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Mastering PostgreSQL 10

You're reading from  Mastering PostgreSQL 10

Product type Book
Published in Jan 2018
Publisher Packt
ISBN-13 9781788472296
Pages 428 pages
Edition 1st Edition
Languages

Table of Contents (15) Chapters

Preface PostgreSQL Overview Understanding Transactions and Locking Making Use of Indexes Handling Advanced SQL Log Files and System Statistics Optimizing Queries for Good Performance Writing Stored Procedures Managing PostgreSQL Security Handling Backup and Recovery Making Sense of Backups and Replication Deciding on Useful Extensions Troubleshooting PostgreSQL Migrating to PostgreSQL Other Books You May Enjoy

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 these 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...

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