Re: ALTER TABLESPACE MOVE command tag tweak

Tom Lane wrote:
> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> > The ALTER TABLESPACE MOVE command affects tables, not tablespaces; and
> > as such, I think event triggers should support that command.  I'm not
> > proposing to change event triggers at this stage, but since IMO we will
> > want to do that in 9.5, we need it to have a different command tag than
> > plain ALTER TABLESPACE.  This is so that check_ddl_tag() can compare
> > the tag with ALTER TABLESPACE and say "unsupported", and ALTER
> > TABLESPACE MOVE and say "supported".  Both are currently spelled the
> > same, which will be a problem.
> 
> > Therefore I propose the attached patch for 9.4.
> 
> Hm.  While the specific change here seems harmless enough, the argument
> for it seems to me to indicate that the very design is broken.  Do you
> expect event triggers to distinguish all the different subflavors of
> ALTER TABLE, for example, on the basis of the command tag?  Backwards
> compatibility is going to prevent us from refining the tag strings
> that much.

Actually, I don't -- I have already implemented ALTER TABLE for event
triggers, and there wasn't any need to tweak the command tags there.
The problem in this particular case is that TABLESPACE is a "global"
object, thus not supported; but ALTER TABLESPACE MOVE is a command that
modifies tables (which *are* supported), not tablespaces.

ALTER TABLESPACE MOVE is a glorified ALTER TABLE.  If ALTER TABLESPACE
MOVE returned ALTER TABLE as a tag, I think it'd work well too; but not
ALTER TABLESPACE.  Individually, since the implementation works by
calling AlterTableInternal(), it already works.

Now if you state that the current design in event_triggers that works by
slicing CommandTag and comparing the pieces is broken, I don't disagree
and I think I have now (in the patch posted in a nearby thread) some
more infrastructure to do it differently.  But even if we do that, I
think we're going to need a way to differentiate ALTER TABLESPACE MOVE
from other forms of ALTER TABLESPACE.  I haven't given this much
thought, though.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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