On Tue, 16 Jun 2020 at 11:45, Amit Langote <amitlangot...@gmail.com> wrote:
> On Fri, Jun 12, 2020 at 9:22 PM Ashutosh Bapat > <ashutosh.bapat....@gmail.com> wrote: > > On Wed, Jun 3, 2020 at 12:44 PM Amit Langote <amitlangot...@gmail.com> > wrote: > > > Are you saying that the planner should take into account the state of > > > the cursor specified in WHERE CURRENT OF to determine which of the > > > tables to scan for the UPDATE? Note that neither partition pruning > > > nor constraint exclusion know that CurrentOfExpr can possibly allow to > > > exclude children of the UPDATE target. > > > > Yes. But it may not be possible to know the value of current of at the > > time of planning since that need not be a plan time constant. This > > pruning has to happen at run time. > > Good point about not doing anything at planning time. > > I wonder if it wouldn't be okay to simply make execCurrentOf() return > false if it can't find either a row mark or a Scan node in the cursor > matching the table being updated/deleted from, instead of giving an > error message? I mean what do we gain by erroring out here instead of > simply not doing anything? Now, it would be nicer if we could do so > only if the table being updated/deleted from is a child table, but it > seems pretty inconvenient to tell that from the bottom of a plan tree > from where execCurrentOf() is called. > A safe guard from a bug where current of is set to wrong table or something. Quite rare bug but if we can fix the problem itself removing a safe guard doesn't seem wise. > The other option would be to have some bespoke "pruning" logic in, > say, ExecInitModifyTable() that fetches the current active table from > the cursor and processes only the matching child result relation. looks better if that works and I don't see a reason why it won't work. > Or > maybe wait until we have run-time pruning for ModifyTable, because the > result relation code restructuring required for that will also be > something we'd need for this. > > I don't see much difference in the final plan with either options. -- Best Wishes, Ashutosh
