alamb commented on code in PR #26640:
URL: https://github.com/apache/datafusion/pull/26640#discussion_r4417526454
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datafusion/optimizer/src/decorrelate.rs:
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@@ -897,6 +923,37 @@ impl PullUpCorrelatedExpr {
}
}
+/// Whether the emptiness of `plan`'s output follows from the emptiness of its
Review Comment:
I don't really understand what emptness means in this context. Is this
saying if the input to the plan is non empty than the output will also be non
empty?
How will we know that for user defined plans?
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datafusion/optimizer/src/decorrelate.rs:
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@@ -897,6 +923,37 @@ impl PullUpCorrelatedExpr {
}
}
+/// Whether the emptiness of `plan`'s output follows from the emptiness of its
+/// input alone, so that `plan` cannot tell apart two non-empty inputs.
Removing
+/// a `Limit` below such a node changes which rows it is given, never whether
it
+/// is given any, and so never whether the subquery is empty.
+///
+/// A `Projection`, a `SubqueryAlias`, a `Sort`, a `Window` and a `Repartition`
+/// each output one row per input row. A `DISTINCT` and a grouped `Aggregate`
output rows
+/// exactly when their input has rows, and an `Aggregate` without a grouping
+/// always outputs its one row, so no `Aggregate` can be emptied by the rows a
+/// `Limit` held back.
+/// A materialized CTE outputs exactly its continuation's rows, and only the
+/// continuation is rewritten, so its emptiness follows that input.
+///
+/// A `Filter` is what this leaves out: it reads values, so the rows the
`Limit`
Review Comment:
I found this really hard to understand -- it seems to be saying that Filter
removes rows ?
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datafusion/optimizer/src/decorrelate_predicate_subquery.rs:
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@@ -2820,6 +2820,137 @@ mod tests {
)
}
+ /// A `Limit` whose row count nothing above it can observe is still
dropped,
Review Comment:
what value do these tests add over just SLT tests?
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