douglasshuang

douglasshuang

High Performance PostgreSQL for Rails: Off-by-one bug in `SCRUB_BATCHES` procedure (page 65)

@andatki

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

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!

Also Liked

douglasshuang

douglasshuang

My pleasure, Thank you!

Where Next?

Popular Pragmatic Bookshelf topics Top

belgoros
Following the steps described in Chapter 6 of the book, I’m stuck with running the migration as described on page 84: bundle exec sequel...
New
ianwillie
Hello Brian, I have some problems with running the code in your book. I like the style of the book very much and I have learnt a lot as...
New
jamis
The following is cross-posted from the original Ray Tracer Challenge forum, from a post by garfieldnate. I’m cross-posting it so that the...
New
simonpeter
When I try the command to create a pair of migration files I get an error. user=> (create-migration "guestbook") Execution error (Ill...
New
joepstender
The generated iex result below should list products instead of product for the metadata. (page 67) iex> product = %Product{} %Pento....
New
brunogirin
When I run the coverage example to report on missing lines, I get: pytest --cov=cards --report=term-missing ch7 ERROR: usage: pytest [op...
New
New
AufHe
I’m a newbie to Rails 7 and have hit an issue with the bin/Dev script mentioned on pages 112-113. Iteration A1 - Seeing the list of prod...
New
ggerico
I got this error when executing the plot files on macOS Ventura 13.0.1 with Python 3.10.8 and matplotlib 3.6.1: programming_ML/code/03_...
New
redconfetti
Docker-Machine became part of the Docker Toolbox, which was deprecated in 2020, long after Docker Desktop supported Docker Engine nativel...
New

Other popular topics Top

ohm
Which, if any, games do you play? On what platform? I just bought (and completed) Minecraft Dungeons for my Nintendo Switch. Other than ...
New
siddhant3030
I’m thinking of buying a monitor that I can rotate to use as a vertical monitor? Also, I want to know if someone is using it for program...
New
DevotionGeo
The V Programming Language Simple language for building maintainable programs V is already mentioned couple of times in the forum, but I...
New
Margaret
Hello everyone! This thread is to tell you about what authors from The Pragmatic Bookshelf are writing on Medium.
1147 29841 760
New
AstonJ
Was just curious to see if any were around, found this one: I got 51/100: Not sure if it was meant to buy I am sure at times the b...
New
AstonJ
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
New
PragmaticBookshelf
Author Spotlight: Peter Ullrich @PJUllrich Data is at the core of every business, but it is useless if nobody can access and analyze ...
New
New
AstonJ
This is a very quick guide, you just need to: Download LM Studio: https://lmstudio.ai/ Click on search Type DeepSeek, then select the o...
New

Sub Categories: