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

jeffmcompsci
Title: Design and Build Great Web APIs - typo “https://company-atk.herokuapp.com/2258ie4t68jv” (page 19, third bullet in URL list) Typo:...
New
Alexandr
Hi everyone! There is an error on the page 71 in the book “Programming machine learning from coding to depp learning” P. Perrotta. You c...
New
alanq
This isn’t directly about the book contents so maybe not the right forum…but in some of the code apps (e.g. turbo/06) it sends a TURBO_ST...
New
jeremyhuiskamp
Title: Web Development with Clojure, Third Edition, vB17.0 (p9) The create table guestbook syntax suggested doesn’t seem to be accepted ...
New
brunogirin
When trying to run tox in parallel as explained on page 151, I got the following error: tox: error: argument -p/–parallel: expected one...
New
brunogirin
When running tox for the first time, I got the following error: ERROR: InterpreterNotFound: python3.10 I realised that I was running ...
New
hazardco
On page 78 the following code appears: <%= link_to ‘Destroy’, product, class: ‘hover:underline’, method: :delete, data: { confirm...
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
a.zampa
@mfazio23 I’m following the indications of the book and arriver ad chapter 10, but the app cannot be compiled due to an error in the Bas...
New
New

Other popular topics Top

DevotionGeo
I know that -t flag is used along with -i flag for getting an interactive shell. But I cannot digest what the man page for docker run com...
New
AstonJ
I’ve been hearing quite a lot of comments relating to the sound of a keyboard, with one of the most desirable of these called ‘thock’, he...
New
AstonJ
Saw this on TikTok of all places! :lol: Anyone heard of them before? Lite:
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
AstonJ
If you get Can't find emacs in your PATH when trying to install Doom Emacs on your Mac you… just… need to install Emacs first! :lol: bre...
New
PragmaticBookshelf
Author Spotlight Rebecca Skinner @RebeccaSkinner Welcome to our latest author spotlight, where we sit down with Rebecca Skinner, auth...
New
New
First poster: bot
zig/http.zig at 7cf2cbb33ef34c1d211135f56d30fe23b6cacd42 · ziglang/zig. General-purpose programming language and toolchain for maintaini...
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
PragmaticBookshelf
A concise guide to MySQL 9 database administration, covering fundamental concepts, techniques, and best practices. Neil Smyth MySQL...
New

Sub Categories: