SQL Server 2017 brings to us some new interesting DMV’s and DMF’s, one of them is sys.dm_db_log_info. This new DMF allows us to manage VLOGs in our databases. The log files are divided in smaller slices called virtual log files, or vlog. Operations such as log truncations happens over an entire vlog. For example, when … 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
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
When healing a sick SQL Server, you must forget the idea that there could ever be a simple correspondence between symptom and disease: The art of troubleshooting is much more the art of discovering, and assembling, the various pieces of the puzzle so that you have a complete understanding of what is going on inside of a server… Read more
