Michael J. Swart

August 4, 2021

What To Avoid If You Want To Use MERGE

Filed under: SQLServerPedia Syndication — Michael J. Swart @ 1:06 pm

Aaron Bertrand has a post called Use Caution with SQL Server’s MERGE Statement. It’s a pretty thorough compilation of all the problems and defects associated with the MERGE statement that folks have reported in the past. But it’s been a few years since that post and in the spirit of giving Microsoft the benefit of the doubt, I revisited each of the issues Aaron brought up.

Some of the items can be dismissed based on circumstances. I noticed that:

  • Some of the issues are fixed in recent versions (2016+).
  • Some of the issues that have been marked as won’t fix have been fixed anyway (the repro script associated with the issue no longer fails).
  • Some of the items are complaints about confusing documentation.
  • Some are complaints are about issues that are not limited to the MERGE statement (e.g. concurrency and constraint checks).

So what about the rest? In what circumstances might I decide to use a MERGE statement? What do I still need to avoid? In 2019 and later, if I’m using MERGE, I want to avoid:

It’s a shorter list than Aaron’s but there’s another gotcha. The same way that some of the items get addressed with time, new issues continue to pop up. For example, temporal tables are a relatively new feature that weren’t a thing when Aaron’s first posted his list. And so I also want to avoid:

If MERGE has trouble with old and new features , then it becomes clear that MERGE is a very complicated beast to implement. It’s not an isolated feature and it multiplies the number of defects that are possible.

Severity of Issues

There’s a large number of issues with a large variety of severity. Some of the issues are minor annoyances, or easily avoidable. Some of them are serious performance issues that are harder to deal with. But a few of the issues can be worse than that! If I ask SQL Server to UPDATE something, and SQL Server responds with (1 row affected) , then it better have affected that row! If it didn’t, then that’s a whole new level of severity.

That leads me to what I think is the worst, unfixed bug. To be safe from it, avoid MERGEs with:

The defect is called Merge statement Delete does not update indexed view in all cases. It’s still active as of SQL Server 2019 CU9 and it leaves your indexed view inconsistent with the underlying tables. For my case, I can’t predict what indexed views would be created in the future, so I would shorten my advice to say avoid MERGEs with:

Conclusion

When I first started writing this post, I thought the gist was going to be “MERGE isn’t so bad if you just avoid these things”. But it still is bad. Aaron’s list is a little out of date but his advice is not. If Aaron updated that post today, the list of defects and issues with MERGE would be different and just as concerning to me as the day he wrote it.

So just to simplify my advice to you. Avoid:

  • MERGE

17 Comments »

  1. I laughed out loud at the bullet point at the end. Nicely done.

    Comment by Brent Ozar — August 5, 2021 @ 12:50 am

  2. I had another issue with MERGE last week. We recently changed the database to RCSI and also had the HOLDLOCK hint in our procedures. It look like this was causing DEADLOCKS (no other database activity at the time of deadlocks so it should be the merge). I removed the HOLDLOCK hint and changed the MERGE to an explicit transaction. This fixed the deadlocks, but I don’t want to jump into the conclusion this locking hit was het root cause. Do you think we need HOLDLOCK in case of an RCSI database?

    Comment by Wilfred van Dijk — August 5, 2021 @ 4:35 am

  3. Great evaluation !
    btw: I just received an urgent call from one of our ETL responsibles, their system goes haywire and it is getting worse. Turns out they use MERGE in EVERY single process, because it is so “convenient”. We take it one beat at a time …

    Comment by alzdba — August 5, 2021 @ 4:40 am

  4. The solution is simple, don’t use indexed views. /s

    Comment by Tom — August 5, 2021 @ 5:14 am

  5. Dang, I was under the impression that almost everything was fixed and what was left was very minor, but this is still quite a lot of stuff. Thanks for the write up.

    Comment by Alex Friedman — August 5, 2021 @ 5:34 am

  6. […] Michael J. Swart has important bullet points: […]

    Pingback by Ways to avoid the MERGE Operator – Curated SQL — August 5, 2021 @ 8:00 am

  7. @Wilfred
    I’ve had a lot of experience with the features you’ve just mentioned: RCSI (on or off), Working with HOLDLOCKs to avoid deadlocks and the MERGE statement.
    In my experience RCSI has only had the effect of reducing deadlocks.
    Deadlocks are an interesting thing to troubleshoot and I can’t give you advice without knowing a *lot* more details. It sounds like a good question for DBA.stackexchange.com.

    Comment by Michael J. Swart — August 5, 2021 @ 11:16 am

  8. @alzdba
    Databases that use MERGE statements can go haywire. But be careful not to turn it into a scapegoat.
    What I’m saying is, I’d be wary of giving advice that sounds like “Avoid MERGE and your problems will go away”

    Comment by Michael J. Swart — August 5, 2021 @ 11:22 am

  9. @Michael J. Swart : great post.

