Tag: SQL Server

Could not update the metadata that indicates database <> is enabled for Change Data Capture. The failure occurred when executing the command ‘SetCDCTracked(Value = 1)’. The error returned was 15517: ‘Cannot execute as the database principal because the principal “dbo” does not exist, this type of principal cannot be impersonated, or you do not have permission.’. Use the action and error to determine the cause of the failure and resubmit the request.

While configuring Change Data Capture on a database that is restored from different environment, we might end up with an error message as below:

Could not update the metadata that indicates database <> is enabled for Change Data Capture. The failure occurred when executing the command ‘SetCDCTracked(Value = 1)’. The error returned was 15517: ‘Cannot execute as the database principal because the principal “dbo” does not exist, this type of principal cannot be impersonated, or you do not have permission.’. Use the action and error to determine the cause of the failure and resubmit the request.

To resolve the issue, we need to change the owner of database and then enable CDC for the database as below. Please note that, the database would have restored irrespective of the error. You just have to run the below statement to avoid similar issues in future.

EXEC sp_changedbowner 'sa'
EXEC sys.sp_cdc_enable_db ;

Ref: https://docs.microsoft.com/en-us/sql/relational-databases/system-stored-procedures/sp-changedbowner-transact-sql?view=sql-server-2017

Decoding hexa error code to windows error message in SQL Server

Problem Statement:
Sometimes, the error messages generated are not friendly to understand the underlined issues. This is going to be an extra effort to google out for most of us with the error code (mostly a hexa value) and identify the corresponding windows error message.

Solution:
A simple solution would be to use NET HELPMSG with the errocode. But if the error code is a hexa value, then we need to identify the corresponding Decimal value to use as a parameter to NET HELPMSG.

Here is a script to identify actual error message from hex error code. This is important to be aware that this approach will only work for Win32 error codes from Microsoft Windows. If the net helpmsg command does not return a result, the cause is likely the error has not originated from Windows, or no longer a valid Win32 process.

DECLARE @hex VARCHAR(64) = '0x80070643'
Set @hex = Right(@hex,4)

DECLARE @rez BIGINT;
SELECT @rez = ISNULL(@rez,0) * 16 + 
CHARINDEX(substring(@hex,n.number+1,1),'0123456789ABCDEF') - 1
FROM MASTER..spt_values n WHERE n.TYPE='P' AND n.number<len(@hex)

/*
	--To use NET HELPMSG, need to enable xp_cmdshell as below:

	EXEC sp_configure 'show advanced options',1
	RECONFIGURE WITH OVERRIDE
	EXEC sp_configure 'xp_cmdshell',1
	RECONFIGURE WITH OVERRIDE
	EXEC sp_configure 'show advanced options',0
	RECONFIGURE WITH OVERRIDE
*/

Declare @Table Table(WindowsErrorMessage nVarchar(MAX))
Declare @s nvarchar(500) = 'NET HELPMSG ' + Cast(@rez as varchar(MAX))
Insert into @Table 
exec xp_cmdshell @s

Select @hex HexaVal,@rez DecimalVal,* From @Table 
where WindowsErrorMessage is not null

How to identify parallel plans in SQL Server

Few years back, on one of my engagements, identified lots of performance issues with an application. As part of a detailed analysis, it is observed there are lots of queries under performing with wait type “CXPACKET”. In short, CXPACKET wait types are involved in parallel query execution. These wait types indicate that the SPID is waiting on a parallel process to complete or start. There are many reasons your query may undergo a parallel execution like missing index, non optimized sql script etc. But, CXPACKET is not ALWAYS a bad thing.We will see about CXPACKET in detail with another post.

Now, We will see how to identify the queries with parallel execution. Once you identified the queries, you can further examine to identify the actual cause of the issues and optimize.

Script

select p.*,q.*,cp.plan_handle
from sys.dm_exec_cached_plans cp
cross apply sys.dm_exec_query_plan(cp.plan_handle) p
cross apply sys.dm_exec_sql_text(cp.plan_handle) as q
where
cp.cacheobjtype = 'Compiled Plan' and
p.query_plan.value('declare namespace
p="http://schemas.microsoft.com/sqlserver/2004/07/showplan";
max(//p:RelOp/@Parallel)', 'float') > 0

Please note that the above query gets information from cached plan in SQL Server. Hence, I will recommend to run a load test with a real time workload and use the script to identify the parallel executing queries. It is also important that the cached plans may get flushed due various factors like memory pressure, recompile of procedures, auto stats updates etc. But, this helped me a lot at times!!!

[Edit – 8th-Sep-202]

How do we know the degree of parallelism of a currently running query?

select session_id, scheduler_id
from sys.dm_os_tasks
where parent_task_address IS NOT NULL
--AND session_id = ? --You can uncomment and give spid as required.

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How do we set up Change Data Capture in SQL Server

This may be an old topic for many who worked with SQL Server, but, As I was doing a proof of concept on CDC recently for one of my projects, thought of sharing about CDC for those who is yet to explore this feature in SQL Server.

What is Change Data Capture?

Change Data Capture or CDC in SQL Server is introduced to track the DML changes and records insert/update/delete operations in respective change tables. The source of truth is Transaction Logs in SQL Server. Capture process jobs will be picking up the required table information from the Transaction Log and records in the Change tables. As these jobs are asynchronous, you may see a latency ideally between the commit and population of change tables. However, this has a performance advantages compared to Trigger based solutions. Similarly there is another clean up job to prune the data (older) from the change tables.

Supporting Editions: Enterprise/Developer/Standard.

How do we implement CDC on a database?

Step 1: : Let us create a database and table to see the proof of Concept


--Create a Test Database
Create Database CDCORigin;

--Create a Table 
create Table TestCDC(FirstName Varchar(100),SecondName Varchar(100))

--Insert values into tables
Insert into TestCDC Values('Ron','David'),('Chirs','Martin')

Step 1: : Enable CDC (Need two steps as below)


EXEC sys.sp_cdc_enable_db
EXEC sys.sp_cdc_enable_table  
@source_schema = N'dbo',  
@source_name   = N'TestCDC',  
@role_name     = NULL

EXEC sys.sp_cdc_add_job @job_type = N'capture'; 
EXEC sys.sp_cdc_add_job @job_type = N'cleanup'; 

You need to make sure that the SQLServerAgent is up and running while configuring the CDC, failing on this, it may end up with not creating the respective jobs in the environment.

Step 3: : Verification of CDC configuration

Once, CDC has set up, we can see the CDC specific tables and Changes tables in Object explorer as below.

In addition, we can also verify the data changes using below scripts.


--Check the CDC tables
Select * From [cdc].[dbo_TestCDC_CT]  --No data

Insert into TestCDC Values('Reddick','Pitt')

--Check the CDC tables
Select * From [cdc].[dbo_TestCDC_CT]

Update TestCDC Set SecondName = 'Pi' Where FirstName='Chirs'

--Check the CDC tables
Select * From [cdc].[dbo_TestCDC_CT]

Delete From TestCDC Where FirstName='Ron'

--Check the CDC tables
Select * From [cdc].[dbo_TestCDC_CT]

Precisely, the __$operation will indicate the action on the table as below.

Hope you enjoyed this post, and I am excited to write more about CDC with my real client experience in next posts. Until then, happy and continuous learning!