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Curated SQL Posts

Dealing with Key Lookups

Published 2021-06-01 by Kevin Feasel

Jared Poche shows a good technique for removing key lookups:

A key lookup is an operation that occurs when a query has used a nonclustered index on a given table, but needs to access more columns to complete the query. It may need to check columns not in that index for additional filters, or it may just need to return that column as part of its result set.

In the simple query above, we’re retrieving 100 rows from the seek against a nonclustered index, then performing a key lookup against the clustered index. There is a nested loops operator between the two and understanding how that operates is important; for each row we receive from the first table, we perform the second operation once. So, in this query we are seeking 100 rows from the nonclustered index, then performing the key lookup 100 times. We go through the index once for each row we return, and you can see the cost of the key lookup operator is 99% of the query.

Read on for more information, including how to eliminate key lookups.

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Whither Code Page 1

Published 2021-06-01 by Kevin Feasel

Solomon Rutzky unravels a mystery:

Collation names, at least in SQL Server, are structured in that they contain various pieces of information detailing some of the properties of the collation. The collation name of SQL_Latin1_General_CP1_CI_AS can be broken down as follows:

– SQL_ This indicates that the collation is a SQL Server collation, while names without this prefix indicate Windows collations

– Latin1_General This is the culture (a.k.a. locale) used for the sorting and comparison rules

– CP1 This is the 8-bit [C]ode [P]age

– CI This indicates [C]ase- [S]ensitive or [I]nsensitive

– AS This indicates [A]ccent- [S]ensitive or [I]nsensitive

Ok, but there’s no code page “1”. So, what’s going on there?

Read on to learn the answer behind this mystery.

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Enumerating Breaking Changes to Power BI Reports

Published 2021-06-01 by Kevin Feasel

Brett Powell gives us a list of things which might cause breaking changes in Power BI reports:

A breaking change, which we can define as any change to a dataset which causes either reports to render errors or the dataset to fail to refresh, can severely impact business workflows and reflect poorly on those responsible for the solution. Given significant investments in other areas of the organization’s data estate such as Azure Synapse Analytics, a simple, easily avoidable oversight in a Power BI deployment may not be tolerated.

Read on for the list.

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Speeding Up Azure Data Factory Pipelines

Published 2021-06-01 by Kevin Feasel

Hiram Fleitas doesn’t have all day to wait for that pipeline to finish:

His issue was pretty much as mentioned on the tile. Our bank’s Azure Data Factory pipeline is running slow moving data from on-prem, we’re copying all tables in a SQL Server database, files from ftp sites and network share drives to Azure SQL DB Managed Instance and to blob storage (our datalake) , do you have some recommendations how to make it go faster? Its around 300GBs and takes over 8 hrs.

So I replied with the following and figured to post it here as it may help others.

Hiram has a video, as well as specific advice to offer.

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What to Do with Temp Tables in Stored Procedures

Published 2021-06-01 by Kevin Feasel

Chad Callihan invokes Betteridge’s Law of Headlines:

Generally speaking, it’s best to put things away that aren’t being used. Don’t keep indexes that aren’t getting utilized because they are taking up disk space and still have to be kept up to date with changes. A table is still loaded up with old data that’s not being used but needs kept? Maybe it’s time for options like an archive database or partitioning.

While it’s not on the same level of importance, one related argument I’ve seen and been in is how to handle temp tables in stored procedures. Do you drop them at the end of a stored procedure or do you leave them to be cleaned up by SQL Server? Is one way better for performance than the other? Let’s do some testing and see what we find out.

Read on for the answer.

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Automate Availability Group Failover for SSISDB 2012 and 2014

Published 2021-06-01 by Kevin Feasel

Alex Stuart shows how to fail over SSISDB in SQL Server 2012 or 2014:

Hopefully not many people are still configuring SSIS instances on SQL 2012 or 2014 – especially HA instances – but if you are, this post is for you.

If you’re running SQL Server 2016 or above, having the SSIS catalog function correctly in an AG is supported by built-in functionality to manage the DMK (database master key). In 2012/2014 however there is no such support. Without intervention this makes SSISDB unable to be opened after a failover, because the DMK isn’t open – leading to errors such as “Please create a master key in the database or open the master key in the session before performing this operation.”

Read on to see how to resolve this error, and then how to do this automatically.

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Querying Delta Lake Files with T-SQL in Azure Synapse Analytics

Published 2021-05-28 by Kevin Feasel

Jovan Popovic shows off a new feature in public preview for Azure Synapse Analytics serverless SQL pools:

You can use Azure Synapse and Azure Databricks to prepare and modify your Delta Lake data sets placed in the Azure Data Lake storage. Once your data engineers have prepared the data, your data analysts can create reports using the tools such as Power BI.

Using the serverless query endpoint in Azure Synapse, you can create a relational layer on top of your Delta Lake files that directly references the location where Azure Synapse and Azure Databricks are used to modify data. This way, you can get the real-time analytics on top of the Delta Lake data set without any need to wait for a pipeline to copy and prepare data.

Read on to see how this works.

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Understanding the STUFF() Function

Published 2021-05-28 by Kevin Feasel

Chad Callihan explains an important function:

I used to always see the STUFF function in passing when reading blogs and kind of move past it without understanding what it was doing. I would see it used and think it’s doing something with a bunch of stuff and kind of skip over it since it wasn’t entirely relevant to what else I was reading. When I read about what the STUFF function actually does, it made a lot more sense as a name. More than dealing with “a bunch of stuff” the STUFF function is used for stuffing a string into another string. Let’s check out a few examples of stuffing data.

Granted, 99% of its importance is in combination with FOR XML PATH() but that’s still important. And we get to see a few other use cases for it as well.

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