Understanding MySQL Replication "fatal error 1236": "Replica has more GTIDs than the source has, using the source's SERVER_UUID"

This MySQL replication error — fatal error 1236 / Replica has more GTIDs than the source has, using the source's SERVER_UUID — shows the importance of thinking before acting.  I am glad a non-DBA Colleague asked me about it, because if he had restarted replication, it would have caused a much bigger mess.

Often, we are tempted — or pushed — to just restart things as quickly as possible.  In this specific case, restarting replication is unsafe as it leads to a replication breakage in the best case, and to silent data corruption in the worse case.  It is a great example of the following two advice : 1) do not make a bigger mess, do not failover, do not reboot, and 2) we cannot just bring the database back up, we have things to do first to make this safe.  Let's explore all these.

Do not uselessly grant CREATE and ALTER TABLE

This lesson should have been learned with the CREATE TABLE of death, but it is worth a refresh.

Do not uselessly grant CREATE and ALTER TABLE

The reason I am posting this reminder is that another crashing bug related to DDL came to my attention.  This bug is only fixed in a recent version of MySQL (probably not affecting 5.6 and 5.7), so if you are running the latest 8.0 or 8.4, you should be fine.  I am not sharing the detail, I might in the future (like I did for the CREATE TABLE of death).  For triggering it, one need CREATE and ALTER TABLE.

The even more scary thing about this bug / crash is that it is also triggered by a rollback.  So after the crash, when MySQL attempts crash recovery, it crashes again when rolling-back uncommitted transaction.  There might still be a way to recover data with innodb_force_recovery, but it would compromise atomicity and is probably a big hurdle.  In this respect, in addition to being a crashing bug, it can be considered a data corruption bug.

A crash in rollback is non-trivial to recover

I gave a talk about a similar situation at SRECon: Autopsy of a Cascading Outage from a MySQL Crashing Bug.  The slides are self-contained and were appreciated.  If you face this or a similar situation, I hope you have a good backup and can replay binlogs (maybe it is also good time to test backups and binlog replay).

Binary Log Compression is Safe since MySQL 8.0.34

This is a quick one.  My attention was recently brought (thanks Simon) on a relatively recent comment (25 Nov 2025) in Bug #103672 - Binlog compression transaction payload event exceeds max allowed packet :

The underlying server bug was fixed in 8.0.34 in BUG#33588473. The server now falls back to writing the transaction without compression, if the compressed size would exceed 1 GiB.

So Binary Log Transaction Compression is safe since MySQL 8.0.34.  But this also means that it is unsafe before, as also clearly mentioned in Bug #103672 :

It is true that an 8.0.33 server (or older) will produce a corrupted binary log and that replicas may have to recover from backup. It would be better to document this clearly and to advise to not use compression on 8.0.33 and older.

However, even if updating the documentation is mentioned in the above quote, my quick check did not find any reference to the risk of using binlog compression before 8.0.34 (including in the documentation of the global variable binlog_transaction_compression).  I thought this deserved wider knowledge, hence this post.

Undo Log Truncation Bug in 8.0 leads to Data Corruption

I am upset about this one : I have a hard time not seeing this as negligence, and it starts to become a pattern...  So please forgive me if this post is not my most diplomatic, because I really think someone deserves a kick in the butt !  But what is all this about...

There is a MySQL bug, which can lead to data corruption, opened for 8.0 in September 2023, fixed in MySQL 8.4.0 (released in February 2024), AND STILL NOT FIXED IN MySQL 8.0.44 !  Luckily for some, it is fixed in Percona Server 8.0.39 (released in October 2024).  As usual, I start this post with some context before diving in the nitty-gritty details.

Performance Regression in MySQL 8.0, Fixed in 8.4, Easy Workaround (innodb_doublewrite_pages)

While doing benchmarks on 5.7 and 8.0, I came across a performance regression in MySQL 8.0 over 5.7 and opened a bug (Bug #111353 : 3x Performance Regression from 5.7 to 8.0 on ALTER TABLE FORCE).  There has been recent activity on this bug, showing an easy workaround.  This, even if it is known since 16 July 2024, has not been talked about much, so this deserves a blog post.

Trusting mysqldump and Insecure MySQL Client Lead to Remote Code Execution

Yes, you read this correctly: because the MySQL client is insecure and allows running arbitrary commands, and because mysqldump blindly trusts the server it is dumping from, a hostile MySQL Server on which mysqldump is executed could trigger arbitrary command execution (also known as a remote code execution).  This post raises awareness on this vulnerability and shows how a secure MySQL client makes things harder to exploit.

Follow-up on an Unprivileged User can Crash your MySQL Server

A year ago, I blogged about An Unprivileged User can Crash your MySQL Server.  At the time, I presented how to protect yourself against this problem without explaining how to generate a crash.  In this post, I am revisiting this vulnerability, not giving the exploit yet, but presenting the fix.  Also, because the default configuration of Group Replication in 5.7 is still vulnerable (it is not in 8.0), I show the adjustment to make to avoid problems.

Duplicate Entry in ALTER TABLE (and in OPTIMIZE TABLE)

A few weeks ago and in MySQL 5.7, I had an ALTER TABLE that failed with a duplicate entry error.  This is old news as it is happening since MySQL 5.6, but I only saw this recently because I am normally using online schema change from the Percona Toolkit (pt-osc) or GitHub's online schema migration (gh-ost).  I do not like that and I am disappointed this has not been improved, so this post is partially a rant, but not only, because I explain why ALTER TABLE is failing and I give solutions to avoid the failure.

An Unprivileged User can crash your MySQL Server

Yes, your read the title correctly: an unprivileged user can crash your MySQL Server.  This applies for the default configuration of MySQL 8.0.21 (and it is probably the case for all MySQL 8 GA versions).  Depending on your configuration, it might also be the case for MySQL 5.7.  This needs malicious intent and a lot of determination, so no need to panic as this will not happen by accident.  I am not sharing the exploit in this post (I might do it later), but I am telling you how to avoid the problem.