I have a workload that does partitioning manually (largely because at the end of each day I want to 'optimize table' for the previous day's partition to address fragmentation). As a result there's a stored procedure, that prepares a statement of the form "create table if not exists table_20181130 like table_template" and runs it. This procedure is called rather a lot and >99% of the time the table already exists so this doesn't do anything. This has been working great up-to and including MariaDB 10.3.9.
Upon upgrading to 10.3.10 (or 11) I find that performance of my system is abysmal. And this seems to be down to this procedure - the create table if not exists doesn't return immediately like it used to if the table exists and any other query has the table open. I can work around this by looking in information_schema.TABLES and avoiding the 'create table if not exists' if the table already exists - however this feels like a regression to me rather than an intentional change, especially as I don't see anything obviously related in the 10.3.10 ChangeLog.
To recreate; in one terminal execute:
... and in another:
In anything before 10.3.10 the second terminal will complete immediately, but in 10.3.10 (or 11) the second terminal will only complete once the select in the first terminal has done so.