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Category: Internals

Comparing Oracle and PostgreSQL Physical Architectures

Published 2025-06-16 by Kevin Feasel

Kellyn Gorman continues a series on learning PostgreSQL for Oracle DBAs:

In the previous post, I covered some high-level areas around installation and architecture, but for this post, we’re going to go a little deeper.  For the seasoned Oracle DBA, this should feel like we’re stepping into a familiar landscape with just a few different rules. While both PostgreSQL and Oracle Database are robust, feature-rich systems, their physical architecture and internal mechanics diverge in key areas, especially around storage structures, memory architecture, and background processing.

In this post, we’ll break down these differences so Oracle DBAs can feel more comfortable with the shift when they transition between the two.

Click through to see how the two differ.

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Optimizing SQL Server via Indirect Checkpoints

Published 2025-06-09 by Kevin Feasel

Jon Russell covers a quiet feature:

A checkpoint is a background process that writes dirty pages to disk. A checkpoint performs a full scan of the pages in the buffer pool, lists all the dirty pages that are yet to be written to disk, and finally writes those pages to disk. In SQL instances that do not have many dirty pages in the buffer pool, this is a trivial operation. However, with SQL instances that have OLTP databases, use more memory and/or involve sequential scanning of all pages, the performance of the system could be impacted.

With SQL Server 2012, indirect checkpoints were introduced. In this case, the dirty page manager manages the dirty page list and keeps track of all the dirty page modifiers of the database. By default, it runs every 60 seconds and tracks the dirty pages that need to be flushed.

Read on to learn more about why indirect checkpointing exists, the kinds of capabilities it offers, and the extent to which you might want to tweak its settings.

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New Objects in SQL Server 2025

Published 2025-05-21 by Kevin Feasel

Brent Ozar does a bit of spelunking:

The first public preview of SQL Server 2025 dropped yesterday, and here’s what’s new under the hood as compared to today’s SQL Server 2022 CU19. Forgive me – this is quite a lengthy post, and it’s mostly a data dump. There are new stored procedures, extended stored procs, views, feature switches, new columns in existing tables, messages, and more.

I suspect that the contents of the new undocumented sys.dm_feature_switches table alone will get y’all really curious because there are 5,910 lines in there (included in this post) that suggest all kinds of wild things Microsoft is currently working on, but hasn’t announced publicly yet.

Read on for the very long list.

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Data Recovery in SQL Server without a Backup

Published 2025-05-13 by Kevin Feasel

Rodrigo Riberio Gomes digs in:

In more than 10 years of experience, I have dealt with cases where someone has performed incorrect operations on a table, such as updating or deleting wrong rows, in a SQL Server database that does not have full backups available. There are multiple reasons for no full backup: corrupted backups, taking too much time to restore, etc.

In this post, I want to show an alternative for these cases, an ace up one’s sleeve, that you can use to recover data. This method also provides a deep understanding of the internal workings of how your SQL Server stores data. So, in addition to learning how to recover data, you will gain more insights into the internals of SQL.

Read on to see how. Rodrigo also points out some limitations or things that would need to change if you have index compression. I consider this a very neat thing you might need to know but never want to use.

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A Dive into Oracle Memory

Published 2025-04-18 by Kevin Feasel

Kellyn Gorman remembers:

When describing Oracle features to folks that are new to the platform or coming from other database platforms, I found it’s best to keep it simple, but take on the important aspects of the technology.  I’m going to take on a few more posts on Oracle internals from the perspective of the individual new to Oracle.

To start, I’ve been having some long discussions, as well as documenting how Oracle memory works and how we monitor it when you don’t have the diagnostic and tuning pack at your disposal.  With the exclusion of these management packs, you have to be intentional in your queries to provide the data, without violating the licensing by using the %_HIST_% views.

Click through for an overview of how Oracle makes use of memory and how you can monitor this usage.

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Understanding Multi-Version Concurrency Control in PostgreSQL

Published 2025-04-03 by Kevin Feasel

Grant Fritchey explains a mechanism:

Let me start by giving you the short version of what MVCC is, and then the rest of the article explains more details. Basically, PostgreSQL is focused on ensuring, as much as possible, that reads don’t block writes and writes don’t block reads. This is done by always, only, inserting rows (tuples). No updates to an existing row. No actual deletes or updates. Instead, it uses a logical delete mechanism, which we’ll get into. This means that data in motion doesn’t interfere with data at rest, meaning a write doesn’t interfere with a read, therefore, less contention & blocking. There’s a lot to how all that works, so let’s get into it.

Click through for the dive. The pattern for MVCC is interesting, though quite different from pretty much any other implementation of concurrency management in a relational database system.

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Identifying an Object Name from a Wait Resource

Published 2025-03-18 by Kevin Feasel

Haripriya Naidu wants to know what object this is:

You run a query to check for locking or blocking in SSMS and find a wait resource in the format (8:1:3610). To identify the object name, you would typically run multiple queries, first find database name, turn on trace flag 3604, then find object id from DBCC PAGE and then find object name from sys.objects.
However, with a new function “sys.dm_db_page_info” introduced in SQL Server 2019, you no longer need to go through these steps. Instead, you can run a single query to get the object name directly.

Read on to see how it all works. This is definitely a lot easier than in the olden days.

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Reviewing Azure SQL DB Internals for Hints at SQL Server 2025

Published 2025-03-14 by Kevin Feasel

Brent Ozar does some digging:

Microsoft staff used to say that Azure SQL DB is “the next version” of the boxed product, meaning that Microsoft tested, tweaked, and proved new features in the cloud while they could still rapidly iterate over code, shipping updates to make the product more reliable and scalable. In practice, that’s not entirely true: some things never leave the cloud, and some things actually ship to SQL Server long before they’re available up in Azure SQL DB.

Today, we’re going to go through documented & undocumented system objects that are available up in the cloud today (March 2025), but keep in mind that some of these may be cloud-only. I’m just dumping out the list of Azure SQL DB objects that aren’t present in SQL Server 2022 CU17.

Read on for those sets, with the caveat that not all of this may necessarily be in SQL Server 2025.

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Lock Escalation in SQL Server

Published 2025-02-20 by Kevin Feasel

Stephen Planck talks lock escalation:

Lock escalation is a built-in mechanism in SQL Server designed to optimize how the database engine manages locks. By reducing thousands of fine-grained locks (such as row or page locks) to a single table-level lock, SQL Server aims to lower overhead in the lock manager. In practice, lock escalation saves system resources but can also introduce challenges in high-concurrency environments. This post explores why lock escalation happens, how it works under the hood, the problems it can cause, and strategies to prevent or mitigate unwanted escalations.

Read on for the full story.

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Checking out the mssqlsystemresource Database

Published 2025-02-17 by Kevin Feasel

Stephen Planck goes into single-user mode:

Have you ever wondered where SQL Server actually stores the definitions for system objects like built-in stored procedures, system views, and functions? Enter the mssqlsystemresource database—often just called the “Resource” database. While this database remains hidden from everyday use, it plays a pivotal role in the internal workings of SQL Server. In this post, we will explore what the Resource database is, why it’s necessary, and what every DBA should know about it.

Read on for a primer on what’s in this database and why we typically shouldn’t mess with it.

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