This is my first pass at a modern (2005/2008) blocking monitoring script. I think it's a decent blocking script to capture information about blocks as they are occurring. Filters can be applied and it wouldn't be hard at all to add on other information such as execution plans, plan hash, etc. SELECT tl.request_session_id AS WaitingSessionID       ,wt.blocking_session_id AS BlockingSessionID       ,wt.resource_description       ,wt.wait_type       ,wt.wait_duration_ms       ,DB_NAME(tl.resource_database_id) AS DatabaseName       ,tl.resource_associated_entity_id AS WaitingAssociatedEntity       ,tl.resource_type AS WaitingResourceType       ,tl.request_type AS WaitingRequestType       ,wrt.[text] AS WaitingTSql       ,btl.request_type BlockingRequestType       ,brt.[text] AS BlockingTsql FROM   sys.dm_tran_locks tl        JOIN sys.dm_os_waiting_tasks wt        ON tl.lock_owner_address = wt.resource_address        JOIN sys.dm_exec_requests wr        ON wr.session_id = tl.request_session_id        CROSS APPLY sys.dm_exec_sql_text(wr.sql_handle) AS wrt        LEFT JOIN sys.dm_exec_requests br        ON br.session_id = wt.blocking_session_id        OUTER APPLY…
I'm working on the chapter on blocking in the new book. Explaining blocking of course means explaining locks. Prior to 2005, to understand locks, you went to sp_lock. Not anymore. Now you can query sys.dm_tran_locks. It's so much more sophisticated than the old system procedure. Best of all, the information within it is simply a view into the internal locking infrastructure, so you're not placing extra load or extra processing on the system to marshal this data. A simple query to get basic locking information would look like this:  SELECT tl.request_session_id            ,tl.resource_database_id            ,tl.resource_associated_entity_id            ,tl.resource_type            ,tl.resource_description            ,tl.request_mode            ,tl.request_status  FROM sys.dm_tran_locks tl That just outputs roughly the same information as sp_lock. Lots more detail, not available in sp_lock, is available if you need it. Things…
I'm just going through the chapter on the procedure cache in the new book and I'm having a blast playing with the dynamic management views and functions that let you access the procedure cache. It's just too much fun and way too easy to get detailed information about the queries in the system, not like the old days. First, you can access the cache itself with the DMV, sys.dm_exec_cached_plans. This shows some of the data describing the plan in cache, but most importantly it provides the plan_handle. You need this for other joins later. You can also use sys.dm_exec_query_stats to get aggregated performance statistics about the plan. It also has the plan_handle and two things new to SQL Server 2008, the query_hash and the query_plan_hash, also known as query fingerprints.…
Blog post #7 in support of Tim Ford’s (b|t) #iwanttohelp, #entrylevel. Read about it here. Sooner or later when you're working with SQL Server, someone is going to complain that the server is slow. I already pointed out the first place you should look when this comes up. But what if they're more precise? What if, you know, or at least suspect, you have a problem with a query? How do you get information about how queries are behaving in SQL Server? Choices For Query Metrics It's not enough to know that you have a slow query or queries. You need to know exactly how slow they are. You must measure. You need to know how long they take to run and you need to know how many resources are…
Knowledge of how your system behaves is vital to better control, maintain, and grow the system. While Azure provides all sorts of wonderful assistance within Azure SQL Database, you're still going to need that same knowledge. When it comes to getting detailed information about Azure SQL Database, the tools are a little more limited than with an on-premises instance of SQL Server, or any virtual instance of SQL Server. There are no trace events. To see individual query calls, recompile events, query store behaviors, and so much more, you're going to have to use Extended Events. I'm going to write a series of posts on using Extended Events with Azure SQL Database. Tradition would call for this first post to be an initial how-to. Instead, I want to take a…
Probably the single most important factor when deciding which query to tune, or actively tuning a query, is how you go about measuring query execution time. SQL Server provides a number of different mechanisms (really, maybe too many) to get this done. However, all measures are not created equally. In fact, they frequently disagree with one another. Let's take a look at this odd phenomenon. Measuring Query Execution Time Before we get into all the choices and compare them, let's baseline on methodology and a query to use. Not sure why, but many people give me blow back when I say "on average, this query runs in X amount of time." The feedback goes "You can't say that. What if it was just blocking or resources or..." I get it.…
I place a lot of emphasis on capturing actual execution plans because of the runtime metrics, but with Trace Flag 7412, we don't need the plan. This is great news, because capturing execution plans, even using extended events, is an expensive proposition. However, using either the query_thread_profile event, or, Trace Flag 7412, we can get the runtime metrics without the plan. Trace Flag 7412 Here's how it works.You can either be running the extended event, query_thread_profile (a debug event, but one documented and supported by Microsoft) or, enable the Trace Flag 7412. I like to use the extended event in a targeted fashion to easily see behaviors on a query without having to capture the plan. You can even capture the information and then combine it with an estimated plan…
Starting with SQL Server 2016 Sp1 CU2, a new way of directly querying statistics, specifically the histogram, has been introduced: dm_db_stats_histogram. We've always been able to query the statistics using DBCC SHOW_STATISTICS. However, the output, three different result sets with three different structures, made automating access to statistics information a pain. Now, we have more capability through dm_db_stats_histogram. dm_db_stats_histogram To access the information in dm_db_stats_histogram, you just have to pass in the object_id and the statistics_id values for the statistics you're interested in like this: SELECT * FROM sys.dm_db_stats_histogram(OBJECT_ID('HumanResources.Employee'), 1) AS ddsh; It's very straight forward to use. The results look like this: Handy right? Now you can query the histogram directly. Yeah, I hear a few of you saying... and this helps me... how? Here's an example. This query…
In Azure SQL Database for quite some time and now available in SQL Server 2017, Microsoft has put a lot of the knowledge they've gleaned from running more databases that any of the rest of us ever will to work with Automatic Tuning. Automatic Tuning The core of automatic tuning at this point in time (because I'm sure it's going to evolve) is the ability of the query engine to spot when a query has generated a new plan and that new plan is causing performance to degrade. This is known as a regression in the plan. It comes from bad parameter sniffing, changes in statistics, cumulative updates, or the big notorious one, the cardinality estimator introduced in SQL Server 2014 (it's been almost four years, I'm not calling it…
Data Definition Language queries don't go through the optimizer, right? While normally, my short answer to this question in the past would have been, yes. However, with testing comes knowledge and I want to be sure about the statement. I'm working with a team of people to completely rewrite the SQL Server Execution Plans book. We'll probably be published in April or May. It's going to be amazing. The structure will be completely different and the book will represent five years of additional knowledge in how execution plans work and how to read and interpret them since the last book was written. However, enough on that. Let's answer the question about Data Definition Language. First of all, we need to quickly define our terms. Data Definition Language (DDL) represents the syntax for queries that build…