P. 73 Example works with postgres

SQLite in memory errors out on the threading
examples, but Postgres does much better. It
still throws errors which need to be caught,
it also proceeds to a satisfactory conclusion.
That is, threading computes an incorrect final
balance.

I'm skipping the last 10 pages as the treatment
of race conditions doesn't have an exercises.
It's very open-ended, and I would need to create
some way to evaluate the execution. As it's not
"n+1" per se, I'll pick this up later when I
need to go deeper into race conditions.

P.73 SQLite3 doesn't handle concurrency well.

This is good example.

P. 67 finish counter caching and chapter 4

Good chapter, will be easy to extend and
go deeper.

Page 60 End Chapter 3.

Page 53 Load then count for efficiency

when the data will be needed anyway.

Page 51 Comments with length and size

Page 49 comments length

Page 49, load and count

Page 44, service class for predefining a query

Page 43, custom queries for posts and voters