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Mastering PostgreSQL 9.6
Mastering PostgreSQL 9.6

Mastering PostgreSQL 9.6: A comprehensive guide for PostgreSQL 9.6 developers and administrators

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Mastering PostgreSQL 9.6

Understanding Transactions and Locking

Locking is an important topic in any kind of database. It is not enough to understand just how it works to write proper or better applications; it is also essential from a performance point of view. Without properly handling locks, your applications might not only be slow, it might also be wrong and behave in an insane way. In my judgment, locking is key to performance and having a good overview will certainly help. Therefore, understanding locking and transaction is important for administrators and developers alike.

In this chapter, you will learn:

  • Basic locking
  • Transactions and transaction isolation
  • Deadlocks
  • Locking and foreign keys
  • Explicit and implicit locking
  • Advisory locks

At the end of the chapter, you will be able to understand and utilize PostgreSQL transactions in the most efficient way possible.

...

Working with PostgreSQL transactions

PostgreSQL provides you with a highly advanced transaction machinery that offers countless features to developers and administrators alike. In this section, it is time to look at the basic concept.

The first important thing to know is this: in PostgreSQL, everything is a transaction. If you send a simple query to the server, it is already a transaction. Here is an example:

test=# SELECT now(), now();
now | now
------------------------------+------------------------------
2016-08-30 12:03:27.84596+02 | 2016-08-30 12:03:27.84596+02
(1 row)

In this case, the SELECT statement will be a separate transaction. If the same command is executed again, different timestamps will be returned.

Keep in mind that the now() function will return the transaction time. The SELECT statement will, therefore, always return two identical timestamps...

Understanding basic locking

In this section, you will learn about basic locking mechanisms. The goal is to make you understand how locking works in general and how to get simple applications right.

To show how things work, a simple table can be created. For demonstration purposes, I will add one row to the table:

test=# CREATE TABLE t_test (id int); 
CREATE TABLE
test=# INSERT INTO t_test VALUES (1);
INSERT 0 1

The first important thing is that tables can be read concurrently. Many users reading the same data at the same time won't block each other. This allows PostgreSQL to handle thousands of users without problems.

Multiple users can read the same data at the same time without blocking each other.

The question now is: what happens if reads and writes occur at the same time? Here is an example:

Transaction 1

Transaction 2

BEGIN;

BEGIN;

UPDATE t_test SET id = id + 1 RETURNING *;
User will...

Making use of FOR SHARE and FOR UPDATE

Sometimes, data is selected from the database, then some processing happens in the application and finally some changes are made back on the database side. This is a classic example of SELECT FOR UPDATE.

Here is an example:

BEGIN; 
SELECT * FROM invoice WHERE processed = false;
** application magic will happen here **
UPDATE invoice SET processed = true ...
COMMIT;

The problem here is that two people might select the same unprocessed data. Changes made to those processed rows will then be overwritten. In short, a race condition will occur.

To solve this problem, developers can make use of SELECT FOR UPDATE. Here is how it works:

BEGIN; 
SELECT * FROM invoice WHERE processed = false FOR UPDATE;
** application magic will happen here **
UPDATE invoice SET processed = true ...
COMMIT;

The SELECT FOR UPDATE will lock rows just like an UPDATE would. This means that no changes can...

Understanding transaction isolation levels

Up to now, you have seen how to handle locking as well as some basic concurrency. In this section, you will learn about transaction isolation. To me, this is one of the most neglected topics in modern software development. Only a small fraction of software developers are actually aware of this issue, which in turn leads to disgusting and mind-boggling bugs.

Here is an example of what can happen:

Transaction 1

Transaction 2

BEGIN;

SELECT sum(balance) FROM t_account;

User will see 300

BEGIN;

INSERT INTO t_account (balance) VALUES (100);

COMMIT;

SELECT sum(balance) FROM t_account;

User will see 400

COMMIT;

Most users would actually expect the left transaction to always return 300 regardless of the second transaction. However, this is not true. By default, PostgreSQL runs in READ COMMITTED transaction isolation mode. This means...

Observing deadlocks and similar issues

Deadlocks are an important issue and can happen in every database I am aware of. Basically, a deadlock will happen if two transactions have to wait on each other.

In this section, you will see how this can happen. Let us suppose we have a table containing two rows:

CREATE TABLE t_deadlock (id int);
INSERT INTO t_deadlock VALUES (1), (2);

The next listing shows what can happen:

Transaction 1

Transaction 2

BEGIN;

BEGIN;

UPDATE t_deadlock SET id = id * 10 WHERE id = 1;

UPDATE t_deadlock SET id = id * 10 WHERE id = 2;

UPDATE t_deadlock SET id = id * 10 WHERE id = 2;

Waiting on transaction 2

UPDATE t_deadlock SET id = id * 10 WHERE id = 1;

Waiting on transaction 2

Waiting on transaction 1

Deadlock will be resolved after one second (deadlock_timeout)

COMMIT;

ROLLBACK;

As soon as the deadlock is detected, the following error message...

Utilizing advisory locks

PostgreSQL has a highly efficient and sophisticated transaction machinery that is capable of handling locks in a really fine grained and efficient way. Some years ago, some people came up with the idea of using this code to synchronize applications with each other. Thus, advisory locks were born.

When using advisory locks, it is important to mention that they won't go away on COMMIT as normal locks do. Therefore, it is really important to make sure that unlocking is done properly and in a totally reliable way.

If you decide to use an advisory lock, what you really lock is a number. So this is not about rows or data: it is really just a number. Here is how it works:

Session 1

Session 2

BEGIN;

SELECT pg_advisory_lock(15);

SELECT pg_advisory_lock(15);

It has to wait

COMMIT;

It still has to wait

SELECT pg_advisory_unlock(15);

It still waiting

Lock...

Optimizing storage and managing cleanup

Transactions are an integral part of the PostgreSQL system. However, transactions come with a small price tag attached. As already shown in this chapter, it can happen that concurrent users will be presented with different data. Not everybody will get the same data returned by a query. In addition to that, DELETE and UPDATE are not allowed to actually overwrite data as ROLLBACK would not work. If you happen to be in the middle of a large DELETE operation, you cannot be sure whether you will be able to COMMIT or not. In addition to that, data is still visible while you do a DELETE, and sometimes data is even visible once your modification has long since finished.

Consequently, this means that cleanup has to happen asynchronously. A transaction can not clean up its own mess and COMMIT/ROLLBACK might be too early to take care of dead rows.

The solution to the problem is VACUUM...

Summary

In this chapter, you learned about transactions, locking and its logical implications, and the general architecture the PostgreSQL transaction machinery can have for storage, concurrency, as well as administration. You saw how rows are locked and which features are present.

In the next chapter, you will learn one of the most important topics in database work: indexing. You will learn about the PostgreSQL query optimizer as well as various types of index and their behavior.

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Key benefits

  • Your one-stop guide to mastering the advanced concepts in PostgreSQL with ease
  • Master query optimization, replication, and high availability with PostgreSQL
  • Extend the functionalities of PostgreSQL to suit your organizational needs with minimum effort

Description

PostgreSQL is an open source database used for handling large datasets (Big Data) and as a JSON document database. It also has applications in the software and web domains. This book will enable you to build better PostgreSQL applications and administer databases more efficiently. We begin by explaining the advanced database design concepts in PostgreSQL 9.6, along with indexing and query optimization. You will also see how to work with event triggers and perform concurrent transactions and table partitioning, along with exploring SQL and server tuning. We will walk you through implementing advanced administrative tasks such as server maintenance and monitoring, replication, recovery and high availability, and much more. You will understand the common and not-so-common troubleshooting problems and how you can overcome them. By the end of this book, you will have an expert-level command of the advanced database functionalities and will be able to implement advanced administrative tasks with PostgreSQL.

Who is this book for?

If you are a PostgreSQL data architect or an administrator who wants to understand how to implement advanced functionalities and master complex administrative tasks with PostgreSQL, then this book is perfect for you. Prior experience of administrating a PostgreSQL database and a working knowledge of SQL is required to make the best use of this book.

What you will learn

  • Get to grips with the advanced features of PostgreSQL 9.6 and handle advanced SQL
  • Make use of the indexing features in PostgreSQL and fine-tune the performance of your queries
  • Work with the stored procedures and manage backup and recovery
  • Master the replication and failover techniques
  • Troubleshoot your PostgreSQL instance for solutions to the common and not-so-common problems
  • Learn how to migrate your database from MySQL and Oracle to PostgreSQL without any hassle
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Language : English
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Frequently bought together


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Total AU$ 258.97
PostgreSQL Administration Cookbook, 9.5/9.6 Edition
AU$90.99
Mastering PostgreSQL 9.6
AU$83.99
PostgreSQL High Availability Cookbook
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Total AU$ 258.97 Stars icon
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Table of Contents

