What contributes to the transaction size on a MySQL UPDATE?

Question

Background

We're running a MySQL 8 INNODB cluster behind some Java services. We received an error from the application because we'd exceeded the group replication transaction size limit (“Error on observer while running replication hook 'before_commit'”). Our limit was set to 150Mb at the time.

Problem

Looking at the transaction involved I don't understand how it might have involved anything like 150Mb.

It involved an update to two tables

update my_table mt 
inner join my_table_aud mta on mt.id = mta.id 
  set mt.boolean_column_1 = TRUE, 
      mt.boolean_column_2 = TRUE, 
      mt.varchar_column = coalesce(mt.varchar_column, ?2), 
      mta.varchar_column = coalesce(mta.varchar_column, ?2) 
where mt.boolean_column_1 = FALSE 
AND mta.rev <= ?1

which involved approximately 100 rows in my_table and maybe 200 rows in my_table_aud. Plus one other simple insert to a different table. The varchar columns were updated with around 10 bytes of data.

However the two tables involved in the UPDATE do both have a different longtext column, which wasn't updated. There would have been on average maybe 1MB in text per row updated in those columns.

The only explanation I can think of for us exceeding the transaction limit would be that the text in longtext columns contributed to the transaction size, even though they were not referenced in the update.

I searched for documentation on what contributes to the transaction size of a transaction in MySQL and haven't been able to find anything useful.

Please can someone help my understanding of how the transaction size limit might have been exceeded in this scenario?

Answer 1

The transaction size is not just based on the amount of data you're actually change, rather the whole state of the row before you made the change is preserved in case of a rollback and therefore the entire row is essentially copied. From further research it sounds like as much as even the entire page of rows is copied to the rollback segment, so depending on how many pages contain the rows you're updating, it could easily be a lot more data generated in the transaction then what you're actually updating.