Hi,
On 18.01.21 23:42, Tom Lane wrote:
David Geier<da...@swarm64.com> writes:
On 18.01.21 19:46, Tom Lane wrote:
Hm. I agree that we shouldn't simply assume that ss_currentRelation
isn't null. However, we cannot make search_plan_tree() descend
through non-leaf CustomScan nodes, because we don't know what processing
is involved there. We need to find a scan that is guaranteed to return
rows that are one-to-one with the cursor output. This is why the function
doesn't descend through join or aggregation nodes, and I see no argument
by which we should assume we know more about what a customscan node will
do than we know about those.
That makes sense. Thanks for the explanation.
OK, cool. I was afraid you'd argue that you really needed your CustomScan
node to be transparent in such cases. We could imagine inventing an
additional custom-scan-provider callback to embed the necessary knowledge,
but I'd rather not add the complexity until someone has a use-case.
I was thinking about that. Generally, having such possibility would be
very useful. Unfortunately, I believe that in my specific case even
having such functionality wouldn't allow for the query to work with
CURRENT OF, because my CSP behaves similarly to a materialize node.
My understanding is only such plans are supported which consist of nodes
that guarantee that the tuple returned by the plan is the unmodified
tuple a scan leaf node is currently positioned on?
Still, if there's interest I would be happy to draft a patch. Instead of
a separate CSP callback, we could also provide an additional flag like
CUSTOMPATH_SUPPORT_CURRENT_OF. The advantage of the callback would be
that we could delay the decision until execution time where potentially
more information is available.
I updated the patch to match your proposal.
WFM, will push in a bit.
regards, tom lane
Best regards,
David
Swarm64
