Re: CLUSTER, REINDEX, VACUUM in "read only" transaction?

Tom Lane wrote:
> So I was looking for other omissions in utility.c, and I noticed that
> check_xact_readonly() doesn't reject CLUSTER, REINDEX, or VACUUM.
> Now the notion of "read only" that we're trying to enforce is pretty
> weak (I think it's effectively "no writes to non-temp tables").
> But I can't see that CLUSTER is a read-only operation even under the
> weakest definitions, and I'm not seeing the rationale for REINDEX or
> VACUUM here either.

I think the way the SQL standard meant the read-only flag is that the 
transaction doesn't change the structure of or the data in the database 
as seen by the next guy.  So all of these commands are OK, I think.

A theoretical use case is that you should be able to do the maximum set 
of useful work in read-only mode on a Slony-I slave.  No I haven't 
checked what Slony does with these three commands, so let me have it. :-)

Other definitions might be OK, but I can't see one offhand that is based 
on the current behavior but disallows these three commands.  "No disk 
writes" or "no big locks" is probably not what the SQL standard meant.