alamb commented on PR #15770:
URL: https://github.com/apache/datafusion/pull/15770#issuecomment-2971510953
> 1. Mutli-column order by not working. This seems like a bug / oversight.
Fundamentally I don't see any reason it shouldn't work, I'll have to
investigate why.
I suggest we file a ticket explaining what is going on so we don't lose the
context, but don't try and fix it in this PR
> 2. Interaction with `EnforceSorting` & co. After grokking the code for a
while I'm pretty convinced this is the only way to do things correctly.
`SortExec` needs to establish a link (via references) with the
`DataSourceExec`, and that needs to happen _before_ `EnforceSorting` runs
because it might modify the plan in the process. So we have no choice but to
teach `EnforceSorting` how to preserve this link. But that's the part I'm not
sure I did correctly since I don't fully understand how `EnforceSorting` works.
There may be other alternatives here. @berkaysynnada suggested that the right
rule ordering might "just work", but neither of us could get it to. We might be
able to split the filter pushdown into two steps: static (cannot make
assumptions about reference links but can modify the plan tree, e.g. for
`FilterExec`) and dynamic (can make reference links but cannot modify the plan
tree, e.g. `SortExec` and `HashJoin`). I think this would consist of adding a
new optimizer rule
`DynamicFilterPushdown` or something like that and maybe a
`Phase::{Dynamic,Static}` argument to the existing method on `ExecutionPlan`.
I just know we have also found EnforceSorting to be fragile (and there have
been quite a few bugs related to limits being lost, etc) so I agree with your
assesment that it is tough to know if it is correct.
I don't understand why `FilterPushdown` can't also push down DynamicFlters
if it was run after `EnforceSorting` but I vaguely remember it being discussed
and rejected before
> 3. Formatting in explain plans. I'm open to suggestions on this!
Likewise I think we should just file a follow on ticket. Maybe we can change
the `Display` impl to be just `<DynamicFilter>` or something to make it clear
it is something special (not just `false` constant)
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