Category: Indexing

Francisco looks at index spools:

When we are analyzing execution plans, we may come across different types of Spool operators – Table Spools, Row Count Spools, Window Spools or Index Spools – that the Query Optimizer chooses for specific purposes. In this post we are going to briefly look into the Index Spool, how it can sometimes lead to suboptimal query performance, and what can be done to easily fix it.

My favorite description of this is Erik Darling’s: spools are SQL Server’s passive-aggressive way of telling you “I’m not saying you need an index but you need an index.”

Erik Darling runs through an exercise:

The more columns you have in a table, the more potential column combinations there are for indexes. Much like columns, indexes tend to get added following the path of least resistance.

Very rarely does someone consider current indexes when deciding to add an index.

Erik’s process is a good one. The real pain comes when there are 40-50 indexes on a table (seriously…) and there are a lot of similar-but-not-quite-similar-enough indexes.

Kendra Little announces a new article:

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.

Read on to understand the impetus behind this article and then go check it out yourself.

Erik Darling made me rhyme. First up, data retrieval via seek:

People. People complain. People complain about SQL Server. That’s probably why I get paid to deal with it, but whatever.

One complaint I get to hear week in and week out is that SQL Server isn’t using someone’s index, or that there are too many index scans and they’re slow.

That might actually be a composite of like twelve complaints, but let’s not start counting.

Erik gives us three reasons why we might not see a seek. But wait, there’s more!

I’m not sure why scans got such a bad rap. Perhaps it’s a leftover from the Bad Old Days© when people worried about logical fragmentation and page splits.

What I mean to say is: scans are often treated with a level of revulsion and focus that distracts people from larger issues, and is often due to some malpractice on their part.

This is true–scans aren’t inherently bad and Erik gives us a better mental model to work with.