Why Won't PostgreSQL Use My Covering Index?

Dear Postgres, Why won’t you use my covering index?

Lately I’ve been learning to tune queries running against PostgreSQL, and it’s pretty delightful. One fun question that I worked through struck me something that most Postgres users probably encounter one way or another: sometimes you may create the perfect index that covers a given query, but the query planner will choose to ignore it and scan the base table.

Why in the world would it do that?

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Comparing Single Column, Multi-Column, and Filtered Statistics in SQL Server

Statistics in SQL Server are simple in theory: they help the optimizer estimate how many rows a query might return.

In practice? Things get weird fast. Especially when you start filtering on multiple columns, or wondering why the optimizer thinks millions of rows are coming back when you know it’s more like a few hundred thousand.

In this post, I’ll walk through examples using single-column, multi-column, and filtered statistics—and show where estimates go off the rails, when they get back on track, and why that doesn’t always mean you need to update everything with FULLSCAN.

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New Article on Performance Tuning with the Missing Indexes Feature in SQL Server

We’ve just published a new article in the SQL docs, Tune nonclustered indexes with missing index suggestions . The article explains what the missing index feature is, limitations of the feature, and how to use missing index DMVs and missing index suggestions in Query Store to tune indexes.

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Index Design Guide Updated: Index Types, B+ Trees, and Row Locators, Oh My

We’ve recently updated the SQL Server and Azure SQL index architecture and design guide. This article is an in-depth guide to indexing in databases using the SQL Server engine, including SQL Server, Azure SQL Database, Azure SQL Managed Instance, and Azure Synapse Analytics.

Our recent update adds a table to categorize the types of indexes discussed in the article, clarifies B-trees vs B+ trees, and describes how row locators (aka “secret columns”) are used in nonclustered indexes.

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Do Clustered Index Columns Count Against Max Nonclustered Key Size?

SQL Server 2016 and Azure SQL Database have a lot of little perks. One little thing that changed is that the maximum key size allowed for nonclustered indexes was raised from 900 bytes to 1700 bytes.

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