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Category: T-SQL

Preventing Execution With PARSEONLY And NOEXEC

Published 2018-12-31 by Kevin Feasel

Solomon Rutzky shows us a way to prevent accidental full script execution:

There are times when I am working on a SQL script that really shouldn’t be executed all at once. Sometimes it’s a series of examples / demos for a presentation or forum answer. Other times it’s just a temporary need while I’m in the process of creating a complex script, but once the script is completed and tested then it should run all at once. In either case, I have accidentally hit F5 too many times when I thought that a certain section of code was highlighted (so only that section would execute) but in fact nothing was highlighted so the script started executing from the very top, and either ran until completion or until I was able to cancel it (if it ran long enough for me to have time to understand what was happening and hit the “cancel” button).
So I needed some way of ensuring that a script would not execute if no section was highlighted.

Read on to learn about PARSEONLY and NOEXEC.

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Using IDENTITY In A SELECT Statement

Published 2018-12-20 by Kevin Feasel

Kenneth Fisher shares something he learned recently about the IDENTITY function:

Now, looking at it a bit more closely you’ll see that this is a function call, not just a property. Now, in my research for this post I did find where I’d mentioned this function briefly in my somewhat comprehensive identity post. Technically I didn’t mention so much as it was mentioned to me in the comments so I added it to the list. I guess I either didn’t look at it closely enough at the time or it’s just one of those cases where I forgot. Either way, it’s worth highlighting now.

Click through to learn more.

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Comparing Data With CHECKSUM

Published 2018-12-20 by Kevin Feasel

David Fowler shows how to use CHECKSUM and CHECKSUM_AGG to compare data:

There are times when we need to compare two tables and figure out if the data matches. I often see a number of ways of doing this suggested, most are quite slow and inefficient. I’d quite like to share a quick and slightly dirty way of doing this using the CHECKSUM and CHECKSUM_AGG functions.

CHECKSUM()
Just a reminder that CHECKSUM() will generate a checksum for an entire row or selection of columns in the row.

CHECKSUM_AGG()
Will generate a checksum for a dataset.

David then has a couple of examples showing these in action.

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Using DISTINCT With XML Data

Published 2018-12-17 by Kevin Feasel

Dave Bland has a workaround for a limitation with processing XML in SQL Server:

Since I was working on an example for my next performance class, I decided to use an example from that session.  In the query below the qp.* part of the column list will return a column named query_plan which is an XML data type.
SELECT DISTINCT pa.,cp.,qp.* 
FROM
(SELECT plan_handle
FROM sys.dm_exec_cached_plans) cp 
CROSS APPLY sys.dm_exec_query_plan (cp.plan_handle ) qp
CROSS APPLY sys.dm_exec_plan_attributes(cp.plan_handle) As pa
WHERE is_cache_key = 1 AND pa.attribute = ‘set_options’
However, when I add the DISTINCT keyword I get the error below.

Read on for a workaround for this.

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Finding The Closest Numeric Match

Published 2018-12-13 by Kevin Feasel

Itzik Ben-Gan has a T-SQL puzzle for us:

As you can see, both T1 and T2 have a numeric column (INT type in this example) called val. The challenge is to match to each row from T1 the row from T2 where the absolute difference between T2.val and T1.val is the lowest. In case of ties (multiple matching rows in T2), match the top row based on val ascending, keycol ascending order. That is, the row with the lowest value in the val column, and if you still have ties, the row with the lowest keycol value. The tiebreaker is used to guarantee determinism.

Click through for the details as well as several solutions.

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Finding The SQL Server Port With T-SQL

Published 2018-12-07 by Kevin Feasel

Jack Vamvas shows us how to find the port SQL Server is listening on using T-SQL:


Question: Without going into the SQL Server Configuration manager via the GUI is there a command oriented method to extract the port number SQL Server is listening on?
 
Answer: There are a few different methods to extract the port number without going into the Configuration interface.
Method 1 – use xp_readererrorlog

Read on for an explanation of this technique as well as links to a couple other methods.  I hadn’t thought about using the error log as a source, but it works.

