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Re: ae2cb44: MDEV-7922 - ERROR 1939 (HY000): Engine PERFORMANCE_SCHEMA failed to discover


Hi, Sergey!

On Apr 07, Sergey Vojtovich wrote:
> On Tue, Apr 07, 2015 at 03:03:39PM +0200, Sergei Golubchik wrote:
> > On Apr 07, svoj@xxxxxxxxxxx wrote:
> > > 
> > > MDEV-7922 - ERROR 1939 (HY000): Engine PERFORMANCE_SCHEMA failed
> > > to discover table
> > 
> > No, I don't like this solution. This is an issue in discovery, there is
> > nothing wrong with how P_S uses it.
> > 
> > I'd rather fix it in TABLE_SHARE::init_from_sql_statement_string()
> > by temporarily setting THD::db to this->db (which is a db where the
> > table will be created).
> Well, I don't like it either. I also thought about temporary substition of
> THD::db. But it may be read by concurrent thread (at least SHOW PROCESSLIST
> does it) and be source for race conditions. Not sure how much these races are
> relevant though.

Relevant. We've had these races with thd->query_string - and quite a few
of them.

But I think they shouldn't be an issue in this case, because you won't
free either thd->db or TABLE_SHARE::db. That is, any concurrent SHOW
PROCESSLIST will see a valid string and will use it just fine. So I
don't think this will cause any issues.


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