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

sdmoralesma
Title: Web Development with Clojure, Third Edition - migrations/create not working: p159 When I execute the command: user=> (create-...
New
raul
Hi Travis! Thank you for the cool book! :slight_smile: I made a list of issues and thought I could post them chapter by chapter. I’m rev...
New
joepstender
The generated iex result below should list products instead of product for the metadata. (page 67) iex> product = %Product{} %Pento....
New
cro
I am working on the “Your Turn” for chapter one and building out the restart button talked about on page 27. It recommends looking into ...
New
leonW
I ran this command after installing the sample application: $ cards add do something --owner Brian And got a file not found error: Fil...
New
patoncrispy
I’m new to Rust and am using this book to learn more as well as to feed my interest in game dev. I’ve just finished the flappy dragon exa...
New
hgkjshegfskef
The test is as follows: Scenario: Intersecting a scaled sphere with a ray Given r ← ray(point(0, 0, -5), vector(0, 0, 1)) And s ← sphere...
New
Charles
In general, the book isn’t yet updated for Phoenix version 1.6. On page 18 of the book, the authors indicate that an auto generated of ro...
New
akraut
The markup used to display the uploaded image results in a Phoenix.LiveView.HTMLTokenizer.ParseError error. lib/pento_web/live/product_l...
New
kolossal
Hi, I need some help, I’m new to rust and was learning through your book. but I got stuck at the last stage of distribution. Whenever I t...
New

Other popular topics Top

AstonJ
If it’s a mechanical keyboard, which switches do you have? Would you recommend it? Why? What will your next keyboard be? Pics always w...
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
PragmaticBookshelf
Rails 7 completely redefines what it means to produce fantastic user experiences and provides a way to achieve all the benefits of single...
New
PragmaticBookshelf
Build efficient applications that exploit the unique benefits of a pure functional language, learning from an engineer who uses Haskell t...
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
AstonJ
If you’re getting errors like this: psql: error: connection to server on socket “/tmp/.s.PGSQL.5432” failed: No such file or directory ...
New
AnfaengerAlex
Hello, I’m a beginner in Android development and I’m facing an issue with my project setup. In my build.gradle.kts file, I have the foll...
New
PragmaticBookshelf
Explore the power of Ash Framework by modeling and building the domain for a real-world web application. Rebecca Le @sevenseacat and ...
New

Sub Categories: