Category: T-SQL

Daniel Hutmacher shows another good use of the APPLY operator:

You can tell by the plan why this is an inefficient query: the SQL expression in the common table expression is executed once for every time that it’s referenced in the code.

Better living through CROSS APPLY

You could store the results of the CTE in a temp table, but where’s the fun in that? Instead, why not use the CTE once, and then return four rows for each row that the CTE spits out? That’s exactly what CROSS APPLY does.

Read the whole thing and appreciate that much more all the nice things you can do with APPLY.

Arun Sirpal shows off some missing functionality in SQL Server:

Being in the cloud does have many benefits, from lower administration to fast scaling but another “side effect” of operating in Azure SQL Database is the cloud first nature of changes. By this I basically mean new features always get pushed to Azure first before the classic on-premises version so some gems come to light.

Here is one for you. Have you ever wanted MySQL’s functinality to apply LEAST() and GREATEST() argument? Well, you can now, in Azure.

I can’t say that I would use this every day or anything, but I have felt the pain of not having it. There are workarounds, though nothing as convenient as syntax. Hopefully this shows up on-prem in the next version of SQL Server.

Shane O’Neill has a bandolier of SQL tricks to show off:

Recently the DBA Team Lead and I were reviewing some SQL code, and we came across some SQL that neither of us had frequently encountered before. This led to a brief watercooler moment where we shared some uncommon SQL that we had seen. Perfect blog post material, I think.

I had previously learned about ODBC date functions from Shane and also learned about CURRENT in this post, so check it out.

Jared Poche is flirting with the dark side:

I work on hundreds of databases with the same schema. They have different data sets and distributions, different sizes, and their statistics are going to update at different times. But if one of them chooses a bad plan, I have to push aside whatever other work to research the high CPU on database xyz.

Consistency is really valuable to me. And in this case, the answer is simple. Yes, I want to scan the fast, small memory-optimized table variable first, and use it to filter the larger, slower table. Adding a join hint or a force order to this query should keeps its plan and performance consistent.

Click through for a few examples of where query hints can be useful, but also where they can fail you in unexpected ways.

Itzik Ben-Gan continues a series on table expressions:

Last month I explained and demonstrated that CTEs get unnested, whereas temporary tables and table variables actually persist data. I provided recommendations in terms of when it makes sense to use CTEs versus when it makes sense to use temporary objects from a query performance standpoint. But there’s another important aspect of CTE optimization, or physical processing, to consider beyond the solution’s performance—how multiple references to the CTE from an outer query are handled. It’s important to realize that if you have an outer query with multiple references to the same CTE, each gets unnested separately. If you have nondeterministic calculations in the CTE’s inner query, those calculations can have different results in the different references.

Say for instance that you invoke the SYSDATETIME function in a CTE’s inner query, creating a result column called dt. Generally, assuming no change in the inputs, a built-in function is evaluated once per query and reference, irrespective of the number of rows involved. If you refer to the CTE only once from an outer query, but interact with the dt column multiple times, all references are supposed to represent the same function evaluation and return the same values. However, if you refer to the CTE multiple times in the outer query, be it with multiple subqueries referring to the CTE or a join between multiple instances of the same CTE (say aliased as C1 and C2), the references to C1.dt and C2.dt represent different evaluations of the underlying expression and could result in different values.

Definitely worth the read.