microbluey commented on PR #5125:
URL: https://github.com/apache/calcite/pull/5125#issuecomment-5094221588
Thanks for looking. Both answered, and I've pushed a commit for the first.
**"What is DEE?"** — fair challenge: the abbreviation appears nowhere else
in the codebase, so it shouldn't be in a comment. It's the standard
relational-algebra name for the relation with no attributes and one tuple (the
identity for cross join; DUM is its empty counterpart), but I've replaced it
with a plain description — "a Values with an empty row type and a single empty
tuple, which returns one row with zero columns" — in `269e562`.
**"Can this be reproduced with a quidem test?"** — I tried, and I don't
think it can, which I should have said in the description rather than leaving
you to ask.
You're right that not every builder-constructible plan surfaces from a real
program. I attempted the SQL routes to a zero-column join input:
```
-- (select) as u -> parse error, not valid syntax
select cast(a.x as varchar) from (values (1)) as a(x)
cross join (select count(*) from (values (1),(2))) as b; -- runs fine
select cast(a.x as varchar) as c from (values (1)) as a(x)
cross join (select 1 from (values (1),(2)) group by ()) as b; -- runs fine
```
Both execute correctly. The aggregate keeps a column in the plan the
validator produces, so the workaround always has a first column to fall back on
and the crash never triggers. `RelFieldTrimmer` deliberately avoids handing
anything a zero-column row type for the same historical reason as this
workaround (see the "Fennel abhors an empty row type" comment at
`RelFieldTrimmer.java:1031`), which is presumably why no SQL path reaches it.
So the reachable route is via `RelBuilder` or an equivalent frontend rather
than Calcite's own SQL: the reporter hit it through substrait-java, where an
`Aggregate(GROUP BY (), measures)` with `Rel.Emit{outputMapping=[]}` yields
exactly this shape on one side of a cross join. Since `PushProjector` guards
defensively elsewhere and the fix is two bounds checks, guarding here seemed
worth it even though Calcite's own parser can't get there — but if you'd rather
this be fixed at the frontend, or declined as unreachable, that's a reasonable
call and I'm happy to close it.
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