Sharing Code and
Experiences
@fabriziomello

About me

I was born in Dom Pedrito in the South of Brazil

But I lived in Bagé since I was 1 year old

Bagé is 236 miles away from Porto Alegre

Porto Alegre is the FISL city

In my hometown Bagé is common
people...
● … be born there
● … grow up there
● … build a family there
● … spend the entire life there
● … and die there.

Until 2001 this lifestyle
fitted with my “old” needs

But I met this pretty woman
and she changed my life

And now I’m a husband and father

Background (1/2)
● IT experience since 1993
○ Programming Languages (Basic, C, Clipper, Pascal,
PHP, Javascript, …)
○ Operating Systems (Windows “argh”, Unix and
Linux)
○ PostgreSQL, Firebird, MySQL, Oracle
○ Agile Methodologies (XP, Lean, Scrum, …)
○ …

Background (2/2)
● Bachelor in Information Systems in 2002
● Entrepeneur at http://timbira.com
● Agile Methodologies Specialization student 2014/2015
● PostgreSQL colaborator since 2008 (Brazilian
community and now the international too)

FOSS and me
● My first contact was using Linux in 1997
● I fell in love with this culture since then
● In 1999 I met PostgreSQL so since then I
knew this would be part of my life
● Because of this decision I had a lot of
troubles, including financial…
● But here I am :-)

Is a global program that
offers students stipends to
write code for open source
projects.
We have worked with the
open source community to
identify and fund exciting
projects for the upcoming
summer.

Connect students to
open source communities

GSoC and PostgreSQL
● Since 2006
● Cool projects
○ Fast GiST index build
○ New phpPgAdmin Plugin Architecture (brazilian)
○ pgAdmin database designer
○ Better indexing for ranges
○ Document collection Foreign-data Wrapper

And now my project ...
PostgreSQL 9.1 introduced a new kind of table
Unlogged Tables

What means “Unlogged”?
First we need to know what means “WAL”
PostgreSQL is Full-ACID and to guarantee data
integrity uses a standard method called
WAL (Write-Ahead Logging)

WAL (Write-Ahead Logging)
“In computer science, write-ahead logging (WAL) is a family
of techniques for providing atomicity and durability (two of
the ACID properties) in database systems.
In a system using WAL, all modifications are written to a log
before they are applied. Usually both redo and undo
information is stored in the log.”
http://en.wikipedia.org/wiki/Write-ahead_logging

Ok, and what means “Unlogged” ?
● Unlogged means that the data written in
these tables is not written to WAL.
● So it makes written really, really fast
compared to written into regular tables.

So I’ll use it to all of my tables...
● However you won’t want to do that, because
● They are neither crash-safe (an unlogged
table is automatically truncated after a crash
or unclean shutdown)
● And they are nor replicated using SR

But there are some cool use cases
● Speed ETL jobs
● Cache
● Session State
● Queues?!
● ...

And now we have the power to ...
● change from UNLOGGED to LOGGED
○ ALTER TABLE name SET LOGGED;
● change from LOGGED to UNLOGGED
○ ALTER TABLE name SET UNLOGGED;

Already committed
commit: f41872d0c1239d36ab03393c39ec0b70e9ee2a3c
author: Alvaro Herrera <alvherre@alvh.no-ip.org>
date: Fri, 22 Aug 2014 14:27:00 -0400
Implement ALTER TABLE .. SET LOGGED / UNLOGGED
This enables changing permanent (logged) tables to unlogged and
vice-versa.
(Docs for ALTER TABLE / SET TABLESPACE got shuffled in an order that
hopefully makes more sense than the original.)
Author: Fabrízio de Royes Mello
Reviewed by: Christoph Berg, Andres Freund, Thom Brown
Some tweaking by Álvaro Herrera

How it works
1. Acquire AcessExclusiveLock
2. Check dependencies
a. Cannot change temp tables
b. Check Foreign Keys
3. Change indexes “relpersistence”
4. Create new heap/toast with new relpersistence
5. Rewrite heap/toast
6. Rewrite indexes

New patch with refactoring
1. Acquire AcessExclusiveLock
2. Check dependencies
a. Cannot change temp tables
b. Check Foreign Keys
3. Create new heap/toast with new relpersistence
(pass down relpersistence to reindex_index)
4. Rewrite heap/toast
5. Rewrite indexes

Currently Caveats
● AccessExclusiveLock
● Rewrite datafiles

Future work
● Don’t rewrite datafiles when wal_level =
minimal
● Unlogged Indexes on Regular Tables
● Unlogged Materialized Views (was reverted
by Tom Lane because of the bad design)

Questions?

Special thanks to
● Stephen Frost (mentor)
● Josh Berkus and Thom Brown (organizers)
● Christoph Berg (patch review)
● Álvaro Herrera (patch review and commit)
● Maristela Kohlrausch de Andrade (my
english teacher)