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

jimmykiang
This test is broken right out of the box… — FAIL: TestAgent (7.82s) agent_test.go:77: Error Trace: agent_test.go:77 agent_test.go:...
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
HarryDeveloper
Hi @venkats, It has been mentioned in the description of ‘Supervisory Job’ title that 2 things as mentioned below result in the same eff...
New
leba0495
Hello! Thanks for the great book. I was attempting the Trie (chap 17) exercises and for number 4 the solution provided for the autocorre...
New
AndyDavis3416
@noelrappin Running the webpack dev server, I receive the following warning: ERROR in tsconfig.json TS18003: No inputs were found in c...
New
jgchristopher
“The ProductLive.Index template calls a helper function, live_component/3, that in turn calls on the modal component. ” Excerpt From: Br...
New
digitalbias
Title: Build a Weather Station with Elixir and Nerves: Problem connecting to Postgres with Grafana on (page 64) If you follow the defau...
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
New
andreheijstek
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

Other popular topics Top

Devtalk
Reading something? Working on something? Planning something? Changing jobs even!? If you’re up for sharing, please let us know what you’...
1052 21915 398
New
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
AstonJ
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
AstonJ
poll poll Be sure to check out @Dusty’s article posted here: An Introduction to Alternative Keyboard Layouts It’s one of the best write-...
New
AstonJ
I ended up cancelling my Moonlander order as I think it’s just going to be a bit too bulky for me. I think the Planck and the Preonic (o...
New
Exadra37
I am asking for any distro that only has the bare-bones to be able to get a shell in the server and then just install the packages as we ...
New
Margaret
Hello everyone! This thread is to tell you about what authors from The Pragmatic Bookshelf are writing on Medium.
1147 29994 760
New
mafinar
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
Help
I am trying to crate a game for the Nintendo switch, I wanted to use Java as I am comfortable with that programming language. Can you use...
New
PragmaticBookshelf
Develop, deploy, and debug BEAM applications using BEAMOps: a new paradigm that focuses on scalability, fault tolerance, and owning each ...
New

Sub Categories: