© Laurenz Albe 2023 The PostgreSQL documentation has some information about row locks. But typically, you cannot see them in pg_locks, and not everybody knows how they work and how to track and debug row locks. This article intends to give some insight into how PostgreSQL row locks work “under the hood”. Why are there […]

Stored procedures are a core concept which can be found in most relational database systems. They have proven to be useful in many areas and have been widely adopted by developers and DBA’s alike. Stored procedures basics In PostgreSQL stored procedures have been around for a number of years. The syntax of this important command […]

What is LISTEN and NOTIFY? LISTEN / NOTIFY is a feature that enables users to listen to what goes on in the database. It is one of the oldest functionalities in PostgreSQL and is still widely used. The main question is: What is the purpose of the asynchronous query interface (LISTEN / NOTIFY), and what […]

Sequences are a core feature of SQL. However, some users are tempted to implement sequences to generate invoices. That’s dangerous and should be avoided. The core question is: Why? What’s the problem with using database-side sequences to put unique invoice numbers to send to clients? Let’s dive in and find out. Getting started with CREATE […]

© Laurenz Albe 2022 I was recently confronted with a nice example of how adding FOR UPDATE to a query can introduce transaction anomalies. This article will explain how that happens and how you can cope with the problem. Be ready to learn some PostgreSQL implementation details! The example Setting the stage We have a […]

© Laurenz Albe 2021 Both cursors and transactions are basic ingredients for developing a database application. This article describes how cursors and transactions interact and how WITH HOLD can work around their limitations. We will also see the dangers involved and how to properly use WITH HOLD cursors in a PL/pgSQL procedure. Cursors in PostgreSQL […]

PostgreSQL is one of the best OLTP databases (OLTP = online transaction processing) in the world. However, it can do more than just OLTP. PostgreSQL offers many additional features relevant to a more OLAP-style workload. One of those features is called “GROUPING SETS”.  Before we dive into the details, I’ve compiled some sample data which […]

Our PostgreSQL blog about “Speeding up count(*)” was widely read and discussed by our followers on the internet. We also saw some people commenting on the post and suggesting using different means to speed up count(*). I want to specifically focus on one of those comments and to warn our readers. max(id) – min(id) will […]

© Laurenz Albe 2020   (Last updated on 2023-01-24) Recently, while troubleshooting PostgreSQL performance problems, I ran into problems with subtransactions twice. So I thought this was a nice topic for another blog post. What are subtransactions? Everybody knows database transactions. In PostgreSQL, which is operating in autocommit mode, you have to start a transaction […]

BY Kaarel Moppel – Having recently witnessed quite an ingenious hack to implement some good old “println” style development debugging from stored procedures into a file, it prompted me to post knowledge to the Interwebs on two other ways how such a goal can be implemented more transparently. Also, with help of some other good […]

Prepared transactions are disabled in PostgreSQL by default, since the parameter max_prepared_transactions has the default value 0. You don’t need prepared transactions in most cases. However, they can cause nasty problems, so I think that everybody who runs a PostgreSQL database should understand them. To illustrate these problems, I’ll show you how to use prepared […]

xmax is a PostgreSQL system column that is used to implement Multiversion Concurrency Control (MVCC). The documentation is somewhat terse: The identity (transaction ID) of the deleting transaction, or zero for an undeleted row version. It is possible for this column to be nonzero in a visible row version. That usually indicates that the deleting […]