Using DBCC INPUTBUFFER

This command shows the last statement executed for a particular SPID. This statement is mostly used for troubleshooting purposes to determine the exact command a particular SPID is running. You must be a member of the sys admin fixed server role, or have VIEW SERVER STATE permission (if SQL 2005+). As for a real world use, I will mostly use this command after using sp_who2 to find out which SPID is taking a lot of resources. To execute, simply replace the SPID 55 below with the one you want to spy on. [cc lang=”sql”] DBCC INPUTBUFFER(55) [/cc] And here are the results: The results you are looking for are displayed in the third column above. This shows the last statement that has been or is currently being executed by the SPID. Another alternative to the DBCC INPUTBUFFER is to use the dm_exec_requests DMV. This query will show the currently executing statement for a SPID: [cc lang=”sql”] SELECT SQLStatement = SUBSTRING ( qt.text, er.statement_start_offset/2, (CASE WHEN er.statement_end_offset = -1 THEN LEN(CONVERT(nvarchar(MAX), qt.text)) * 2 ELSE er.statement_end_offset END – er.statement_start_offset)/2 ) FROM sys.dm_exec_requests er CROSS APPLY sys.dm_exec_sql_text(er.sql_handle) as qt WHERE er.session_id = 54 [/cc] This DMV will only work on SQL 2005+ and you also need the VIEW SERVER STATE permission as a minimum. The one advantage DBCC INPUTBUFFER has over this, is the DMV will only show the currently executing request and not the previously executed request so you need to be a little quicker to catch it. The above DMV […]

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Monitor Queries against a Table

I recently had a need to monitor inserts against a particular table in order to determine what kind of locks they were acquiring. Being that we could not run traces on the system, I had to resort to a roll-your-own approach to monitoring the table. The goal was to determine what kind of locking was occurring, and it would also be nice to be able to associate it to the executing query. The following code uses DMVs in order to trap a sample of the queries running against it. Please note that this will not trap all queries, however it will work in getting a good number of samples. This script will run in an endless loop so be sure to hit cancel at some point. Also, for some reason the SQL Statement will not always be trapped. It was not that important for me, as I mostly needed the locks, however if someone figures it out, please post. [cc lang=”sql”] — Capture query activity against a table using DMVs DECLARE @TableName varchar(255); — Specify the table you want to monitor SET @TableName = ‘Sales.SalesOrderDetail’; DECLARE @ObjectID int; SET @ObjectID = (SELECT OBJECT_ID(@TableName)); IF OBJECT_ID(‘tempdb..##Activity’) IS NOT NULL BEGIN DROP TABLE ##Activity; END; — Create table SELECT TOP 0 * INTO ##Activity FROM sys.dm_tran_locks WITH (NOLOCK); — Add additional columns ALTER TABLE ##Activity ADD SQLStatement VARCHAR(MAX), SQLText VARCHAR(MAX), LoginName VARCHAR(200), HostName VARCHAR(50), Transaction_Isolation VARCHAR(100), DateTimeAdded DATETIME; DECLARE @Rowcount INT = 0; WHILE 1 = 1 BEGIN INSERT INTO ##Activity SELECT […]

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Execution Plans

The execution plans SQL Server creates and uses is a huge topic with many intricacies. Now I have never spoken to anyone on the SQL Server Development team, however I would imagine that there are some extremely sharp people in the query optimization team. In a nutshell, this is where the magic happens in SQL Server. Overview An execution plan is a behind-the-scenes look at the road a query takes in order to deliver it’s final result. They are generated from the underlying data statistics combined with what the query and it’s parameters are trying to accomplish. When the initial query is read, the execution plan generation engine or “Query Optimizer” searches for the best way to deliver the results of the query in the quickest way possible. To do this, it needs to know what the underlying data looks like. This is facilitated by the statistics that are stored for each table, column, and index. With these statistics in hand, the goal is to eliminate the largest number of records as quickly as possible, and iterate through this process until the final result is delivered. That said, it is not an easy job. There are many variables that come into play when determining a query’s path. A few of these include the selection of indexes, join algorithms, join order, parallelism. Displaying the Plan As SQL developers, it is our job to understand how to read execution plans so we can see how the query is being executed. To view […]

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Understanding SQL Server Deadlocks

In order to fix deadlocks, it’s critical to understand why they occur. I’ve gotten some push back sometimes from readers when I write articles that do not give cookie cutter solutions, but the “teach a man to fish” adage couldn’t be more true for deadlocks. All of it revolves around locking, which is a central piece to SQL Server’s default transaction isolation level of read-committed. Now what is read committed? Some people are critical of this isolation level because with a different isolation level like Snapshot, it is much less likely that deadlocks occur because each transaction utilizes it’s own version of what a table looks like at the very time the transaction begins. There are trade-offs with shapshot however. Snapshot isolation is much more disk intensive because these “snapshots” need to be persisted in a temporary data store. So if you are ever considering enabling snapshot isolation as your default isolation level for SQL Server you will want to make sure you have a beefy RAID 10 tempdb storage. So with read-committed we have less tempdb data swapping, but more locking. There are many different kinds of locks and many different objects that can get locked. Each of these types of locks may or may not be compatible with other types of locks. Locks are acquired so that users have a consistent view of what the data looks like at the time they are querying it. Imagine user 1 begins updating a table to inactivate all the records while […]

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The object name ‘x’ contains more than the maximum number of prefixes. The maximum is 2.

This error usually comes up when you are trying to perform a SELECT..INTO across a linked server. The problem is that a 4 part name cannot be used as the “INTO” parameter. This will fail: [cc lang=”sql”] SELECT * INTO [remoteserver].RemoteDB.dbo.sysdtspackages FROM MSDB.dbo.sysdtspackages [/cc] The trick is to create the table on the remote server, then perform an INSERT INTO. [cc lang=”sql”] INSERT INTO [remoteserver].RemoteDB.dbo.sysdtspackages SELECT * FROM MSDB.dbo.sysdtspackages [/cc]

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