Dubious Database Design

My talk from DjangoCon US 2015.

Transcript

1. DUBIOUS Database DESIGN
2. Andrew Godwin Hi, I'm Author of 1.7 Django & South migrations Senior Software Engineer at Only hates MySQL a little
3. “Do this. Don't ask why.”
4. Learning from failure.
5. Spacelog 1
6. Spacelog 1
7. “Redis is fast!”
9. Spacelog 1
10. Read-only, forever.
11. “Redis is fast!”
12. GET chapter-1 GET chapter-2
13. GET entry-123 GET entry-124 GET entry-125
14. ZRANGEBYSCORE .... GET entry-123 GET entry-124 GET entry-125
15. Request page Look up key range Multi-get key range Get speaker details
16. SELECT ... JOIN ... WHERE ...
17. Ignoring JOIN 2
18. “Joins are slow!”
19. {"id": 11, "post": "abc", "author": 1} {"id": 12, "post": "def", "author": 2} {"id": 13, "post", "ghi", "author": 3} {"id": 1, "name": "Andrew"} {"id": 2, "name": "Brenda"} {"id": 3, "name": "Carol"}
20. n number of authors m × number of posts
21. scan all posts build dict of author -> posts scan all authors and emit with posts
22. HASH JOIN
23. { "id": 11, "post":"abc", "author": {"name": "Andrew"} }
24. { "id": 11, "post":"abc", "author": { "name": "Andrew", "last_seen": 120993013, } }
25. The server's running, it's fine! 3
26. Write new save file Write new save file Delete old save file
27. Write new save file Write new save file ?
28. Tell payment processor to send Mark as processing Find unpaid clients Mark as paid
29. Tell payment processor to send Mark as processing Find unpaid clients Mark as paid
30. The Fastidious Modeller 4
31. TwitterUser FacebookUser LinkedInUser EmailUser
32. SELECT ... FROM TwitterUser SELECT ... FROM EmailUser SELECT ... FROM LinkedInUser
35. The database isn't magic.
36. The Table Lover 5
37. “How do I make tables at runtime?”
39. Tables/columns per language Tables/columns per customer Configurable CMS columns
40. Columns per language 300 - 400 language variants x A couple of translated cols per table x
41. DDL is very expensive.
42. Use JSON, hstore, or EAV-style table!
43. 6 The Cold Boot
44. Decent cache hit rate Application servers mostly utilised
45. Great engineering!
46. What would happen if I deleted the entire cache?
48. The Optimist 7
49. Sharded PostgreSQL ElasticSearch Riak Redis Flat files + + + +
50. Don't forget redundancy. And backups.
51. What happens if just one dies?
52. n services = n points of failure
53. The Primary Optimist 8
54. “The highest value PK is the most recent”
55. “Autoincrement will work and scale forever”
56. “IDs are numbers we can do maths on”
57. The Function Lover 9
58. "Why waste time fetching columns and rendering them separately?"
59. CREATE FUNCTION
60. CREATE FUNCTION ... import jinja2
61. bit.ly/whynotpg
63. SELECT render(template, id) FROM pages WHERE %s ~ url;
65. There's a reason behind every rule.
66. Ask why, or try yourself. Don't write it off without context.
67. Thanks. Andrew Godwin @andrewgodwin