Category: Internals

Jared Poche looks at hash matching:

When I began working at Microsoft, I was very much a novice at performance troubleshooting. There was a lot to learn, and hash match joins were pointed out to me multiple times as the potential cause for a given issue. So, for a while I had it in my head, “hash match == bad”. But this really isn’t the case.

Hash matches aren’t inefficient; they are the best way to join large result sets together. The caveat is that you have a large result set, and that itself may not be optimal. Should it be returning this many rows? Have you included all the filters you can? Are you returning columns you don’t need?

I might throw in one caveat about hash match joins and being the best performers for two really large datasets joining together: merge join can be more efficient so long as both sets are guaranteed to be ordered in the same way without an explicit sort operator. That last clause is usually the kicker.

Jared Poche explains my favorite type of join:

Nested loops joins are the join operator you are likely to see the most often. It tends to operate best on smaller data sets, especially when the first of the two tables being joined has a small data set.

In row mode, the first table returns rows one at a time to the join operator. The join operator then performs a seek\scan against the second table for each row passed in from the first table. It searches that table based on the data provided by the first table, and the columns defined in our ON or WHERE clauses.

Read on for more information about nested loop joins.

Erik Darling continues looking at plan operators. Erik starts with spools:

Spools are temporary structures that get stuck over in tempdb. They’re a bit like temp tables, though they don’t have any of the optimizations and enhancements. For example, loading data into a spool is a row-by-row operation.

The structure that spools use varies a bit. Table spools use a “clustered index”, but it’s not built on any of the columns in your data. Index spools use the same thing, but it’s defined on columns in your data that the optimizer thinks would make some facet of the query faster.

Definitely a must-read and a good way of explaining things. In my words, spools aren’t necessarily a problem but if you have a problem, spools are often at the root.

Erik Darling is also Overdrawn at the Memory Bank:

Whoever called memory a “bank” was a smart cookie. Everything you get from RAM is a loan.

In SQL Server, queries can get memory loaned to them while they execute. The most common reasons for memory grants are Sorts and Hashes. You may also see them for an Optimized Nested Loops Join, but whatever.

Memory is such an important aspect of query and overall server performance that it really helps to understand when there’s pressure on it, and where it’s coming from.

Check out both.

David Alcock giveth:

The full optimisation stage is where the optimiser uses a bag of tricks to optimise our query (surprise, surpise), well technically it has three bags of tricks that are named optimisation phases that each contain a collection of transformation rules (which I cover in this post that you should never do). The optimiser is not limited to using just one of the phases and each has a set criteria which determines if the optimiser can use that particular phase.

In order to see what how the optimiser is using these phases we need to enable Trace Flag 8675 as well as Trace Flag 3604 which will redirect the output to the query messages tab in Management Studio:

And David Alcock taketh away:

Now it has to be said it’s undocumented for a reason, the reason is that it’s really not a good idea to do this. In fact enabling this trace flag is such a bad idea that it will probably cause no end of issues with query performance…so let’s do it, but before we do let me add yet again that please don’t do this! Disabling optimisation features is a really bad idea, just like we did in this post – the purpose for this demo is just to show that we can, and how dangerous it can get.

This is fun to learn and interesting when doing advanced troubleshooting, but maybe not something you want to do very often.