
douglasshuang
High Performance PostgreSQL for Rails: Off-by-one bug in `SCRUB_BATCHES` procedure (page 65)
The SCRUB_BATCHES
procedure has an off-by-one bug that causes it to skip one row after each batch. Here is output showing the incorrect behavior:
owner@localhost:5432 rideshare_development# CALL SCRUB_BATCHES();
NOTICE: current_id: 1 - Number of rows updated: 1000
NOTICE: current_id: 1002 - Number of rows updated: 1000
NOTICE: current_id: 2003 - Number of rows updated: 1000
NOTICE: current_id: 3004 - Number of rows updated: 1000
NOTICE: current_id: 4005 - Number of rows updated: 1000
NOTICE: current_id: 5006 - Number of rows updated: 1000
NOTICE: current_id: 6007 - Number of rows updated: 1000
NOTICE: current_id: 7008 - Number of rows updated: 1000
NOTICE: current_id: 8009 - Number of rows updated: 1000
NOTICE: current_id: 9010 - Number of rows updated: 1000
NOTICE: current_id: 10011 - Number of rows updated: 1000
NOTICE: current_id: 11012 - Number of rows updated: 1000
NOTICE: current_id: 12013 - Number of rows updated: 1000
NOTICE: current_id: 13014 - Number of rows updated: 1000
NOTICE: current_id: 14015 - Number of rows updated: 1000
NOTICE: current_id: 15016 - Number of rows updated: 1000
NOTICE: current_id: 16017 - Number of rows updated: 1000
NOTICE: current_id: 17018 - Number of rows updated: 1000
NOTICE: current_id: 18019 - Number of rows updated: 1000
NOTICE: current_id: 19020 - Number of rows updated: 1000
NOTICE: current_id: 20021 - Number of rows updated: 190
CALL
owner@localhost:5432 rideshare_development# SELECT id, email FROM users WHERE id IN (1000, 1001, 1002);
id | email
------+-----------------------------------------
1000 | 6ddd3da1abbacd649a2853d9@email.com
1001 | Jenee-Robel-driver-1000@email.com
1002 | efa11a15129ea1bed93f96@email.com
(3 rows)
The bug is in this assignment statement at the end of the loop body:
current_id := current_id + batch_size + 1;
It should read as follows:
current_id := current_id + batch_size;
Here is the output after the correction:
owner@localhost:5432 rideshare_development# CALL SCRUB_BATCHES();
NOTICE: current_id: 1 - Number of rows updated: 1000
NOTICE: current_id: 1001 - Number of rows updated: 1000
NOTICE: current_id: 2001 - Number of rows updated: 1000
NOTICE: current_id: 3001 - Number of rows updated: 1000
NOTICE: current_id: 4001 - Number of rows updated: 1000
NOTICE: current_id: 5001 - Number of rows updated: 1000
NOTICE: current_id: 6001 - Number of rows updated: 1000
NOTICE: current_id: 7001 - Number of rows updated: 1000
NOTICE: current_id: 8001 - Number of rows updated: 1000
NOTICE: current_id: 9001 - Number of rows updated: 1000
NOTICE: current_id: 10001 - Number of rows updated: 1000
NOTICE: current_id: 11001 - Number of rows updated: 1000
NOTICE: current_id: 12001 - Number of rows updated: 1000
NOTICE: current_id: 13001 - Number of rows updated: 1000
NOTICE: current_id: 14001 - Number of rows updated: 1000
NOTICE: current_id: 15001 - Number of rows updated: 1000
NOTICE: current_id: 16001 - Number of rows updated: 1000
NOTICE: current_id: 17001 - Number of rows updated: 1000
NOTICE: current_id: 18001 - Number of rows updated: 1000
NOTICE: current_id: 19001 - Number of rows updated: 1000
NOTICE: current_id: 20001 - Number of rows updated: 210
CALL
owner@localhost:5432 rideshare_development# SELECT id, email FROM users WHERE id IN (1000, 1001, 1002);
id | email
------+-----------------------------------------
1000 | 4b44cfd50568b6a9c54@email.com
1001 | fc7266ea448226ed25ccdc7f1@email.com
1002 | 6fdbe4620e31b4464b1b5fd95c64c@email.com
(3 rows)
Marked As Solved

