Watching Brent Ozar’s 2017 PASS Summit session on Youtube the other day, I learned that the Top N Sort operation in SQL Server behaves dramatically differently, depending on how many rows you want from the TOP.Continue reading
Last row per group
A very common challenge in T-SQL development is filtering a result so it only shows the last row in each group (partition, in this context). Typically, you’ll see these types of queries for SCD 2 dimension tables, where you only want the most recent version for each dimension member. With the introduction of windowed functions in SQL Server, there are a number of ways to do this, and you’ll see that performance can vary considerably.
Segment and Sequence Project
For windowed functions, SQL Server introduces two new operators in the execution plan; Segment and Sequence Project. If you’ve tried looking them up in the documentation, you’ll know that it’s not exactly perfectly obvious how they work. Here’s my stab at clarifying what they actually do.
Using TOP n WITH TIES
Here’s an interesting feature to that may greatly simplifie a few queries. TOP n WITH TIES works pretty much like TOP, except it orders the top records “densely” to include tied values.
Creating a top-10 list and aggregating the remainder
This article will show you how to create a “top 10” list of data, as well as providing an aggregate on the remainder of the data. There are a few different approaches to this.