Re: ExecRTCheckPerms() and many prunable partitions - Mailing list pgsql-hackers

On Fri, Jul 2, 2021 at 12:45 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Amit Langote <amitlangote09@gmail.com> writes:
> > The problem is that it loops over the entire range table even though
> > only one or handful of those entries actually need their permissions
> > checked.  Most entries, especially those of partition child tables
> > have their requiredPerms set to 0, which David pointed out to me in
> > [2], so what ExecCheckRTPerms() does in their case is pure overhead.
>
> > An idea to fix that is to store the RT indexes of the entries that
> > have non-0 requiredPerms into a separate list or a bitmapset in
> > PlannedStmt.
>
> I think perhaps we ought to be more ambitious than that, and consider
> separating the list of permissions-to-check from the rtable entirely.
> Your patch hardly qualifies as non-invasive, plus it seems to invite
> errors of omission, while if we changed the data structure altogether
> then the compiler would help find any not-updated code.
>
> But the main reason that this strikes me as possibly a good idea
> is that I was just taking another look at the complaint in [1],
> where I wrote
>
> >> I think it's impossible to avoid less-than-O(N^2) growth on this sort
> >> of case.  For example, the v2 subquery initially has RTEs for v2 itself
> >> plus v1.  When we flatten v1 into v2, v2 acquires the RTEs from v1,
> >> namely v1 itself plus foo.  Similarly, once vK-1 is pulled up into vK,
> >> there are going to be order-of-K entries in vK's rtable, and that stacking
> >> makes for O(N^2) work overall just in manipulating the rtable.
> >>
> >> We can't get rid of these rtable entries altogether, since all of them
> >> represent table privilege checks that the executor will need to do.
>
> Perhaps, if we separated the rtable from the required-permissions data
> structure, then we could avoid pulling up otherwise-useless RTEs when
> flattening a view (or even better, not make the extra RTEs in the
> first place??), and thus possibly avoid that exponential planning-time
> growth for nested views.
>
> Or maybe not.  But I think we should take a hard look at whether
> separating these data structures could solve both of these problems
> at once.

Ah, okay.  I'll think about decoupling the permission checking stuff
from the range table data structure.

Thanks for the feedback.

I'll mark the CF entry as WoA, unless you'd rather I just mark it RwF.

-- 
Amit Langote
EDB: http://www.enterprisedb.com