    Comment by Mitchell Wheat — August 9, 2021 @ 10:25 pm

  10. To be honest, I use MERGE often, even with partitioned columnstore tables in a simple recovey DWH database and had no problems (SQL 2016 SP2 CU ).

    If I look onto the bug regarding columnstore (https://feedback.azure.com/forums/908035-sql-server/suggestions/34791778-sql-server-2016-sp2-cui-bug-merge-and-insert-wi), I wonder, whyever someone would use such a silly syntax.

    He does an INSERT INTO TWO SELECT … FROM (MERGE TWO…. OUTPUT)
    So he puts the MERGE into a FROM clause (didn’t know that this is possible) AND is inserting the output of the MERGE into the same table which he was just MERGING. To be honest, I understand, that such a “concept” could cause errors, but since I’d never do something as this, I see no reason not to use MERGE on columnstores.
    BTW: a simple MERGE INTO TWO … OUTPUT … INTO TWO would what I’d have done and – if this still causes error (because output target = merge target), I’d insert the output into a #temptable first

    The simple dupicate key with filtered indexes is only for < SQL 2016 and the partitioning bug for partitioned tables with only a single partition (not splitted yet) and can't be tested, because it is only available in the webarchiv (where you can't access the attachments).

    So I see no reason to not use MERGE when / where it would saves me time (depends as always) because I have to do insert / updates / deletes with the same (big) set of data and don't want to join the same tables three times.

    Comment by Thomas Franz — August 10, 2021 @ 5:54 am

  11. @Thomas Franz
    Okay, granted, the partition bug has been resolved (I missed that link). And the syntax of the columnstore bug is bizarre and falls under the category of easily avoidable.
    The other issues still concern me (like I said, especially that last one).

    You’ve been using MERGE in your data warehouse without issues. Chances are you’re going to continue to see no further issues. For my own experience, about 10 years ago, I was affected by incorrect results found in a join in a MERGE statement. The specific issue I encountered has since been fixed but I’m still not convinced that MERGE is worth reconsidering, especially when there alternatives.

    Comment by Michael J. Swart — August 10, 2021 @ 9:59 am

  12. “In the spirit of giving Microsoft the benefit of the doubt”? I’m doing that less and less lately. In fact I’ve been considering moving our flagship product to PostgreSQL in a future release.

    Comment by Dave Boltman — August 10, 2021 @ 7:32 am

  13. If we stopped using features of SQL server that somebody somewhere thought was bad, we wouldn’t use it at all.

    Comment by Todd Chittenden — August 10, 2021 @ 12:47 pm

  14. @Todd, I expect you’re being a bit facetious, but what you said is not true at all. I mean no-one complains about SELECT/INSERT/UPDATE/DELETE.
    Besides, features aren’t separated into good/bad, dangerous/safe. There’s varying degrees of value and varying degrees of risk.
    For me I’ve been burned before by the MERGE statement before and I’m sharing my thoughts. I’m not by nature a pessimist or a grouch and the criticism of the MERGE statement is valid.

    Comment by Michael J. Swart — August 10, 2021 @ 12:53 pm

  15. Michael, thanks for the update. I’d like to offer another reason to avoid MERGE that is more managerial than technical. Many shops allow SQL to be modified by app developers or others who are not hard-core SQL gurus. Selecting an obscure feature to save a few lines of code is tempting, but may be counter-productive if it’s not easily understood by all who may subsequently touch it. I ignored that principle once to use MERGE in a stored procedure, only to regret it after a junior developer modified an input that caused an insert/delete recursion storm inside the MERGE in production. Keeping code simple and understandable is often more valuable than an elegant technical solution.

    Comment by L. P. Hineman — August 11, 2021 @ 9:46 am

  16. @ L.P. Hineman
    Oh absolutely, I like to think of everything you wrote as “maintainability” and it’s an often overlooked value.
    Simplicity is good for maintainability, it’s good for SQL Server’s query optimizer.
    Also I would argue that simple and understandable code is exactly what makes a technical solution elegant. 🙂

    Comment by Michael J. Swart — August 11, 2021 @ 11:05 am

  17. My case *for* MERGE is that, for follow-on devs maintaining my original code, the intent is clear. But then again, I’ve only been bitten once (if the target needs filtering, do it upstream – use say a CTE – don’t do it in the join… plan is death). I’ll probably keep using MERGE, and then one day have horrible regret. 😉

    Comment by Paul Holmes — August 11, 2021 @ 8:04 pm

RSS feed for comments on this post. TrackBack URL

Leave a comment

Powered by WordPress