Metadata locks
Metadata locks are a particular type of lock that has been supported since MariaDB 5.5. Transactions acquire metadata locks when they access a table or view for the first time. This includes non-transactional tables such as Aria tables. Metadata locks prevent transactions from dropping the locked object or modifying the structure. This is very important because if a transaction is using a table, you want to be sure that the table columns (or even the whole table) will not disappear in the middle. In some cases, stored programs are also locked.
If a connection tries to execute a DDL statement (such as ALTER TABLE) on a table that has a metadata lock, the connection will be put on hold until the locks are released. However, metadata ...
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