From: Zardosht Kasheff
Date: April 8 2011 11:36pm
Subject: allowing other threads to write to a table during "create index"
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Hello all,

In MySQL 5.1, a storage engine can allow other threads to write to a
table while an index (via handler::add_index) is being created. To do
so, one can set the lock_type to TL_WRITE_ALLOW_WRITE when
handler::store_lock is called.

In MySQL 5.5, this does not work, because a "metadata lock", or MDL
has been added to the system. Is there any way to get this to work in
MySQL 5.5?

Thanks
-Zardosht