In this post we continue looking at the Cardinality Estimator (CE). The article explores some join estimation algorithms in the details, however this is not a comprehensive join estimation analysis, the goal of this article is to give a reader a flavor of join estimation in SQL Server.

The complexity of the CE process is that it should predict the result without any execution (at least in the current versions), in other words it should somehow model the real execution and based on that modeling get the number of rows. Depending on the chosen model the predicted result may be closer to the real one or not. One model may give very good results in one type of situations, but will fail in the other, the second one may fail the first set and succeed in the second one. That is why SQL server uses different approaches when estimating different types of operations with different properties. Joins are no exception to this.

Yesterday I came across a question on one of SQL forums, that I may rephrase like:

“Does a query plan compilation depend on how busy SQL Server is”.

Before we go further, I should explicitly mention that we talk about a Compiled plan, not an Executable plan. Plan execution will of course depend on how busy server is, for example, the query may wait for the memory grant to start execution, or execution may be slow because there are no cached pages in the Buffer Pool etc.

However, the question was about a Compiled plan: does the shape of a plan depend on the server load.

From the first glance it should not. But…

Sometimes, when I saw expressions like ‘Expr1002’ or ‘WindowCount1007’ or something similar in the columns Output List of a query plan, I asked myself, is there a way to project those columns into the final result to look at the values. That question first came to me out of curiosity when I was playing with window aggregate functions and a Window Spool plan operator in SQL Server 2012, I wanted to look into the Window Spool to understand, how it performs an aggregation.

Interestingly, that SQL Server 2016 CTP3.0 allows us to look deep inside into the iterator and observe the data flowing through it. Let’s turn on an “x-ray machine” and take a look.

Most of the people know about the so-called “Parameter Sniffing”. This topic was discussed in many aspects in a number of great articles. It is interesting that not only parameters might be “sniffed” during the first execution, but also a runtime constant functions. Let’s look at the example.

Test Data

I will use a test server and administrator account to run the script below, be sure you have enough privileges on your test server if you want to try out the script below.

In this blog post, we will look at one more Nested Loops (NL) Join Post Optimization Rewrite. This time we will talk about parallel NL and Few Outer Rows Optimization.

For the demonstration purposes, I will use the enlarged version of AdventureWorks2014. In the sample query, I will also use the trace flag (TF) 8649 – this TF forces parallel plan when possible and is very convenient here, as we need one for the demo. There are also a few other undocumented TFs: TF 3604 – direct diagnostic output to console, TF 8607 – get a physical operator tree, before Post Optimization Rewrite, TF 7352 – get a tree after Post Optimization Rewrite phase.

The sample query is asking for some data based on the period’s table.

Continuing my blog post series after 24HOP Russia “Query Processor Internals – Joins”. In this (and the next one) blog post, we will talk about the Nested Loop Post Optimization Rewrite optimizations.

Some of you may know that a Nested Loop join algorithm preserves order of the outer table.

Some time ago, on the 24HOP Russia I was talking about the Query Processor internals and joins. Despite I had three hours, I felt the lack of time, and something left behind, because it is a huge topic, if you try to cover it in different aspects in details. With the few next articles, I’ll try to describe some interesting parts of my talk in more details. I will start with Hash Join execution internals.

The Hash Match algorithm is one of the three available algorithms for joining two tables together. However, it is not only about joining. You may observe a complete list of the logical operations that Hash Match supports in the documentation: