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
