From: Sergei Golubchik
Date: July 22 2010 9:50am
Subject: Re: HA_ONLINE_ADD_INDEX v. HA_ONLINE_ADD_INDEX_NO_WRITES
List-Archive: http://lists.mysql.com/internals/37992
Message-Id: <20100722095031.GB17731@janus.mylan>
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Hi, Zardosht!

On Jul 20, Zardosht Kasheff wrote:
> Hello all,
> 
> I was looking at alter table flags, and was wondering what the
> difference between these two flags are. Their name seems to imply that
> declaring HA_ONLINE_ADD_INDEX means the storage engine can accomodate
> writes while an index is being added, whereas
> HA_ONLINE_ADD_INDEX_NO_WRITES cannot.
> 
> Looking at the code in 5.1.46, I cannot find a difference. The only
> change I see is in the function mysql_alter_table, the value of
> need_lock_for_indexes is false if the engine exposes
> HA_ONLINE_ADD_INDEX, and true if the engine exposes
> HA_ONLINE_ADD_INDEX_NO_WRITES. This variable, need_lock_for_indexes,
> does not seem to actually be used anywhere.
> 
> So, my question is, despite what their names imply (stating if a table
> lock is necessary for add index), is there any functional difference
> for storage engines that set one of these flags and not the other?

It looks like there is none.

Originally there was supposed to be a difference - there was one in the
original changeset. But later it was removed, perhaps it was found to be
unnecessary.
 
Regards,
Sergei