andatki
Author of High Performance PostgreSQL for Rails
Hi @douglasshuang. Thank you for the detailed bug report. I was able to verify what you’re saying and agree that the last assignment within the loop should not add 1. I took that off as you suggested and the print output matches what you have.
I’m submitting a fix and hopefully it makes it into a future edition of the book. Thanks again for taking the time to notice this and prepare a detailed write-up!
1
Also Liked

douglasshuang
My pleasure, Thank you!
1
Popular Pragmatic Bookshelf topics

As per the title, thanks.
New

Running the examples in chapter 5 c under pytest 5.4.1 causes an AttributeError: ‘module’ object has no attribute ‘config’.
In particula...
New

Hi,
build fails on:
bracket-lib = “~0.8.1”
when running on Mac Mini M1 Rust version 1.5.0:
Compiling winit v0.22.2
error[E0308]: mi...
New

Page 28: It implements io.ReaderAt on the store type.
Sorry if it’s a dumb question but was the io.ReaderAt supposed to be io.ReadAt?
...
New

#book-python-testing-with-pytest-second-edition
Hi. Thanks for writing the book. I am just learning so this might just of been an issue ...
New

I’m running Android Studio “Arctic Fox” 2020.3.1 Patch 2, and I’m embarrassed to admit that I only made it to page 8 before running into ...
New

Modern Front-End Development for Rails - application does not start after run bin/setup (page xviii)
After some hassle, I was able to finally run bin/setup, now I have started the rails server but I get this error message right when I vis...
New

Modern front-end development for Rails, second edition - Struggling to get the first chapter to work
After running /bin/setup, the first error was: The foreman' command exists in these Ruby versions: That was easy to fix: gem install fore...
New

I just bought this book to learn about Android development, and I’m already running into a major issue in Ch. 1, p. 20: “Update activity...
New

From page 13:
On Python 3.7, you can install the libraries with pip by running these commands inside a Python venv using Visual Studio ...
New
Other popular topics

Reading something? Working on something? Planning something? Changing jobs even!?
If you’re up for sharing, please let us know what you’...
New

Which, if any, games do you play? On what platform?
I just bought (and completed) Minecraft Dungeons for my Nintendo Switch. Other than ...
New

No chair. I have a standing desk.
This post was split into a dedicated thread from our thread about chairs :slight_smile:
New

SpaceVim seems to be gaining in features and popularity and I just wondered how it compares with SpaceMacs in 2020 - anyone have any thou...
New

Curious to know which languages and frameworks you’re all thinking about learning next :upside_down_face:
Perhaps if there’s enough peop...
New

In case anyone else is wondering why Ruby 3 doesn’t show when you do asdf list-all ruby :man_facepalming: do this first:
asdf plugin-upd...
New

Use WebRTC to build web applications that stream media and data in real time directly from one user to another, all in the browser.
...
New

Author Spotlight
Mike Riley
@mriley
This month, we turn the spotlight on Mike Riley, author of Portable Python Projects. Mike’s book ...
New

This is cool!
DEEPSEEK-V3 ON M4 MAC: BLAZING FAST INFERENCE ON APPLE SILICON
We just witnessed something incredible: the largest open-s...
New

Fight complexity and reclaim the original spirit of agility by learning to simplify how you develop software. The result: a more humane a...
New
Categories:
Sub Categories:
Popular Portals
- /elixir
- /rust
- /ruby
- /wasm
- /erlang
- /phoenix
- /keyboards
- /rails
- /python
- /js
- /security
- /go
- /swift
- /vim
- /clojure
- /emacs
- /haskell
- /java
- /svelte
- /onivim
- /typescript
- /kotlin
- /crystal
- /c-plus-plus
- /tailwind
- /react
- /gleam
- /ocaml
- /flutter
- /elm
- /vscode
- /ash
- /opensuse
- /html
- /centos
- /php
- /deepseek
- /zig
- /scala
- /textmate
- /sublime-text
- /lisp
- /react-native
- /nixos
- /debian
- /agda
- /kubuntu
- /arch-linux
- /django
- /ubuntu
- /revery
- /deno
- /manjaro
- /spring
- /nodejs
- /diversity
- /lua
- /julia
- /slackware
- /c