In "Description" subsection of documentation of "UNLOCK TABLES" statement as present in web page https://mariadb.com/kb/en/library/transactions-unlock-tables/, the following statement exists:
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Another use for UNLOCK TABLES is to release the global read lock acquired with the FLUSH TABLES WITH READ LOCK statement, which enables you to lock all tables in all databases.
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I think comma before "which" is redundant and may confuse people like me. Since English statement clause starting with the word "which" is directly related to previous clause containing "FLUSH TABLES...statement", comma is redundant.