Performance

Nikola Dimitrijevic

SQL Server performance counters (Batch Requests/sec or Transactions/sec): what to monitor and why

June 5, 2018 by

When maintaining SQL Server, it is essential to get an accurate perception of how busy it is. Two metrics that are often considered as indicators of how busy SQL Server is are Batch Requests/sec and Transaction/sec. When those metrics trend higher, they often affect all other metrics and make them go higher as well. While they could look similar, they are using a different type of starting point for measurement; the batches and transactions. So, to correctly understand those important metrics, lets first try to understand what the batches and transactions in SQL Server are and what are the differences between the two

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Dmitry Piliugin

Overpopulated Primary Key and CE Model Variation in SQL Server

April 4, 2018 by

In this blog post, we are going to talk about another cardinality estimation model enhancement in SQL Server 2014 – Overpopulated Primary Key (OPK).

Consider a fact table that contains information about some sales, for example, and a date dimension table. Usually, a fact table contains the data about the current year and past years, but a dimension table usually contains the data for the next few years also.

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Dmitry Piliugin

Join Containment Assumption and CE Model Variation in SQL Server

April 3, 2018 by

In this post we are going to talk about one of the model assumptions, that was changed in the new cardinality estimation mechanism in SQL Server 2014 – Join Containment Assumption.

You may find some information about this assumption in the Ian Jose’s blog post: Query Processor Modelling Extensions in SQL Server 2005 SP1, there you may find the description of the so-called simple assumption and base assumption. Another source of available information is a white paper from Joseph Sack Optimizing Your Query Plans with the SQL Server 2014 Cardinality Estimator.

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Dmitry Piliugin

Filtered Stats and CE Model Variation in SQL Server

April 2, 2018 by

In this blog post, we are going to view some interesting model variation, that I’ve found while exploring the new CE.

A model variation is a new concept in the cardinality estimation framework 2014, that allows easily turn on and off some model assumptions and cardinality estimation algorithms. Model variations are based on a mechanism of pluggable heuristics and may be used in special cases. I think they are left for Microsoft support to be able to address some client’s CE issues pointwise.

Today we are going to view some interesting model variation, that creates filtered statistics on-the-fly. I should give a disclaimer here.

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Dmitry Piliugin

Cardinality Estimation Framework Version Control in SQL Server

March 30, 2018 by

This is a small post about how you may control the cardinality estimator version and determine which version was used to build a plan.

The version of the cardinality framework is determined by the query database context, where the database has a specific compatibility level.

When you create a database in SQL Server 2014 it has the latest compatibility level equals 120 by default. If you issue a query in that database context, the new cardinality version will be used. You may verify this by inspecting the plan property “CardinalityEstimationModelVersion” of the root language element (the one with the green icon), SELECT, for example.

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Dmitry Piliugin

Cardinality Estimation Process in SQL Server

March 29, 2018 by

In this post, we are going to take a deeper look at the cardinality estimation process. We will use SQL Server 2014, the main concepts might also be applied to the earlier versions, however, the process details are different.

Calculators

The algorithms responsible for performing the cardinality estimation in SQL Server 2014 are implemented in the classes called Calculators.

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Dmitry Piliugin

Cardinality Estimation Concepts in SQL Server

March 28, 2018 by

In this blog post we are going to talk about the principles and the main concepts which are used by the optimizer to perform an estimation, and also, we will do a little bit math, so be prepared.

Base Statistics

A cardinality estimation mechanism, as a first step, usually uses base statistics to estimate the expected number of rows that should be returned from the base table. You may look at these statistics using DBCC command – DBCC SHOW_STATISTICS.

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Dmitry Piliugin

Cardinality Estimation Place in the Optimization Process in SQL Server

March 27, 2018 by

In this blog post, I’m going to look at the place of the Cardinality Estimation Process in the whole Optimization Process. We’ll see some internals, that will show us, why the Query Optimizer is so sensitive to the cardinality estimation. To understand that we should observe the main steps that a server performs when the query is sent for execution.

Plan Construction Process

Below you may see the picture of the general plan-building process in SQL Server from the moment when a server receives a query till the storage engine is ready to retrieve the data. Take a quick look first, and next I’ll provide explanations.

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Dmitry Piliugin

Cardinality Estimation Role in SQL Server

March 26, 2018 by

This post opens a series of blog posts dedicated to my observations of the new cardinality estimator in SQL Server 2014. But, before we jump to the new features, I’d like to provide some background, to make the next posts clearer.

We’ll start by discussing the role of Cardinality Estimation in SQL Server, trying to answer – what is it and why it is needed.

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Ahmad Yaseen

SQL Server table hints – WITH (NOLOCK) best practices

February 14, 2018 by

SQL Server table hints are a special type of explicit command that are used to override the default behavior of the SQL Server query optimizer during the T-SQL query execution This is accomplished by enforcing a specific locking method, a specific index or query processing operation, such index seek or table scan, to be used by the SQL Server query optimizer to build the query execution plan. The table hints can be added to the FROM clause of the T-SQL query, affecting the table or the view that is referenced in the FROM clause only.

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Prashanth Jayaram

Top 8 new (or enhanced) SQL Server 2017 DMVs and DMFs for DBAs

January 17, 2018 by

Dynamic management views (DMVs) and dynamic management functions (DMFs) are system views and system functions that return metadata of the system state. On querying the related system objects, database administrators can understand the internals of SQL Server. It allows us to monitor the performance of the SQL Server instance, and diagnose issues with it.

SQL Server 2017 ships with a number of new and enhanced dynamic management views and dynamic management functions that will help DBAs monitor the health and performance of SQL Server instances. A few existing DMV’s such as sys.dm_os_sys_info and sys.dm_db_file_space_usage have been enhanced. Some have also been newly built and available only for SQL Server 2017.

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Esat Erkec

How to use sargable expressions in T-SQL queries; performance advantages and examples

December 22, 2017 by

The challenge

One of the main tasks of a SQL Server database administrator is performance tuning. Sometimes, though, coders or developers don’t always prioritize database performance or query optimization. Here is a typical scenario

  • Imagine that developers create a new table and then insert some records in a test environment and test their queries to retrieve data from it
  • The query executed successfully and does not exhibit any symptoms of performance problems
  • The developer team release this table and query into production
  • One day you take a telephone from your colleague and he says my report is very slow
  • Bingo! In production, this table contains a lot of records and this is resulting in performance bottlenecks when querying it
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Ahmad Yaseen

SQL Server read-ahead mechanism; concept and performance gains

December 21, 2017 by

The user’s read requests in SQL Server are managed and controlled by the SQL Server Relational Engine, that is responsible for determining the most optimized access method, such as index scan or table scan, to retrieve the requested data. These read requests are also optimized internally by the SQL Server Storage Engine, the buffer manager components specifically, that is responsible for determining the general read pattern to be performed.

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Ahmad Yaseen

SQL Server Data Type Conversion Methods and performance comparison

October 3, 2017 by

When you define SQL Server database tables, local variables, expressions or parameters, you should specify what kind of data will be stored in those objects, such as text data, numbers, money or dates. This attribute is called the Data Type. SQL Server provides us with a big library of system data types that define all types of data that can be used with SQL Server, from which we can choose the data type that is suitable for the data we will store in that object. You can also define your own customized user defined data type using T-SQL script. SQL Server data types can be categorized into seven main categories:

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