1.
PostgreSQL worst practices
at FOSDEM PGDay Brussels 2017
Ilya Kosmodemiansky
ik@postgresql-consulting.com

2.
Best practices are just boring
• Never follow them, try worst practices
• Only those practices can really help you to screw the things up
most effectively
• PostgreSQL consultants are nice people, so try to make them
happy

3.
How it works?
• I have a list, a little bit more than 100 worst practices
• I do not make this stuff up, all of them are real-life examples
• I reshuffle my list every time before presenting and extract
some amount of examples
• Well, there are some things, which I like more or less, so it is
not a very honest shuffle

4.
0. Do not use indexes (a test one!)
• Basically, there is no difference between full table scan and
index scan
• You can check that. Just insert 10 rows into a test table on
your test server and compare.
• Nobody deals with more than 10 row tables in production!

5.
1. Use ORM
• All databases share the same syntax
• You must write database-independent code
• Are there any benefits, which are based on database specific
features?
• It always good to learn a new complicated technology

6.
2. Move joins to your application
• Just select * a couple of tables into the application written in
your favorite programming language
• Than join them at the application level

7.
2. Move joins to your application
• Just select * a couple of tables into the application written in
your favorite programming language
• Than join them at the application level
• Now you only need to implement nested loop join, hash join
and merge join as well as query optimizer and page cache

8.
3. Be in trend, be schema-less
• You do not need to design the schema
• You need only one table, two columns: id bigserial and extra
jsonb
• JSONB datatype is pretty effective in PostgreSQL, you can
search in it just like in a well-structured table
• Even if you put a 100M of JSON in it
• Even if you have 1000+ tps

9.
4. Be agile, use EAV
• You need only 3 tables: entity, attribute, value

10.
4. Be agile, use EAV
• You need only 3 tables: entity, attribute, value
• At some point add the 4th: attribute_type

11.
4. Be agile, use EAV
• You need only 3 tables: entity, attribute, value
• At some point add the 4th: attribute_type
• Whet it starts to work slow, just call those four tables The
Core and add 1000+ tables with denormalized data

12.
4. Be agile, use EAV
• You need only 3 tables: entity, attribute, value
• At some point add the 4th: attribute_type
• Whet it starts to work slow, just call those four tables The
Core and add 1000+ tables with denormalized data
• If it is not enough, you can always add value_version

13.
5. Try to create as many indexes as you can
• Indexes consume no disk space
• Indexes consume no shared_bufers
• There is no overhead on DML if one and every column in a
table covered with bunch of indexes
• Optimizer will definitely choose your index once you created it
• Keep calm and create more indexes

14.
6. Always keep all your time series data
• Time series data like tables with logs or session history should
be never deleted, aggregated or archived, you always need to
keep it all

15.
6. Always keep all your time series data
• Time series data like tables with logs or session history should
be never deleted, aggregated or archived, you always need to
keep it all
• You will always know where to check, if you run out of disk
space

16.
6. Always keep all your time series data
• Time series data like tables with logs or session history should
be never deleted, aggregated or archived, you always need to
keep it all
• You will always know where to check, if you run out of disk
space
• You can always call that Big Data

17.
6. Always keep all your time series data
• Time series data like tables with logs or session history should
be never deleted, aggregated or archived, you always need to
keep it all
• You will always know where to check, if you run out of disk
space
• You can always call that Big Data
• Solve the problem using partitioning... one partition for an
hour or for a minute

18.
7. Turn autovacuum off
• It is quite auxiliary process, you can easily stop it
• There is no problem at all to have 100Gb data in a database
which is 1Tb in size
• 2-3Tb RAM servers are cheap, IO is a fastest thing in modern
computing
• Besides of that, everyone likes BigData

19.
8. Keep master and slave on different hardware
• That will maximize the possibility of unsuccessful failover

20.
8. Keep master and slave on different hardware
• That will maximize the possibility of unsuccessful failover
• To make things worser, you can change only slave-related
parameters at slave, leaving defaults for shared_buffers etc.

21.
9. Put a synchronous replica to remote DC
• Indeed! That will maximize availability!

22.
9. Put a synchronous replica to remote DC
• Indeed! That will maximize availability!
• Especially, if you put the replica to another continent

23.
10. Reinvent Slony
• If you need some data replication to another database, try to
implement it from scratch

24.
10. Reinvent Slony
• If you need some data replication to another database, try to
implement it from scratch
• That allows you to run into all problems, PostgreSQL have
had since introducing Slony

25.
11. Use as many count(*) as you can
• Figure 301083021830123921 is very informative for the end
user
• If it changes in a second to 30108302894839434020, it is still
informative
• select count(*) from sometable is a quite light-weighted query
• Tuple estimation from pg_catalog can never be precise
enough for you

26.
12. Never use graphical monitoring
• You do not need graphs
• Because it is an easy task to guess what was happened
yesterday at 2 a.m. using command line and grep only

27.
13. Never use Foreign Keys
(Use local produced instead!)
• Consistency control at application level always works as
expected
• You will never get data inconsistency without constraints
• Even if you already have a bullet proof framework to maintain
consistency, could it be good enough reason to use it?

28.
14. Always use text type for all columns
• It is always fun to reimplement date or ip validation in your
code
• You will never mistakenly convert ”12-31-2015 03:01AM” to
”15:01 12 of undef 2015” using text fields

29.
15. Always use improved ”PostgreSQL”
• Postgres is not a perfect database and you are smart
• All that annoying MVCC staff, 32 bit xid and autovacuum
nightmare look like they look because hackers are oldschool
and lazy
• Hack it in a hard way, do not bother yourself with submitting
your patch to the community, just put it into production
• It is easy to maintain such production and keep it compatible
with ”not perfect” PostgreSQL upcoming versions

30.
16. Postgres likes long transactions
• Always call external services from stored procedures (like
sending emails)

31.
16. Postgres likes long transactions
• Always call external services from stored procedures (like
sending emails)
• Oh, it is arguable... It can be, if 100% of developers were
familiar with word timeout

32.
16. Postgres likes long transactions
• Always call external services from stored procedures (like
sending emails)
• Oh, it is arguable... It can be, if 100% of developers were
familiar with word timeout
• Anyway, you can just start transaction and go away for
weekend

33.
17. Load your data to PostgreSQL in a smart manner
• Write your own loader, 100 parallel threads minimum

34.
17. Load your data to PostgreSQL in a smart manner
• Write your own loader, 100 parallel threads minimum
• Never use COPY - it is specially designed for the task

35.
18. Even if you want to backup your database...
• Use replication instead of backup

36.
18. Even if you want to backup your database...
• Use replication instead of backup
• Use pg_dump instead of backup

37.
18. Even if you want to backup your database...
• Use replication instead of backup
• Use pg_dump instead of backup
• Write your own backup script

38.
18. Even if you want to backup your database...
• Use replication instead of backup
• Use pg_dump instead of backup
• Write your own backup script
• As complicated as possible, combine all external tools you
know

39.
18. Even if you want to backup your database...
• Use replication instead of backup
• Use pg_dump instead of backup
• Write your own backup script
• As complicated as possible, combine all external tools you
know
• Never perform a test recovery

40.
Do not forget
That was WORST practice talk

41.
Questions or ideas? Share your story!
ik@postgresql-consulting.com
(I’am preparing this talk to be open sourced)