Re: API change advice: Passing plan invalidation info from the rewriter into the planner?

On Tue, Jun 24, 2014 at 12:27 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> I feel like we are getting to the point of simply talking past each
> other and so I'll try anew, and I'll include my understanding of how the
> different approaches would address the specific use-case you outlined
> up-thread.

Thanks, you're right, and this is a good write-up.

> Single policy
> -------------
> The current implementation approach only allows a single policy to be
> included.
> [...snip...]
> For the case where a sales rep isn't also a partner, you could simplify
> this to:
>
> WHERE
> sales_rep_id = 16384
>
> but I'm not sure that really buys you much? With the bitmap heap
> scan, if one side of the OR ends up not returning anything then it
> doesn't contribute to the blocks which have to be scanned. The index
> might still need to be scanned, although I think you could avoid even
> that with an EXISTS check to see if the user is a partner at all.
> That's not to say that a bitmap scan is equivilant to an index scan, but
> it's certainly likely to be far better than a sequential scan.

True, but the wins could be much larger if one policy is WHERE
sales_rep_id = (SELECT oid FROM pg_roles WHERE rolname = current_user)
and the other policy is WHERE complexfn(). I'll also throw out a +1
for Dean's comments on this topic.

> Multiple, Non-overlapping policies
> ----------------------------------
> Preventing the overlap of policies ends up being very complicated if
> many dimensions are allowed. For the simple case, perhaps only the
> 'current role' dimension is useful. I expect that going down that
> route would very quickly lead to requests for other dimensions (client
> IP, etc) which is why I'm not a big fan of it, but if that's the
> concensus then let's work out the syntax and update the patch and move
> on.

I think role is good enough. That's the primary identifier for all
access-control related decisions, so it should be good enough here,
too.

I don't really understand your concerns about overlapping policies
being complex. If you've got a couple of WHERE clauses, combining
them with OR is not hard. Now, the query optimizer may have trouble
with it, but on the whole I expect to win more than we lose, by
entirely excluding some branches of an OR for users for whom entirely
policies can be excluded.

> Overall, while I'm interested in defining where this is going in a way
> which allows us implement an initial RLS capability while avoiding
> future upgrade issues, I am perfectly happy to say that the 9.5 RLS
> implementation may not be exactly syntax-compatible with 9.6 or 10.0.

Again, I think that's completely non-viable. Are you going to tell
people they can't pg_upgrade, and they can't dump-and-reload either,
without manual fiddling? There's no way that's going to be accepted.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company