13 Chapters
PostgreSQL Overview Chevron down icon Chevron up icon
Understanding Transactions and Locking Chevron down icon Chevron up icon
Making Use of Indexes Chevron down icon Chevron up icon
Handling Advanced SQL Chevron down icon Chevron up icon
Log Files and System Statistics Chevron down icon Chevron up icon
Optimizing Queries for Good Performance Chevron down icon Chevron up icon
Writing Stored Procedures Chevron down icon Chevron up icon
Managing PostgreSQL Security Chevron down icon Chevron up icon
Handling Backup and Recovery Chevron down icon Chevron up icon
Making Sense of Backups and Replication Chevron down icon Chevron up icon
Deciding on Useful Extensions Chevron down icon Chevron up icon
Troubleshooting PostgreSQL Chevron down icon Chevron up icon
Migrating to PostgreSQL Chevron down icon Chevron up icon

Customer reviews

Rating distribution
Full star icon Full star icon Half star icon Empty star icon Empty star icon 2.7
(3 Ratings)
5 star 33.3%
4 star 0%
3 star 0%
2 star 33.3%
1 star 33.3%
Joao FirJur Apr 01, 2019
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Gostei da qualidade geral do livro, bem como do fato de ser uma obra bem completa - do meu ponto-de-vista.
Amazon Verified review Amazon
PK Jan 07, 2018
Full star icon Full star icon Empty star icon Empty star icon Empty star icon 2
My main gripe with this book isn’t that it’s entirely useless – it isn’t – but rather that it doesn’t come close to being what it claims to be. Whatever it is, it certainly isn’t ‘a comprehensive guide for PostgreSQL developers and administrators’. Nor is it likely to help anyone ‘Master' PostgreSQL 9.6 (let alone the up-to-date version of the application). Instead, the book is a bunch of tips, some of which might be of value, others not so much. Rather than covering everything systematically, the author has written about those areas which he knows something about and has left the rest out.For example, the chapter ‘Writing Stored Procedures’ doesn’t discuss PL/V8 or PL/R – except to say that they are ‘beyond the scope of this chapter’. Clearly, they are relevant and should have been covered. Material on PL/V8 would have been particularly helpful, since this is not mentioned in any of the (five) other Packt books on PostgreSQL which I have. The GitHub page gives no help at all with installation on Windows, and documentation beyond that is scarce.The rest of the chapter on stored procedures rambles aimlessly through a few features of PL/pgSQL and touches on PL/Python and PL/Perl. In the few examples that were given, keywords are used without introduction, and the code is explained barely or not at all. Another irritant of this chapter is the constant and detailed references to Oracle code. The author is obviously familiar with Oracle: later in the book, he devotes several pages to migrating databases from Oracle and MySQL – but nothing about migrating from MS SQL. Getting up to speed with MS SQL would presumably have taken a bit more trouble than he was prepared to put in.Other parts of the book are equally sloppy and incomplete. For example, the discussion on pgpass on p.268 mentions that it ‘can be found in the %APPDATA%postgresqlpgpass.config path’. Since this isn’t a path, it’s obviously not there. It’s actually in the file (not the path) %APPDATA%\postgresql\pgpass.conf. Given that this is in the official documentation, there's no excuse for not checking it before bunging it in the book and causing frustration to his readers. Some sources say that the file should be in UNIX format. It's precisely this kind of detail which the author of a 'comprehensive' guide to PostgreSQL should get right. The password problem is a big one for Windows users, because PSQL defaults to the currently-logged-in user, who might not even have a password.The book doesn’t claim to be about PostgreSQL 10. However, PG 10 is mentioned in the text, showing that it was available at the time of writing. For example, the section on WAL vaguely mentions the idea of ‘tuning’ the transaction log in PG 10. Unfortunately it fails to mention that it’s been moved to a different directory in this version. Again, information which would be genuinely useful is simply missing.I haven’t read through the entire book yet, and it’s possible that there are some gems. What I have found, however, is superficial and chaotic. There are also endless typos and grammatical mistakes throughout the book, as is usual for Packt Publishing.Frankly, I am tired of computer books written by authors who don't care and published by publishing companies who care even less. This book cost $55 and was barely worth half that.
Amazon Verified review Amazon
Olivier Dec 11, 2017
Full star icon Empty star icon Empty star icon Empty star icon Empty star icon 1
J'espèrai avoir des explications plus détaillées que ce que l'ont trouve sur internet. Hélas, il ne m'a pas été possible de parcourir ce livre car la version kindle ne présente que peu de pages et je me rends compte que le livre présente chaque fonction avec un rapide exemple très simple, sans plus avant de passer à la fonction suivante. Je regrette de ne pouvoir le rendre.
Amazon Verified review Amazon
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