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
page 37
ANTLRInputStream input = new ANTLRInputStream(is);
as of ANTLR 4 .8 should be:
CharStream stream = CharStreams.fromStream(i...
New
Title: Design and Build Great Web APIs - typo “https://company-atk.herokuapp.com/2258ie4t68jv” (page 19, third bullet in URL list)
Typo:...
New
Title: Hands-On Rust (Chapter 11: prefab)
Just played a couple of amulet-less games. With a bit of debugging, I believe that your can_p...
New
I can’t setup the Rails source code. This happens in a working directory containing multiple (postgres) Rails apps.
With:
ruby-3.0.0
s...
New
On the page xv there is an instruction to run bin/setup from the main folder. I downloaded the source code today (12/03/21) and can’t see...
New
In case this helps anyone, I’ve had issues setting up the rails source code. Here were the solutions:
In Gemfile, change
gem 'rails'
t...
New
The allprojects block listed on page 245 produces the following error when syncing gradle:
“org.gradle.api.GradleScriptException: A prob...
New
Title: Agile Web Development with Rails 7: (page 70)
I am running windows 11 pro with rails 7.0.3 and ruby 3.1.2p20 (2022-04-12 revision...
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
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
Stop developing web apps with yesterday’s tools. Today, developers are increasingly adopting Clojure as a web-development platform. See f...
New
Learn from the award-winning programming series that inspired the Elixir language, and go on a step-by-step journey through the most impo...
New
You might be thinking we should just ask who’s not using VSCode :joy: however there are some new additions in the space that might give V...
New
Tailwind CSS is an exciting new CSS framework that allows you to design your site by composing simple utility classes to create complex e...
New
Oh just spent so much time on this to discover now that RancherOS is in end of life but Rancher is refusing to mark the Github repo as su...
New
Think Again 50% Off Sale »
The theme of this sale is new perspectives on familiar topics.
Enter coupon code ThinkAgain2021 at checkout t...
New
Hello everyone! This thread is to tell you about what authors from The Pragmatic Bookshelf are writing on Medium.
New
Create efficient, elegant software tests in pytest, Python's most powerful testing framework.
Brian Okken @brianokken
Edited by Kat...
New
This is going to be a long an frequently posted thread.
While talking to a friend of mine who has taken data structure and algorithm cou...
New
If you want a quick and easy way to block any website on your Mac using Little Snitch simply…
File > New Rule:
And select Deny, O...
New
Categories:
Sub Categories:
Popular Portals
- /elixir
- /rust
- /wasm
- /ruby
- /erlang
- /phoenix
- /keyboards
- /python
- /js
- /rails
- /security
- /go
- /swift
- /vim
- /clojure
- /java
- /emacs
- /haskell
- /svelte
- /onivim
- /typescript
- /kotlin
- /c-plus-plus
- /crystal
- /tailwind
- /react
- /gleam
- /ocaml
- /elm
- /flutter
- /vscode
- /ash
- /html
- /opensuse
- /zig
- /centos
- /deepseek
- /php
- /scala
- /react-native
- /lisp
- /sublime-text
- /textmate
- /nixos
- /debian
- /agda
- /django
- /deno
- /kubuntu
- /arch-linux
- /nodejs
- /spring
- /ubuntu
- /revery
- /manjaro
- /diversity
- /julia
- /lua
- /markdown
- /quarkus









