its own LRU in something it calls the shared buffers. And
you can setup any shared buffers size.
▶ PostgreSQL uses mul process model. So, connec on is more expensive since unless you
use pgBouncer or other external connec on pool.
⁴https://www.postgresql.org/message-id/flat/a86bd200-ebbe-d829-e3ca-0c4474b2fcb7%40ohmu.fi
Alexander Korotkov Наш ответ Uber’у 30 / 31

31. Thank you for a en on!
Alexander Korotkov Наш ответ Uber’у 31 / 31