In my last post, I explained how SQL Server can use an index to efficiently locate rows that qualify for a predicate. When deciding whether to use an index, SQL Server considers several factors. These factors include checking whether the index covers...

Scans and seeks are the iterators that SQL Server uses to read data from tables and indexes. These iterators are among the most fundamental ones that we support. They appear in nearly every query plan. What is the difference between a scan and a seek...

In this post, I’ll give an overview of three interesting properties of iterators that can affect query performance: memory, non-blocking vs. blocking, and dynamic cursor support. Memory All iterators require some small fixed amount of memory to...

In my last post, I explained that SQL Server executes a query by compiling it into an iterator tree which we call a query plan. So, if we want to understand iterators and query execution better, we need a way to look at query plans. Fortunately, SQL Server...

What are iterators? SQL Server breaks queries down into a set of fundamental building blocks that we call operators or iterators. Each iterator implements a single basic operation such as scanning data from a table, updating data in a table, filtering...

About two years ago I presented a talk to a couple of SQL Server user groups where I explained in general terms how SQL Server executes queries. I explained how to read showplan and how some of the most common operators (such as index scans and seeks...