You have to grant the user mysql executable-rights to all the parent-folders of the piwik installation and down to tmp/assets. Read or write doesn't help if it doesn't have executable. In fact, it does not need read or write access on any of these folders ;)
This fixed it for me. I had the same issue as @johniem, that the content of the folder could not be listed (what in addition needs read-access to tmp/assets for mysql, but that's not needed to execute the SQL command).
This fixed it for me. I had the same issue as @johniem, that the content of the folder could not be listed (what in addition needs read-access to tmp/assets for mysql, but that's not needed to execute the SQL command).