Category: Query Tuning

Jared Poche talks about the ability to use either predicate on a join for filtering:

You may recall the transitive property from elementary school math class. It states:

If A = B, and B = C, then A = C

The SQL Server optimizer can and will use this property, and it can lead to issues in your queries. When I’m writing a query, I have a clear idea of how I want it to operate. But using the transitive property, SQL Server has additional options one might not expect, and this may occasionally cause things to go awry

Click through for an example. Most of the time, the optimizer is smart about using transitivity, but sometimes it can go wrong.

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Erik Darling notes that adding an ORDER BY clause to a query can change the underlying query plan:

Sometimes I think it’s interesting how adding a seemingly useless or harmless thing to a query can change the query plan.

Here’s a quick example using an Order By on an indexed column.

Your mileage may vary on whether that’s a good thing.

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Grant Fritchey reminds us to look at the first operation when viewing an execution plan:

The first time you see a new execution plan that you’re examining to fix a performance problem, something broken, whatever, you should always start by looking at the first operator.

First Operator

The first operator is easily discerned (with an exception). It’s the very first thing you see in a graphical execution plan, at the top, on the left. It says SELECT in this case:

It’s easy to overlook, but Grant gives some good reasons not to do so.

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Erik Darling points out how difficult it can sometimes be to get batch mode processing on rowstore tables:

I’m excited about this feature. I’m not being negative, here. I just want you, dear reader, to have reasonable expectations about it.

This isn’t a post about it making a query slower, but I do have some demos of that happening. I want to show you an example of it not kicking in when it probably should. I’m going to use an Extended Events session that I first read about on Dmitry Pilugin’s blog here. It’ll look something like this.

Read on for a demonstration of the point.

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