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Continuing The Advent Of Code In T-SQL

Published 2018-12-06 by Kevin Feasel

Wayne Sheffield has a few more posts in the Advent of Code series.  His latest edition:

In Day 5, we find ourselves working with the polymers of the new Santa suit. A polymer (the input file), consists of units, represented by upper and lower case letters. Adjacent units of the same letter, but of different polarity (case), cancel each other out. This may lead to other units that can then cancel each other out. The goal is to reduce the polymer to as small as possible, and report back the reduced size.

Tasks:

  1. Perform a case-sensitive search/replace for each letter of the alphabet. The search is for a pair of the same letter, where one is upper case, and the other is lower case.
  2. Recursively perform this operation until the string can no longer be reduced.

In my opinion, the key part to this is that the operation needs to be performed recursively. I can think of only two ways to recursively perform an operation in SQL Server:

  1. A recursive common table expression (cte).
  2. Using a WHILE loop.

I don’t like using either of these mechanisms in SQL Server – they both perform operations in a “Row-By-Agonizing-Row” method, instead of a more set-based approach. However, set-based recursion usually performs extremely poorly. So, I’m going to use a while loop.

The recursion requirement does limit things a bit; otherwise I could see putting something together with the LEAD() window function.

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Understanding Recursive CTEs Via Execution Plans

Published 2018-12-04 by Kevin Feasel

Hugo Kornelis shows us how SQL Server’s database engine implements recursive common table expressions:

I am pretty sure that (almost) everyone reading this blog knows that a CTE (Common Table Expression) is an independent subquery that can be named and then referenced (multiple times if needed) in the main query. This makes CTEs an invaluable tool to increase the readability of complex queries. Almost everything we can do with a CTE can equally well be done by using subqueries directly in the query but at the cost of losing readability.

However, there is also one feature that is actually unique to CTEs: recursion. This blog post is not about what this feature is, what it does, or how to write the code. A brief description is given as part of the complete explanation of CTEs in Books Online, and many bloggers have written about recursive CTEs as well.

For this post, I’ll assume the reader knows what a recursive CTE is, and jump into the execution plan to learn how this recursion is actually implemented.

This is (as usual) a great article, and helps explain why recursive CTEs can be slow.

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Advent Of Code Day One, In T-SQL Form

Published 2018-12-04 by Kevin Feasel

Wayne Sheffield walks through several useful techniques for T-SQL developers:

And here we have a T-SQL solution for Day 1 of the Advent of Code challenge. The key tasks that we can learn from today are:

  • Loading a file.

  • Split a string on a delimiter.

  • Including additional rows into a result set (adding the first zero with a UNION ALL).

  • Multiplying (duplicating) a result set multiple times.

  • Performing a running total calculation.

  • Assigning a sequential number to a set of rows in a specific order.

  • Use of the GROUP BY and HAVING clauses while performing an aggregation.

Read the whole thing.

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Testing Scalar UDF Performance In SQL Server 2019 CTP 2.1

Published 2018-11-30 by Kevin Feasel

Brian Hansen takes a look at the scalar UDF performance improvements in SQL Server 2019:

In one of my sessions, Set Me Up: How to Think in Sets, I discuss a variety of performance-inhibiting query constructs, including scalar UDFs. I thought it would be interesting to take the simple scalar function that I use in the demo and see what kind of difference that scalar inlining might make.

First, I restored the CorpDB database that I use in the session to my SQL Server 2019 CTP 2.1 instance and initially set the compatibility level to 140. I also ran script 001 from the demo to create the needed database tables (no need to create the CLR objects for this test). I then ran script 030 to execute the scalar UDF test. In a nutshell, this script

  • creates a UDF

  • runs a query that calls the UDF about 13,000 times, capturing the time required to do so

  • repeated this test five times

  • discards the fastest and slowest tests

  • reports the average time for the remaining three tests

If I’m reading Brian’s notes right, it’s still slower than writing the set-based solution yourself, but a huge improvement over the prior scalar function performance.

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