T-SQL window functions, introduced in 2005 with enhancements in 2012, are great additions to the T-SQL language. In this article, Kathi Kellenberger explains what you need to know to get good performance when using these functions.… Read more
One of the most important tools for query tuning is the ability to view execution plans. They are even portable; you can send an execution plan to get advice from someone who doesn't have access to the underlying data. Have you even considered that sensitive information might travel along with the plans? Hugo Kornelis explains how execution plans affect data protection. … Read more
Developer shops may decide to use Read Committed Snapshot Isolation (RCSI) to reduce contention, and possibly improve performance, but it is not without its own pitfalls. In this article, Uwe Ricken describes a situation where long running transactions caused a severe performance degradation when RCSI was in use.… Read more
The query store gives us a novel way of identifying those queries that are causing performance problems when they are parameterized by SQL Server for reuse. Although it is relatively simple to ensure that certain troublesome queries avoid the problem, it is laborious to identify these queries. Additionally, Query Store gives us the means to fix the problem for groups of queries by means of plan guides without changing the DDL at all. Dennes Torres explains the details… Read more
Once you have Query Store enabled on your databases, runtime statistics are generated for your queries; but what about the natively-compiled stored procedures and memory optimised tables that come with In-Memory OLTP? Do you get the full range of runtime statistics? This is an intriguing question that Enrico explores and answers.… Read more
Distribution statistics are used by SQL Server's Query Optimiser to determine a good execution plan for your SQL query. You don't need to know about them to execute queries, but the better you understand them, the better you can optimise your queries and sort out performance problems. Robert Sheldon once more provides a simple guide.… Read more
Sometimes, in the quest for raw SQL performance, you are forced to sacrifice legibility and maintainability of your code, unless you then document your code lavishly. Phil Factor's SQL Speed Phreak challenge produced some memorable code, but can SQL features introduced since then help to produce code that performs as well and is also easy to understand? Aunty Kathi investigates.… Read more
Relational databases go out of their way to execute SQL, however bad the crimes against Codd and relational theory within the query. The 'conditional join', can be executed but at great cost. As always, it is much better to sit back and restate the problem in a set-based way. The results can be rewarding.… Read more
Before SQL Server had SQL window functions, SQL Server developers had to use all manner of tricks and algorithms to come up with ways to rapidly process large amounts of data. Do we need these techniques now that we can express a SQL task in terms of window functions? Kathi revisits a famous SQL challenge to find out… Read more
The speed of a slow SQL Query can almost always be improved. In SQL Server, the query optimizer determines the best way of executing the query, based on the evidence it has. The same query can be executed in many different ways as the data size increases, new indexes become available, or as the data distribution changes. If the appropriate index doesn't exist or can't be used, then SQL Server shrugs and does the best it can. Tony Davis explains how to find out what a query needs to perform w… Read more
It is always bad news if your SQL queries are having to use the SORT operator. It is worse news if you get a warning that sort operations are spilling onto TempDB. If you have a busy, slow TempDB, then the effect on performance can be awful. You should check your query plans to try to eliminate SORTs and never leave a SORT warning unheeded. Fabiano Amorim shows the range of ways of getting information on what is going on with a query that is doing a SORT and when requests are made for memory.… Read more
When you're developing database applications, it pays to check for index scans in the SQL Server query plan cache. Once you've identified the queries, what next? Dennes Torres gives some preliminary guidelines on how to find out why these index scans are being chosen for these queries and how to make the queries run faster and more efficiently.… Read more
When SQL Server 2014 was released, it included Hekaton, Microsoft's much talked about memory-optimized engine that brings In-Memory OLTP into play. With memory-optimized tables 30 times faster than disk-based tables, higher performance is promised - but at what cost? Jonathan Watts looks at the features that have improved, and those that need careful consideration.… Read more
Performance tuning and optimization definitely have their place in minimizing SQL Server Licensing costs - by helping keep CPU utilization low. But it's important to remember that the fastest and most efficient query possible is the one that you never execute against your SQL Server. That might sound trite, but it's at the heart of caching - which is key to helping organizations save significant money on SQL Server licensing costs while simultaneously enabling better application performance and increased scalability. … Read more
As a coder, the more you know about the business, the better code you're going to write. The more you know about the language in which you code regularly, the better code you're going to write. Similarly, if you're tasked with writing code against a relational database engine, you're going to need to know more about that engine. If you can examine and understand execution plans, you can achieve better understanding of the database system and you will write better database code. Grant Fritchey shows you how.… Read more
Dwain Camps show that, depending on the size and characteristics of some hierarchical data, six different methods of traversal can each be the fastest at some point. He illustrates convincingly that It is dangerous to generalize from just one set of test data, and it is foolish to assume that, just because SQL code looks neat, it will perform well.… Read more
One of the first things one needs to understand well about SQL Server is indexes, but somehow many of the basic questions don't often get asked or answered much on forums. Rob Sheldon answers some of these shy-making questions about SQL Server Indexes and indexing: the ones we somehow feel silly asking in public, and think twice about doing so.… Read more
Database administrators tend to rely more on science than intuition, and may therefore seem exasperatingly cautious to other IT people. A relational database is a complex machine, however, that is asked to deal with intricate problems. When things go wrong, it usually takes knowledge, deduction and a systematic approach to track down the root core of problems and to determine the best fix.… Read more
Imagine it. You've been asked to fix a dire performance problem with a SQL Server database. You find a severe case of 'Cache Bloat' due to ad-hoc queries, but you can't fix the code itself. What should you do? Specify forced parameterization? Perhaps a better idea would be to use guide plans.… Read more
When a SQL Server instance deadlocks, it can be anything from minor irritation to something far more severe. In this article, Gail Shaw looks at how you can identify common types of deadlock, the difference between a deadlock and severe blocking, and how to avoid and fix the most common deadlock types.… Read more
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