Can SQL Server piece together two different indexes in a single-table query, rather than just giving up and scanning a suboptimal clustered index? The short answer is: yes, in a fairly narrow band of conditions.
Continue readingIntermediate
Watch out for Merge Interval with date range Index Seeks
In my last post, I found that DATEDIFF, DATEADD and the other date functions in SQL Server are not as datatype agnostic as the documentation would have you believe. Those functions would perform an implicit datatype conversion to either datetimeoffset or datetime (!), which would noticeably affect the CPU time of a query.
Well, today I was building a query on an indexed date range, and the execution plan contained a Merge Interval operator. Turns out, this operator brings a few unexpected surprises to your query performance. The good news is, it’s a relatively simple fix.
Continue readingDATEDIFF performs implicit conversions
As I was performance tuning a query, I found that a number of date calculation functions in SQL Server appear to be forcing a conversion of their date parameters to a specific datatype, adding computational work to a query that uses them. In programming terms, it seems that these functions do not have “overloads”, i.e. different code paths depending on the incoming datatype.
So let’s take a closer look at how this manifests itself.
Continue readingComputing the modulus from very large numbers
… and what of this all has to do with IBAN numbers.
The modulus is the remainder of a division of two integers*. Suppose you divide 12 by 4, the result is 3. But divide 11 by 4, and the result is 2.75. This could also be expressed by saying that 11/4 is 2 with a remainder of 3. Computing that 3 is the work of the modulo operator, which in T-SQL is represented by the % operator.
Let’s explore how to compute the modulus of large numbers in SQL Server, and how this is useful in the real world.
Continue readingTurn your list into human-readable intervals
If you’ve worked with reporting, you’ve probably come across the following problem. You have a list of values, say “A, B, C, D, K, L, M, N, R, S, T, U, Z” that you want to display in a more user-friendly, condensed manner, “A-D, K-N, R-U, Z”.
Today, we’re going to look at how you can accomplish this in T-SQL, and what this has to do with window functions and gaps and islands.
Continue readingSTRING_SPLIT(), but for quoted names
Can you apply gaps and islands logic on a string? Sure you can.
Continue readingHow to add “created” and “updated” timestamps without triggers
You have a table that you want to add “created” and “updated” timestamp columns to, but you can’t update the application code to update those columns. In the bad old times, you had to write a trigger to do the hard work for you. Triggers introduce additional complexity and potentially even a performance impact.
So here’s a nicer way to do it, trigger-free.
Continue readingHow MERGE on two different rows can still deadlock you
I recently ran into a curious deadlock issue. I have a process that performs a lot of updates in a “state” table using multiple, concurrent connections. The business logic in the application guarantees that two connections won’t try to update the same item, so we shouldn’t ever run into any locking issues. And yet, we keep getting deadlocks.
What’s going on here? Hint: it has to do with isolation levels and range locks.
Continue readingConnecting a SQL Server client on Linux using Active Directory authentication
or: How I learned to stop worrying, and love all-caps domain names.
I’m a complete beginner at Linux, so I should preface this post with the fact that these are my humble notes after hours of pulling my hair. It’s not really a fully-fledged how-to article, and there are lot of things I’m not covering. But I figured it may help someone out there at some point.
Continue readingIs a sort faster when the data is already sorted?
Whenever SQL Server needs to sort a data stream, it will use the Sort operator to reorder the rows of the stream. Sorting data is an expensive operation because it entails loading part or all of the data into memory and shifting that data back and forth a couple of times. The only time SQL Server doesn’t sort the data is when it already knows the data to be ordered correctly, like when it has already passed a Sort operator or it’s reading from an appropriately sorted index.
But what happens if the data is ordered correctly, but SQL Server doesn’t know about it? Let’s find out.
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