SQLServerFast

Can you trust your constraints?

Hugo Kornelis
24 Comments
SQL Server allows you to temporarily disable your CHECK and FOREIGN KEY constraints. I would not recommend this as part of your regular production schedule, but there are certainly cases where this comes in handy.   One example is if you’re copying thousands of rows from another table, and you already know with 100% certainty that the data doesn’t violate any constraint. The overhead of checking could impact performance of your process; in the worst case making the execution time exceed your maintenance window. Disabling the constraints can result in a tremendous reduction of execution time.   Another example would…
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Back with a vengeance!

Hugo Kornelis
2 Comments
It’s been quite a while since my last blog post. Almost three months, in fact. In case you have been wondering if I was ill, dead, or abducted by aliens – not at all. The reality was far worse: Adam Machanic has coerced me into contributing a chapter on spatial data for his excellent book “Expert SQL Server 2005 Development”, due out in May (rather than the original plan to ship in March, possibly due to a certain author of a certain chapter not meeting any of his deadlines…).   Anyway, the chapter is finished and I now finally find…
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The Beatles versus the Stones – the explanation

Hugo Kornelis
No Comments
On December 31 of last year, I posted this brain teaser, promising to post the answer “in a few days”. Apparently, 15 is a few J.   In case you have forgotten what the puzzle was about and are too lazy to click the link above, the bottom line is that I created and populated two tables, with the same schema but different content. One held the first and last name of each of the Beatles, the other held first and last name of each of the Rolling Stones. I then queried both tables, using the same query in both…
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The Beatles versus the Stones

Hugo Kornelis
23 Comments
Here’s a nice brain teaser, just before the end of the year. Despite the title, it is related to SQL Server, not to music!   A common misconception amongst SQL Server users is that a clustered index on a table will ensure that data is returned in the order implied by that index. I have lost count of the number of times I had to disprove this notion.   Of course, there are many cases where the rows returned by a query will be in the order of the clustered index. Here’s a quick illustration, using the lineup that The…
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The table scan from hell

Hugo Kornelis
9 Comments
Greg Linwood, a fellow SQL Server MVP, has started a series of articles in which he attempts to prove that having a clustered index on each table is not a good practice. However, he has failed to include the effects of fragmentation into account, so I decided to run some tests for myself. One of those test had rather upsetting results.   First a bit of background, for those of you who have never read a book by Kalen Delaney. A table without clustered index (also known as a “heap”) can be read in two ways: using a bookmark lookup…
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Just stuff it!

Hugo Kornelis
12 Comments
I’m sure you’ve all heard it, and probably even said it, many times: “SQL Server sucks at string manipulation”. And with good reason – it is true. But not quite as true as many seem to believe.   I notice that many people who complain about SQL Server lacking string manipulation are themselves unaware of the string functions that SQL Server does have. Most know LIKE, LEFT, RIGHT, and SUBSTRING. Some also know CHARINDEX, maybe even REPLACE. But how many of you know and use PATINDEX, REVERSE, QUOTENAME, and LTRIM, to name just a few?   The string function that…
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The prime number challenge – great waste of time!

Hugo Kornelis
10 Comments
No sane person would even consider using SQL Server to construct a list of prime numbers. So just to prove that I’m not sane (as if there could be any doubt!), this post will be about finding prime numbers.   First a bit of history. Ward Pond wrote about efficient ways to populate a table with one million GUIDs. I posted a comment with a slightly more efficient algorithm. And that was quickly followed by a new post from Ward, tweaking my syntax even further. And that’s when Denis the SQL Menace lived true to his name by posting this…
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Snapshot isolation: A threat for integrity? (Part 4)

Hugo Kornelis
5 Comments
In the previous parts of this series, I have shown how SQL Server prevents violations of foreign key constraints by silently disabling snapshot isolation. I have also demonstrated how you can use the same technique inside your trigger code, to keep snapshot isolation from damaging your custom integrity rules. Now, in the final part of this series, I will investigate some less common techniques for preserving integrity. Note that I would normally not recommend any of these techniques, but I do see them often enough in newsgroup postings. Apparently, they are used.   First, I’ll set up the tables again.…
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Fun with ambiguous table names

Hugo Kornelis
1 Comment
Earlier today, I realised that Microsoft has forgotten to include some keywords in the list of reserved keywords. Now, a wise developer will still take care to omit those names when naming tables – but a bored developer can have loads of fun exploring the effects!   The keywords I am referring to are inserted and deleted. Everyone who ever coded a trigger knows that they refer to the pseudo-tables that hold the before and after image of all rows affected by the triggering DML statement. But since they’re not reserved keywords, it’s perfectly legal to name a column “inserted”.…
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Snapshot isolation: A threat for integrity? (Part 3)

Hugo Kornelis
4 Comments
Last month, I showed how snapshot integrity can really mess up your triggers, promised you a workaround, and then went on holiday. Some events in my life kept me from posting the promised workaround for longer than intended, so I hope that you haven’t been holding your breath for it! J   The workaround is actually quite simple. Just remember the first part of this series, where I showed how SQL Server 2005 prevents violation of foreign key constraints when using snapshot isolation – it automatically and silently switches to a less concurrent isolation level. Of course, if Microsoft can…
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Hugo Kornelis

Married, father of 2 children.

Awarded 13x MVP (2006-2016; 2019-now), in the areas SQL Server and Data Platform.

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