Constraints: A Developer's Secret Weapon | PostgreSQL Conference EU 2018 | Will Leinweber
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Of all my regrets while building database-backed applications, not fully embracing database constraints sooner is one of the bigger ones. For far too long, I treated the db as a little more than a dumb key-value store with joins. Strong constraints turn your database into the last line of defense against bad data. Applications change several orders of magnitude more often than db schemas, and each change is a chance for corruption. Constraints in Postgres come in many forms, from literally using the CONSTRAINT keyword to a large choice of datatypes. You'll come away from this talk understanding the pros and cons of type of constraint, and be ready to transform your application into a safer and more resilient system.
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Constraints: A Developer's Secret Weapon | PostgreSQL Conference EU 2018 | Will Leinweber
- 4. INTRO
- 5. Will Leinweber @leinweber CONSTRAINTS maybe not the most exciting topic just want DB to safely store&retrieve data stern parent saying "No!"
- 6. Will Leinweber @leinweber RAILS changed the landscape before: spaghetti or mountains of xml after: convention instead of configuration
- 7. Will Leinweber @leinweber RAILS embrace constraints separate code concerns pluralize table names primary key is called "id" …etc
- 8. Will Leinweber @leinweber RAILS "big dumb hash in the sky"
- 9. Will Leinweber @leinweber MEANWHILE… learning how to make web applications
- 10. Will Leinweber @leinweber LEARNING 3NF examples hard to extrapolate to my problem
- 12. Will Leinweber @leinweber TODAY think about database schema first work backwards to models and api
- 13. Will Leinweber @leinweber MOTIVATION database is the last line of defense
- 14. Will Leinweber @leinweber MOTIVATION code change frequency >>> schema change frequency
- 15. Will Leinweber @leinweber MOTIVATION ~1 year old app 71 migrations 1203 releases after ~2 years 90 migrations 1454 releases
- 16. Will Leinweber @leinweber MOTIVATION logical corruption is more likely to come from app bug
- 17. Will Leinweber @leinweber BUGS bugs that can be caught by schema are particularly dangerous
- 18. Will Leinweber @leinweber BUGS delayed problem hard to find cause can last for months
- 20. Will Leinweber @leinweber MOTIVATION don't write bad data in the first place
- 22. Will Leinweber @leinweber TRADITIONAL CONSTRAINTS NOT NULL
- 23. Will Leinweber @leinweber TRADITIONAL CONSTRAINTS CREATE UNIQUE INDEX ON users (email);
- 24. Will Leinweber @leinweber PARTIAL UNIQUE CREATE UNIQUE INDEX ON users (email) WHERE deleted_at IS NULL;
- 25. Will Leinweber @leinweber ENFORCE ASSUMPTIONS "This property should always "Should never be…" be positive"be present"be between 0 and 1"be a fibonacci number"
- 26. DATATYPES
- 27. Will Leinweber @leinweber DATATYPES not often thought of as constraints constrain what type of data gets in
- 28. Will Leinweber @leinweber DATATYPES numbers are actually numbers booleans are actually booleans …etc
- 29. Will Leinweber @leinweber DATATYPES …other databases
- 30. Will Leinweber @leinweber ENUMS CREATE TYPE state AS ENUM ('creating', 'running'); ALTER TYPE state ADD VALUE 'deleting';
- 31. Will Leinweber @leinweber RANGES SELECT i, i <@ '[1,10)'::int4range included FROM (VALUES (1),(5),(10)) as v(i); i | included ----+---------- 1 | t 5 | t 10 | f
- 32. Will Leinweber @leinweber RANGES int4range, int8range, numrange tsrange, tstzrange daterange
- 34. Will Leinweber @leinweber SMALL OPTIMIZATION CREATE TABLE somename ( aws_id text COLLATE "C" NOT NULL ); -- i-0598c7d356eba48d7
- 35. Will Leinweber @leinweber OTHER TYPES uuid macaddr, inet, cidr array, hstore geometric
- 36. Will Leinweber @leinweber DATATYPES downside of using JSONB
- 37. FOREIGN KEYS
- 38. Will Leinweber @leinweber FOREIGN KEYS CREATE TABLE posts( user_id int NOT NULL REFERENCES users(id), …
- 39. Will Leinweber @leinweber OPTIONS REFERENCES foo ON DELETE/UPDATE NO ACTION / RESTRICT CASCADE SET NULL / DEFAULT
- 40. Will Leinweber @leinweber PROBLEMS WITH TESTING transaction vs. truncation/deleting SET CONSTRAINTS all DEFERRED;
- 41. CHECK
- 42. Will Leinweber @leinweber CHECK custom logic
- 43. Will Leinweber @leinweber ENSURE POSITIVE NUMBER CREATE TABLE products( name text, price int CHECK(price > 0) );
- 44. Will Leinweber @leinweber REFERENCE OTHER COLUMN CREATE TABLE products( name text, price int CHECK(price > 0), sale_p int CHECK(sale_p > 0), CHECK (price > sale_p) );
- 45. Will Leinweber @leinweber PERCENTS scale float DEFAULT 1.0 NOT NULL, CHECK (scale >= 0 & scale <= 1)
- 46. Will Leinweber @leinweber MORE CHECK ((json_col->>'i_prop')::int > 0)
- 47. Will Leinweber @leinweber USER DEFINED FUNCTIONS CREATE OR REPLACE FUNCTION is_fib(i int) RETURNS boolean AS $$ DECLARE a integer := 5*i*i+4; b integer := 5*i*i-4; asq integer := sqrt(a)::int; bsq integer := sqrt(b)::int; BEGIN RETURN asq*asq=a OR bsq*bsq=b; end $$ LANGUAGE plpgsql IMMUTABLE STRICT;
- 48. Will Leinweber @leinweber USER DEFINED FUNCTIONS # CREATE TABLE onlyfib( i int CHECK (is_fib(i)) ); CREATE TABLE # insert into onlyfib values (5), (8); INSERT 0 2 # insert into onlyfib values (6); ERROR: new row for relation "onlyfib" violates check constraint "onlyfib_i_check" DETAIL: Failing row contains (6).
- 49. Will Leinweber @leinweber DOMAINS # CREATE DOMAIN fib AS int CHECK (is_fib(VALUE)); # CREATE TABLE onlyfib(i fib); # insert into onlyfib values (5),(8); INSERT 0 2 # insert into onlyfib values (6); ERROR: value for domain fib violates check constraint "fib_check"
- 51. EXCLUSION
- 52. Will Leinweber @leinweber && OVERLAP select '[ 1,10)'::int4range && '[15,20)'::int4range; f select '[1,10)'::int4range && '[9,20)'::int4range; t
- 53. Will Leinweber @leinweber EXCLUSION CREATE TABLE billings ( formation_id uuid NOT NULL, validity_period tstzrange NOT NULL, price_per_month integer NOT NULL );
- 54. Will Leinweber @leinweber EXCLUSION ALTER TABLE billings ADD CONSTRAINT billings_excl EXCLUDE USING gist ( formation_id WITH =, validity_period WITH && );
- 55. Will Leinweber @leinweber ERROR MESSAGE ERROR: conflicting key value violates exclusion constraint "constraint name" DETAIL: Key (id, range)=(<new row>) conflicts with existing key (id, range)=(<existing row>).
- 56. RECAP
- 57. Will Leinweber @leinweber 3 TAKEAWAYS database is your last line of defense postgres has some really great constraints datatypes are constraints too